statistics: add statistic variables

The StatisticVariable class hierarchy encapsulates the concept
of a dive-variable, which can be plotted in charts either as
dependend or independend variable.

There are three types of these variables:
1) discrete: For example dive buddies or suit type.
2) continuous: Has a notion of linear metric - can be
   used as histogram or scatter plot axis.
3) numeric: Like continuous, but allows for operations
   such as calculating the mean or the sum over numerous
   dives.

All variables support binning. The bins are defined per
variable.

Continuous variables can be converted into an arbitrary
double value, which is used to be plotted on a continuous
axis.

Moreover, numeric variables support a number of operations,
which depend on the variable.

Since binning is based on different types, the code is rather
template-heavy. Of course, this could be solved with
unions/variants and runtime-polymorphism, but using templates
was just much quicker. Notably, this uses the CRTP
(curiously recurring template pattern) where a subclass
passes itself as argument to the baseclass. This is a weird
kind of "reverse inheritance".

The StatsTranslations class is a dummy class which will
be used to collect all translations of the statistics
module.

This includes changes by Dirk to fix compilation of the
downloader.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

5 files changed, 1888 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4cb3effe1..f359aa823 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -320,6 +320,7 @@ if (NOT SUBSURFACE_TARGET_EXECUTABLE MATCHES "DownloaderExecutable")
 add_subdirectory(profile-widget)
 add_subdirectory(map-widget)
 add_subdirectory(mobile-widgets)
+add_subdirectory(stats)
 endif()
 add_subdirectory(backend-shared)
 
@@ -363,6 +364,7 @@ if (SUBSURFACE_TARGET_EXECUTABLE MATCHES "MobileExecutable")
 		subsurface_models_mobile
 		subsurface_commands
 		subsurface_corelib
+		subsurface_stats
 		kirigamiplugin
 		${SUBSURFACE_LINK_LIBRARIES}
 	)
diff --git a/stats/CMakeLists.txt b/stats/CMakeLists.txt
new file mode 100644
index 000000000..1eba8377f
--- /dev/null
+++ b/stats/CMakeLists.txt
@@ -0,0 +1,15 @@
+# the stats-graph widget
+include_directories(.
+	${CMAKE_CURRENT_BINARY_DIR}
+	${CMAKE_BINARY_DIR}
+)
+
+set(SUBSURFACE_STATS_SRCS
+	statsvariables.h
+	statsvariables.cpp
+)
+
+source_group("Subsurface statistics sourcecode" FILES ${SUBSURFACE_STATS_SRCS})
+
+add_library(subsurface_stats STATIC ${SUBSURFACE_STATS_SRCS})
+target_link_libraries(subsurface_stats ${QT_LIBRARIES})
diff --git a/stats/statstranslations.h b/stats/statstranslations.h
new file mode 100644
index 000000000..dadfc83af
--- /dev/null
+++ b/stats/statstranslations.h
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0
+// Dummy object for stats-module related translations
+
+#ifndef STATS_TRANSLATIONS_H
+#define STATS_TRANSLATIONS_H
+
+#include <QObject>
+
+class StatsTranslations : public QObject
+{
+	Q_OBJECT
+};
+
+#endif
diff --git a/stats/statsvariables.cpp b/stats/statsvariables.cpp
new file mode 100644
index 000000000..9551b5386
--- /dev/null
+++ b/stats/statsvariables.cpp
@@ -0,0 +1,1708 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "statsvariables.h"
+#include "statstranslations.h"
+#include "core/dive.h"
+#include "core/divemode.h"
+#include "core/divesite.h"
+#include "core/gas.h"
+#include "core/pref.h"
+#include "core/qthelper.h" // for get_depth_unit() et al.
+#include "core/string-format.h"
+#include "core/subsurface-time.h"
+#include <cmath>
+#include <limits>
+#include <QLocale>
+
+#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
+#define SKIP_EMPTY Qt::SkipEmptyParts
+#else
+#define SKIP_EMPTY QString::SkipEmptyParts
+#endif
+
+static const constexpr double NaN = std::numeric_limits<double>::quiet_NaN();
+
+// Typedefs for year / quarter or month binners
+using year_quarter = std::pair<unsigned short, unsigned short>;
+using year_month = std::pair<unsigned short, unsigned short>;
+
+// Small helper template: add an item to an unsorted vector, if its not already there.
+template<typename T>
+static void add_to_vector_unique(std::vector<T> &v, const T &item)
+{
+	if (std::find(v.begin(), v.end(), item) == v.end())
+		v.push_back(item);
+}
+
+// Small helper: make a comma separeted list of a vector of QStrings
+static QString join_strings(const std::vector<QString> &v)
+{
+	QString res;
+	for (const QString &s: v) {
+		if (!res.isEmpty())
+			res += ", ";
+		res += s;
+	}
+	return res;
+}
+
+// Note: usually I dislike functions defined inside class/struct
+// declarations ("Java style"). However, for brevity this is done
+// in this rather template-heavy source file more or less consistently.
+
+// Templates to define invalid values for variables and test for said values.
+// This is used by the binners: returning such a value means "ignore this dive".
+template<typename T> T invalid_value();
+template<> int invalid_value<int>()
+{
+	return std::numeric_limits<int>::max();
+}
+template<> double invalid_value<double>()
+{
+	return std::numeric_limits<double>::quiet_NaN();
+}
+template<> QString invalid_value<QString>()
+{
+	return QString();
+}
+template<> StatsQuartiles invalid_value<StatsQuartiles>()
+{
+	double NaN = std::numeric_limits<double>::quiet_NaN();
+	return { NaN, NaN, NaN, NaN, NaN };
+}
+
+static bool is_invalid_value(int i)
+{
+	return i == std::numeric_limits<int>::max();
+}
+
+static bool is_invalid_value(double d)
+{
+	return std::isnan(d);
+}
+
+static bool is_invalid_value(const QString &s)
+{
+	return s.isEmpty();
+}
+
+// Currently, we don't support invalid dates - should we?
+static bool is_invalid_value(const year_quarter &)
+{
+	return false;
+}
+
+static bool is_invalid_value(const dive_site *d)
+{
+	return !d;
+}
+
+static bool is_invalid_value(const StatsOperationResults &res)
+{
+	return !res.isValid();
+}
+
+// Describes a gas-content bin. Since we consider O2 and He, these bins
+// are effectively two-dimensional. However, they are given a linear order
+// by sorting lexicographically.
+// This is different from the general type of the gas, which is defined in
+// gas.h as the gastype enum. The latter does not bin by fine-grained
+// percentages.
+struct gas_bin_t {
+	// Depending on the gas content, we format the bins differently.
+	// Eg. in the absence of helium and inreased oxygen, the bin is
+	// formatted as "EAN32". The format is specified by the type enum.
+	enum class Type {
+		Air,
+		Oxygen,
+		EAN,
+		Trimix
+	} type;
+	int o2, he;
+	static gas_bin_t air() {
+		return { Type::Air, 0, 0 };
+	}
+	static gas_bin_t oxygen() {
+		return { Type::Oxygen, 0, 0 };
+	}
+	static gas_bin_t ean(int o2) {
+		return { Type::EAN, o2, 0 };
+	}
+	static gas_bin_t trimix(int o2, int h2) {
+		return { Type::Trimix, o2, h2 };
+	}
+};
+
+// We never generate gas_bins of invalid gases.
+static bool is_invalid_value(const gas_bin_t &)
+{
+	return false;
+}
+
+static bool is_invalid_value(const std::vector<StatsValue> &v)
+{
+	return v.empty();
+}
+
+static bool is_invalid_value(const StatsQuartiles &q)
+{
+	return std::isnan(q.min);
+}
+
+bool StatsQuartiles::isValid() const
+{
+	return !is_invalid_value(*this);
+}
+
+// Define an ordering for gas types
+// invalid < air < ean (including oxygen) < trimix
+// The latter two are sorted by (helium, oxygen)
+// This is in analogy to the global get_dive_gas() function.
+static bool operator<(const gas_bin_t &t1, const gas_bin_t &t2)
+{
+	if (t1.type != t2.type)
+		return (int)t1.type < (int)t2.type;
+	switch (t1.type) {
+	default:
+	case gas_bin_t::Type::Oxygen:
+	case gas_bin_t::Type::Air:
+		return false;
+	case gas_bin_t::Type::EAN:
+		return t1.o2 < t2.o2;
+	case gas_bin_t::Type::Trimix:
+		return std::tie(t1.o2, t1.he) < std::tie(t2.o2, t2.he);
+	}
+}
+
+static bool operator==(const gas_bin_t &t1, const gas_bin_t &t2)
+{
+	return std::tie(t1.type, t1.o2, t1.he) ==
+	       std::tie(t2.type, t2.o2, t2.he);
+}
+
+static bool operator!=(const gas_bin_t &t1, const gas_bin_t &t2)
+{
+	return !operator==(t1, t2);
+}
+
+// First, let's define the virtual destructors of our base classes
+StatsBin::~StatsBin()
+{
+}
+
+StatsBinner::~StatsBinner()
+{
+}
+
+QString StatsBinner::unitSymbol() const
+{
+	return QString();
+}
+
+StatsVariable::~StatsVariable()
+{
+}
+
+QString StatsBinner::name() const
+{
+	return QStringLiteral("N/A"); // Some dummy string that should never reach the UI
+}
+
+QString StatsBinner::formatWithUnit(const StatsBin &bin) const
+{
+	QString unit = unitSymbol();
+	QString name = format(bin);
+	return unit.isEmpty() ? name : QStringLiteral("%1 %2").arg(name, unit);
+}
+
+QString StatsBinner::formatLowerBound(const StatsBin &bin) const
+{
+	return QStringLiteral("N/A"); // Some dummy string that should never reach the UI
+}
+
+QString StatsBinner::formatUpperBound(const StatsBin &bin) const
+{
+	return QStringLiteral("N/A"); // Some dummy string that should never reach the UI
+}
+
+double StatsBinner::lowerBoundToFloat(const StatsBin &bin) const
+{
+	return 0.0;
+}
+
+double StatsBinner::upperBoundToFloat(const StatsBin &bin) const
+{
+	return 0.0;
+}
+
+bool StatsBinner::preferBin(const StatsBin &bin) const
+{
+	return true;
+}
+
+// Default implementation for discrete variables: there are no bins between discrete bins.
+std::vector<StatsBinPtr> StatsBinner::bins_between(const StatsBin &bin1, const StatsBin &bin2) const
+{
+	return {};
+}
+
+QString StatsVariable::unitSymbol() const
+{
+	return {};
+}
+
+QString StatsVariable::diveCategories(const dive *) const
+{
+	return QString();
+}
+
+int StatsVariable::decimals() const
+{
+	return 0;
+}
+
+double StatsVariable::toFloat(const dive *d) const
+{
+	return invalid_value<double>();
+}
+
+QString StatsVariable::nameWithUnit() const
+{
+	QString s = name();
+	QString symb = unitSymbol();
+	return symb.isEmpty() ? s : QStringLiteral("%1 [%2]").arg(s, symb);
+}
+
+QString StatsVariable::nameWithBinnerUnit(const StatsBinner &binner) const
+{
+	QString s = name();
+	QString symb = binner.unitSymbol();
+	return symb.isEmpty() ? s : QStringLiteral("%1 [%2]").arg(s, symb);
+}
+
+const StatsBinner *StatsVariable::getBinner(int idx) const
+{
+	std::vector<const StatsBinner *> b = binners();
+	if (b.empty())
+		return nullptr;
+	return idx >= 0 && idx < (int)b.size() ? b[idx] : b[0];
+}
+
+std::vector<StatsOperation> StatsVariable::supportedOperations() const
+{
+	return {};
+}
+
+// Attn: The order must correspond to the StatsOperation enum
+static const char *operation_names[] = {
+	QT_TRANSLATE_NOOP("StatsTranslations", "Median"),
+	QT_TRANSLATE_NOOP("StatsTranslations", "Mean"),
+	QT_TRANSLATE_NOOP("StatsTranslations", "Time-weighted mean"),
+	QT_TRANSLATE_NOOP("StatsTranslations", "Sum")
+};
+
+QStringList StatsVariable::supportedOperationNames() const
+{
+	std::vector<StatsOperation> ops = supportedOperations();
+	QStringList res;
+	res.reserve(ops.size());
+	for (StatsOperation op: ops)
+		res.push_back(operationName(op));
+	return res;
+}
+
+StatsOperation StatsVariable::idxToOperation(int idx) const
+{
+	std::vector<StatsOperation> ops = supportedOperations();
+	if (ops.empty()) {
+		qWarning("Stats variable %s does not support operations", qPrintable(name()));
+		return StatsOperation::Median; // oops!
+	}
+	return idx < 0 || idx >= (int)ops.size() ? ops[0] : ops[idx];
+}
+
+QString StatsVariable::operationName(StatsOperation op)
+{
+	int idx = (int)op;
+	return idx < 0 || idx >= (int)std::size(operation_names) ? QString()
+								 : operation_names[(int)op];
+}
+
+double StatsVariable::mean(const std::vector<dive *> &dives) const
+{
+	StatsOperationResults res = applyOperations(dives);
+	return res.isValid() ? res.mean : invalid_value<double>();
+}
+
+std::vector<StatsValue> StatsVariable::values(const std::vector<dive *> &dives) const
+{
+	std::vector<StatsValue> vec;
+	vec.reserve(dives.size());
+	for (dive *d: dives) {
+		double v = toFloat(d);
+		if (!is_invalid_value(v))
+			vec.push_back({ v, d });
+	}
+	std::sort(vec.begin(), vec.end(),
+		  [](const StatsValue &v1, const StatsValue &v2)
+		  { return v1.v < v2.v; });
+	return vec;
+}
+
+QString StatsVariable::valueWithUnit(const dive *d) const
+{
+	QLocale loc;
+	double v = toFloat(d);
+	if (is_invalid_value(v))
+		return QStringLiteral("-");
+	return QString("%1 %2").arg(loc.toString(v, 'f', decimals()),
+				    unitSymbol());
+}
+
+// Small helper to calculate quartiles - section of intervals of
+// two consecutive elements in a vector. It's not strictly correct
+// to interpolate linearly. However, on the one hand we don't know
+// the actual distribution, on the other hand for a discrete
+// distribution the quartiles are ranges. So what should we do?
+static double q1(const StatsValue *v)
+{
+	return (3.0*v[0].v + v[1].v) / 4.0;
+}
+static double q2(const StatsValue *v)
+{
+	return (v[0].v + v[1].v) / 2.0;
+}
+static double q3(const StatsValue *v)
+{
+	return (v[0].v + 3.0*v[1].v) / 4.0;
+}
+
+StatsQuartiles StatsVariable::quartiles(const std::vector<dive *> &dives) const
+{
+	return quartiles(values(dives));
+}
+
+// This expects the value vector to be sorted!
+StatsQuartiles StatsVariable::quartiles(const std::vector<StatsValue> &vec)
+{
+	size_t s = vec.size();
+	if (s == 0)
+		return invalid_value<StatsQuartiles>();
+	switch (s % 4) {
+	default:
+		// gcc doesn't recognize that we catch all possible values. disappointing.
+	case 0:
+		return { vec[0].v, q3(&vec[s/4 - 1]), q2(&vec[s/2 - 1]), q1(&vec[s - s/4 - 1]), vec[s - 1].v };
+	case 1:
+		return { vec[0].v, vec[s/4].v, vec[s/2].v, vec[s - s/4 - 1].v, vec[s - 1].v };
+	case 2:
+		return { vec[0].v, q1(&vec[s/4]), q2(&vec[s/2 - 1]), q3(&vec[s - s/4 - 2]), vec[s - 1].v };
+	case 3:
+		return { vec[0].v, q2(&vec[s/4]), vec[s/2].v, q2(&vec[s - s/4 - 2]), vec[s - 1].v };
+	}
+}
+
+StatsOperationResults StatsVariable::applyOperations(const std::vector<dive *> &dives) const
+{
+	StatsOperationResults res;
+	std::vector<StatsValue> val = values(dives);
+
+	double sumTime = 0.0;
+	res.count = (int)val.size();
+	res.median = quartiles(val).q2;
+
+	if (res.count <= 0)
+		return res;
+
+	for (auto [v, d]: val) {
+		res.sum += v;
+		res.mean += v;
+		sumTime += d->duration.seconds;
+		res.timeWeightedMean += v * d->duration.seconds;
+	}
+
+	res.mean /= res.count;
+	res.timeWeightedMean /= sumTime;
+	return res;
+}
+
+StatsOperationResults::StatsOperationResults() :
+	count(0), median(0.0), mean(0.0), timeWeightedMean(0.0), sum(0.0)
+{
+}
+
+bool StatsOperationResults::isValid() const
+{
+	return count > 0;
+}
+
+double StatsOperationResults::get(StatsOperation op) const
+{
+	switch (op) {
+	case StatsOperation::Median: return median;
+	case StatsOperation::Mean: return mean;
+	case StatsOperation::TimeWeightedMean: return timeWeightedMean;
+	case StatsOperation::Sum: return sum;
+	case StatsOperation::Invalid:
+	default: return invalid_value<double>();
+	}
+}
+
+std::vector<StatsScatterItem> StatsVariable::scatter(const StatsVariable &t2, const std::vector<dive *> &dives) const
+{
+	std::vector<StatsScatterItem> res;
+	res.reserve(dives.size());
+	for (dive *d: dives) {
+		double v1 = toFloat(d);
+		double v2 = t2.toFloat(d);
+		if (is_invalid_value(v1) || is_invalid_value(v2))
+			continue;
+		res.push_back({ v1, v2, d });
+	}
+	std::sort(res.begin(), res.end(),
+		  [](const StatsScatterItem &i1, const StatsScatterItem &i2)
+		  { return std::tie(i1.x, i1.y) < std::tie(i2.x, i2.y); }); // use std::tie() for lexicographical comparison
+	return res;
+}
+
+template <typename T, typename DivesToValueFunc>
+std::vector<StatsBinValue<T>> bin_convert(const StatsVariable &variable, const StatsBinner &binner, const std::vector<dive *> &dives,
+					  bool fill_empty, DivesToValueFunc func)
+{
+	std::vector<StatsBinDives> bin_dives = binner.bin_dives(dives, fill_empty);
+	std::vector<StatsBinValue<T>> res;
+	res.reserve(bin_dives.size());
+	for (auto &[bin, dives]: bin_dives) {
+		T v = func(dives);
+		if (is_invalid_value(v) && (res.empty() || !fill_empty))
+			continue;
+		res.push_back({ std::move(bin), v });
+	}
+	if (res.empty())
+		return res;
+
+	// Check if we added invalid items at the end.
+	// Note: we added at least one valid item.
+	auto it = res.end() - 1;
+	while (it != res.begin() && is_invalid_value(it->value))
+		--it;
+	res.erase(it + 1, res.end());
+	return res;
+}
+
+std::vector<StatsBinQuartiles> StatsVariable::bin_quartiles(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const
+{
+	return bin_convert<StatsQuartiles>(*this, binner, dives, fill_empty,
+					   [this](const std::vector<dive *> &d) { return quartiles(d); });
+}
+
+std::vector<StatsBinOp> StatsVariable::bin_operations(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const
+{
+	return bin_convert<StatsOperationResults>(*this, binner, dives, fill_empty,
+						  [this](const std::vector<dive *> &d) { return applyOperations(d); });
+}
+
+std::vector<StatsBinValues> StatsVariable::bin_values(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const
+{
+	return bin_convert<std::vector<StatsValue>>(*this, binner, dives, fill_empty,
+						    [this](const std::vector<dive *> &d) { return values(d); });
+}
+
+// Silly template, which spares us defining type() member functions.
+template<StatsVariable::Type t>
+struct StatsVariableTemplate : public StatsVariable {
+	Type type() const override { return t; }
+};
+
+// A simple bin that is based on copyable value and can be initialized from
+// that value. This template spares us from writing one-line constructors.
+template<typename Type>
+struct SimpleBin : public StatsBin {
+	Type value;
+	SimpleBin(const Type &v) : value(v) { }
+
+	// This must not be called for different types. It will crash with an exception.
+	bool operator<(StatsBin &b) const {
+		return value < dynamic_cast<SimpleBin &>(b).value;
+	}
+
+	bool operator==(StatsBin &b) const {
+		return value == dynamic_cast<SimpleBin &>(b).value;
+	}
+};
+using IntBin = SimpleBin<int>;
+using StringBin = SimpleBin<QString>;
+using GasTypeBin = SimpleBin<gas_bin_t>;
+
+// A general binner template that works on trivial bins that are based
+// on a type that is equality and less-than comparable. The derived class
+// must possess:
+//  - A to_bin_value() function that turns a dive into a value from
+//    which the bins can be constructed.
+//  - A lowerBoundToFloatBase() function that turns the value form
+//    into a double which is understood by the StatsVariable.
+// The bins must possess:
+//  - A member variable "value" of the type it is constructed with.
+// Note: this uses the curiously recurring template pattern, which I
+// dislike, but it is the easiest thing for now.
+template<typename Binner, typename Bin>
+struct SimpleBinner : public StatsBinner {
+public:
+	using Type = decltype(Bin::value);
+	std::vector<StatsBinDives> bin_dives(const std::vector<dive *> &dives, bool fill_empty) const override;
+	std::vector<StatsBinCount> count_dives(const std::vector<dive *> &dives, bool fill_empty) const override;
+	const Binner &derived() const {
+		return static_cast<const Binner &>(*this);
+	}
+	const Bin &derived_bin(const StatsBin &bin) const {
+		return dynamic_cast<const Bin &>(bin);
+	}
+};
+
+// Wrapper around std::lower_bound that searches for a value in a
+// vector of pairs. Comparison is made with the first element of the pair.
+// std::lower_bound does a binary search and this is used to keep a
+// vector in ascending order.
+template<typename T1, typename T2>
+auto pair_lower_bound(std::vector<std::pair<T1, T2>> &v, const T1 &value)
+{
+	return std::lower_bound(v.begin(), v.end(), value,
+	       			[] (const std::pair<T1, T2> &entry, const T1 &value) {
+					return entry.first < value;
+				});
+}
+
+// Register a dive in a (bin_value, value) pair. The second value can be
+// anything, for example a count or a list of dives. If the bin does not
+// exist, it is created. The add_dive_func() function increase the second
+// value accordingly.
+template<typename BinValueType, typename ValueType, typename AddDiveFunc>
+void register_bin_value(std::vector<std::pair<BinValueType, ValueType>> &v,
+			const BinValueType &bin,
+			AddDiveFunc add_dive_func)
+{
+	// Does that value already exist?
+	auto it = pair_lower_bound(v, bin);
+	if (it == v.end() || it->first != bin)
+		it = v.insert(it, { bin, ValueType() });	// Bin does not exist -> insert at proper location.
+	add_dive_func(it->second);				// Register dive
+}
+
+// Turn a (bin-value, value)-pair vector into a (bin, value)-pair vector.
+// The values are moved out of the first vectors.
+// If fill_empty is true, missing bins will be completed with a default constructed
+// value.
+template<typename Bin, typename Binner, typename BinValueType, typename ValueType>
+std::vector<StatsBinValue<ValueType>>
+value_vector_to_bin_vector(const Binner &binner, std::vector<std::pair<BinValueType, ValueType>> &value_bins,
+						      bool fill_empty)
+{
+	std::vector<StatsBinValue<ValueType>> res;
+	res.reserve(value_bins.size());
+	for (const auto &[bin_value, value]: value_bins) {
+		StatsBinPtr b = std::make_unique<Bin>(bin_value);
+		if (fill_empty && !res.empty()) {
+			// Add empty bins, if any
+			for (StatsBinPtr &bin: binner.bins_between(*res.back().bin, *b))
+				res.push_back({ std::move(bin), ValueType() });
+		}
+		res.push_back({ std::move(b), std::move(value)});
+	}
+	return res;
+}
+
+template<typename Binner, typename Bin>
+std::vector<StatsBinDives> SimpleBinner<Binner, Bin>::bin_dives(const std::vector<dive *> &dives, bool fill_empty) const
+{
+	// First, collect a value / dives vector and then produce the final vector
+	// out of that. I wonder if that is premature optimization?
+	using Pair = std::pair<Type, std::vector<dive *>>;
+	std::vector<Pair> value_bins;
+	for (dive *d: dives) {
+		Type value = derived().to_bin_value(d);
+		if (is_invalid_value(value))
+			continue;
+		register_bin_value(value_bins, value,
+				   [d](std::vector<dive *> &v) { v.push_back(d); });
+	}
+
+	// Now, turn that into our result array with allocated bin objects.
+	return value_vector_to_bin_vector<Bin>(*this, value_bins, fill_empty);
+}
+
+template<typename Binner, typename Bin>
+std::vector<StatsBinCount> SimpleBinner<Binner, Bin>::count_dives(const std::vector<dive *> &dives, bool fill_empty) const
+{
+	// First, collect a value / counts vector and then produce the final vector
+	// out of that. I wonder if that is premature optimization?
+	using Pair = std::pair<Type, int>;
+	std::vector<Pair> value_bins;
+	for (const dive *d: dives) {
+		Type value = derived().to_bin_value(d);
+		if (is_invalid_value(value))
+			continue;
+		register_bin_value(value_bins, value, [](int &i){ ++i; });
+	}
+
+	// Now, turn that into our result array with allocated bin objects.
+	return value_vector_to_bin_vector<Bin>(*this, value_bins, fill_empty);
+}
+
+// A simple binner (see above) that works on continuous (or numeric) variables
+// and can return bin-ranges. The binner must implement an inc() function
+// that turns a bin into the next-higher bin.
+template<typename Binner, typename Bin>
+struct SimpleContinuousBinner : public SimpleBinner<Binner, Bin>
+{
+	using SimpleBinner<Binner, Bin>::derived;
+	std::vector<StatsBinPtr> bins_between(const StatsBin &bin1, const StatsBin &bin2) const override;
+
+	// By default the value gives the lower bound, so the format is the same
+	QString formatLowerBound(const StatsBin &bin) const override {
+		return derived().format(bin);
+	}
+
+	// For the upper bound, simply go to the next bin
+	QString formatUpperBound(const StatsBin &bin) const override {
+		Bin b = SimpleBinner<Binner,Bin>::derived_bin(bin);
+		derived().inc(b);
+		return formatLowerBound(b);
+	}
+
+	// Cast to base value type so that the derived class doesn't have to do it
+	double lowerBoundToFloat(const StatsBin &bin) const override {
+		const Bin &b = SimpleBinner<Binner,Bin>::derived_bin(bin);
+		return derived().lowerBoundToFloatBase(b.value);
+	}
+
+	// For the upper bound, simply go to the next bin
+	double upperBoundToFloat(const StatsBin &bin) const override {
+		Bin b = SimpleBinner<Binner,Bin>::derived_bin(bin);
+		derived().inc(b);
+		return derived().lowerBoundToFloatBase(b.value);
+	}
+};
+
+// A continuous binner, where the bin is based on an integer value
+// and subsequent bins are adjacent integers.
+template<typename Binner, typename Bin>
+struct IntBinner : public SimpleContinuousBinner<Binner, Bin>
+{
+	void inc(Bin &bin) const {
+		++bin.value;
+	}
+};
+
+// An integer based binner, where each bin represents an integer
+// range with a fixed size.
+template<typename Binner, typename Bin>
+struct IntRangeBinner : public IntBinner<Binner, Bin> {
+	int bin_size;
+	IntRangeBinner(int size)
+		: bin_size(size)
+	{
+	}
+	QString format(const StatsBin &bin) const override {
+		int value = IntBinner<Binner, Bin>::derived_bin(bin).value;
+		QLocale loc;
+		return StatsTranslations::tr("%1–%2").arg(loc.toString(value * bin_size),
+							  loc.toString((value + 1) * bin_size));
+	}
+	QString formatLowerBound(const StatsBin &bin) const override {
+		int value = IntBinner<Binner, Bin>::derived_bin(bin).value;
+		return QStringLiteral("%L1").arg(value * bin_size);
+	}
+	double lowerBoundToFloatBase(int value) const {
+		return static_cast<double>(value * bin_size);
+	}
+};
+
+template<typename Binner, typename Bin>
+std::vector<StatsBinPtr> SimpleContinuousBinner<Binner, Bin>::bins_between(const StatsBin &bin1, const StatsBin &bin2) const
+{
+	const Bin &b1 = SimpleBinner<Binner,Bin>::derived_bin(bin1);
+	const Bin &b2 = SimpleBinner<Binner,Bin>::derived_bin(bin2);
+	std::vector<StatsBinPtr> res;
+	Bin act = b1;
+	derived().inc(act);
+	while (act.value < b2.value) {
+		res.push_back(std::make_unique<Bin>(act));
+		derived().inc(act);
+	}
+	return res;
+}
+
+// A binner template for discrete variables that where each dive can belong to
+// multiple bins. The bin-type must be less-than comparable. The derived class
+// must possess:
+//  - A to_bin_values() function that turns a dive into a value from
+//    which the bins can be constructed.
+// The bins must possess:
+//  - A member variable "value" of the type it is constructed with.
+template<typename Binner, typename Bin>
+struct MultiBinner : public StatsBinner {
+public:
+	using Type = decltype(Bin::value);
+	std::vector<StatsBinDives> bin_dives(const std::vector<dive *> &dives, bool fill_empty) const override;
+	std::vector<StatsBinCount> count_dives(const std::vector<dive *> &dives, bool fill_empty) const override;
+	const Binner &derived() const {
+		return static_cast<const Binner &>(*this);
+	}
+	const Bin &derived_bin(const StatsBin &bin) const {
+		return dynamic_cast<const Bin &>(bin);
+	}
+};
+
+template<typename Binner, typename Bin>
+std::vector<StatsBinDives> MultiBinner<Binner, Bin>::bin_dives(const std::vector<dive *> &dives, bool) const
+{
+	// First, collect a value / dives vector and then produce the final vector
+	// out of that. I wonder if that is premature optimization?
+	using Pair = std::pair<Type, std::vector<dive *>>;
+	std::vector<Pair> value_bins;
+	for (dive *d: dives) {
+		for (const Type &val: derived().to_bin_values(d)) {
+			if (is_invalid_value(val))
+				continue;
+			register_bin_value(value_bins, val,
+					   [d](std::vector<dive *> &v) { v.push_back(d); });
+		}
+	}
+
+	// Now, turn that into our result array with allocated bin objects.
+	return value_vector_to_bin_vector<Bin>(*this, value_bins, false);
+}
+
+template<typename Binner, typename Bin>
+std::vector<StatsBinCount> MultiBinner<Binner, Bin>::count_dives(const std::vector<dive *> &dives, bool) const
+{
+	// First, collect a value / counts vector and then produce the final vector
+	// out of that. I wonder if that is premature optimization?
+	using Pair = std::pair<Type, int>;
+	std::vector<Pair> value_bins;
+	for (const dive *d: dives) {
+		for (const Type &s: derived().to_bin_values(d)) {
+			if (is_invalid_value(s))
+				continue;
+			register_bin_value(value_bins, s, [](int &i){ ++i; });
+		}
+	}
+
+	// Now, turn that into our result array with allocated bin objects.
+	return value_vector_to_bin_vector<Bin>(*this, value_bins, false);
+}
+
+// A binner that works on string-based bins whereby each dive can
+// produce multiple strings (e.g. dive buddies). The binner must
+// feature a to_bin_values() function that produces a vector of
+// QStrings and bins that can be constructed from QStrings.
+// Other than that, see SimpleBinner.
+template<typename Binner, typename Bin>
+struct StringBinner : public MultiBinner<Binner, Bin> {
+public:
+	QString format(const StatsBin &bin) const override {
+		return dynamic_cast<const Bin &>(bin).value;
+	}
+};
+
+// ============ The date of the dive by year, quarter or month ============
+// (Note that calendar week is defined differently in different parts of the world and therefore omitted for now)
+
+double date_to_double(int year, int month, int day)
+{
+	struct tm tm = { 0 };
+	tm.tm_year = year;
+	tm.tm_mon = month;
+	tm.tm_mday = day;
+	timestamp_t t = utc_mktime(&tm);
+	return t / 86400.0; // Turn seconds since 1970 to days since 1970, if that makes sense...?
+}
+
+struct DateYearBinner : public IntBinner<DateYearBinner, IntBin> {
+	QString name() const override {
+		return StatsTranslations::tr("Yearly");
+	}
+	QString format(const StatsBin &bin) const override {
+		return QString::number(derived_bin(bin).value);
+	}
+	int to_bin_value(const dive *d) const {
+		return utc_year(d->when);
+	}
+	double lowerBoundToFloatBase(int year) const {
+		return date_to_double(year, 0, 0);
+	}
+};
+
+using DateQuarterBin = SimpleBin<year_quarter>;
+
+struct DateQuarterBinner : public SimpleContinuousBinner<DateQuarterBinner, DateQuarterBin> {
+	QString name() const override {
+		return StatsTranslations::tr("Quarterly");
+	}
+	QString format(const StatsBin &bin) const override {
+		year_quarter value = derived_bin(bin).value;
+		return StatsTranslations::tr("%1 Q%2").arg(QString::number(value.first),
+							   QString::number(value.second));
+	}
+	// As histogram axis: show full year for new years and then Q2, Q3, Q4.
+	QString formatLowerBound(const StatsBin &bin) const override {
+		year_quarter value = derived_bin(bin).value;
+		return value.second == 1
+			? QString::number(value.first)
+			: StatsTranslations::tr("Q%1").arg(QString::number(value.second));
+	}
+	double lowerBoundToFloatBase(year_quarter value) const {
+		return date_to_double(value.first, (value.second - 1) * 3, 0);
+	}
+	// Prefer bins that show full years
+	bool preferBin(const StatsBin &bin) const override {
+		year_quarter value = derived_bin(bin).value;
+		return value.second == 1;
+	}
+	year_quarter to_bin_value(const dive *d) const {
+		struct tm tm;
+		utc_mkdate(d->when, &tm);
+
+		int year = tm.tm_year;
+		switch (tm.tm_mon) {
+		case 0 ... 2: return { year, 1 };
+		case 3 ... 5: return { year, 2 };
+		case 6 ... 8: return { year, 3 };
+		default:      return { year, 4 };
+		}
+	}
+	void inc(DateQuarterBin &bin) const {
+		if (++bin.value.second > 4) {
+			bin.value.second = 1;
+			++bin.value.first;
+		}
+	}
+};
+
+using DateMonthBin = SimpleBin<year_month>;
+
+struct DateMonthBinner : public SimpleContinuousBinner<DateMonthBinner, DateMonthBin> {
+	QString name() const override {
+		return StatsTranslations::tr("Monthly");
+	}
+	QString format(const StatsBin &bin) const override {
+		year_month value = derived_bin(bin).value;
+		return QString("%1 %2").arg(monthname(value.second), QString::number(value.first));
+	}
+	// In histograms, output year for fill years, month otherwise
+	QString formatLowerBound(const StatsBin &bin) const override {
+		year_month value = derived_bin(bin).value;
+		return value.second == 0 ? QString::number(value.first)
+					 : QString(monthname(value.second));
+	}
+	double lowerBoundToFloatBase(year_quarter value) const {
+		return date_to_double(value.first, value.second, 0);
+	}
+	// Prefer bins that show full years
+	bool preferBin(const StatsBin &bin) const override {
+		year_month value = derived_bin(bin).value;
+		return value.second == 0;
+	}
+	year_month to_bin_value(const dive *d) const {
+		struct tm tm;
+		utc_mkdate(d->when, &tm);
+		return { tm.tm_year, tm.tm_mon };
+	}
+	void inc(DateMonthBin &bin) const {
+		if (++bin.value.second > 11) {
+			bin.value.second = 0;
+			++bin.value.first;
+		}
+	}
+};
+
+static DateYearBinner date_year_binner;
+static DateQuarterBinner date_quarter_binner;
+static DateMonthBinner date_month_binner;
+struct DateVariable : public StatsVariableTemplate<StatsVariable::Type::Continuous> {
+	QString name() const {
+		return StatsTranslations::tr("Date");
+	}
+	double toFloat(const dive *d) const override {
+		return d->when / 86400.0;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &date_year_binner, &date_quarter_binner, &date_month_binner };
+	}
+};
+
+// ============ Dive depth, binned in 5, 10, 20 m or 15, 30, 60 ft bins ============
+
+struct DepthBinner : public IntRangeBinner<DepthBinner, IntBin> {
+	bool metric;
+	DepthBinner(int bin_size, bool metric) : IntRangeBinner(bin_size), metric(metric)
+	{
+	}
+	QString name() const override {
+		QLocale loc;
+		return StatsTranslations::tr("in %1 %2 steps").arg(loc.toString(bin_size),
+								   get_depth_unit(metric));
+	}
+	QString unitSymbol() const override {
+		return get_depth_unit(metric);
+	}
+	int to_bin_value(const dive *d) const {
+		return metric ? d->maxdepth.mm / 1000 / bin_size
+			      : lrint(mm_to_feet(d->maxdepth.mm)) / bin_size;
+	}
+};
+
+static DepthBinner meter_binner5(5, true);
+static DepthBinner meter_binner10(10, true);
+static DepthBinner meter_binner20(20, true);
+static DepthBinner feet_binner15(15, false);
+static DepthBinner feet_binner30(30, false);
+static DepthBinner feet_binner60(60, false);
+struct DepthVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+	QString name() const override {
+		return StatsTranslations::tr("Max. Depth");
+	}
+	QString unitSymbol() const override {
+		return get_depth_unit();
+	}
+	int decimals() const override {
+		return 1;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		if (prefs.units.length == units::METERS)
+			return { &meter_binner5, &meter_binner10, &meter_binner20 };
+		else
+			return { &feet_binner15, &feet_binner30, &feet_binner60 };
+	}
+	double toFloat(const dive *d) const override {
+		return prefs.units.length == units::METERS ? d->maxdepth.mm / 1000.0
+							   : mm_to_feet(d->maxdepth.mm);
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::Sum };
+	}
+};
+
+// ============ Bottom time, binned in 5, 10, 30 min or 1 h bins ============
+
+struct MinuteBinner : public IntRangeBinner<MinuteBinner, IntBin> {
+	using IntRangeBinner::IntRangeBinner;
+	QString name() const override {
+		return StatsTranslations::tr("in %1 min steps").arg(bin_size);
+	}
+	QString unitSymbol() const override {
+		return StatsTranslations::tr("min");
+	}
+	int to_bin_value(const dive *d) const {
+		return d->duration.seconds / 60 / bin_size;
+	}
+};
+
+struct HourBinner : public IntBinner<HourBinner, IntBin> {
+	QString name() const override {
+		return StatsTranslations::tr("in hours");
+	}
+	QString format(const StatsBin &bin) const override {
+		return QString::number(derived_bin(bin).value);
+	}
+	QString unitSymbol() const override {
+		return StatsTranslations::tr("h");
+	}
+	int to_bin_value(const dive *d) const {
+		return d->duration.seconds / 3600;
+	}
+	double lowerBoundToFloatBase(int hour) const {
+		return static_cast<double>(hour);
+	}
+};
+
+static MinuteBinner minute_binner5(5);
+static MinuteBinner minute_binner10(10);
+static MinuteBinner minute_binner30(30);
+static HourBinner hour_binner;
+struct DurationVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+	QString name() const override {
+		return StatsTranslations::tr("Duration");
+	}
+	QString unitSymbol() const override {
+		return StatsTranslations::tr("min");
+	}
+	int decimals() const override {
+		return 0;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &minute_binner5, &minute_binner10, &minute_binner30, &hour_binner };
+	}
+	double toFloat(const dive *d) const override {
+		return d->duration.seconds / 60.0;
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::Sum };
+	}
+};
+
+// ============ SAC, binned in 2, 5, 10 l/min or 0.1, 0.2, 0.4, 0.8 cuft/min bins ============
+
+struct MetricSACBinner : public IntRangeBinner<MetricSACBinner, IntBin> {
+	using IntRangeBinner::IntRangeBinner;
+	QString name() const override {
+		QLocale loc;
+		return StatsTranslations::tr("in %1 %2/min steps").arg(loc.toString(bin_size),
+								       get_volume_unit());
+	}
+	QString unitSymbol() const override {
+		return get_volume_unit(true) + StatsTranslations::tr("/min");
+	}
+	int to_bin_value(const dive *d) const {
+		if (d->sac <= 0)
+			return invalid_value<int>();
+		return d->sac / 1000 / bin_size;
+	}
+};
+
+// "Imperial" SACs are annoying, since we have to bin to sub-integer precision.
+// We store cuft * 100 as an integer, to avoid troubles with floating point semantics.
+
+struct ImperialSACBinner : public IntBinner<ImperialSACBinner, IntBin> {
+	int bin_size;
+	ImperialSACBinner(double size)
+		: bin_size(lrint(size * 100.0))
+	{
+	}
+	QString name() const override {
+		QLocale loc;
+		return StatsTranslations::tr("in %1 %2/min steps").arg(loc.toString(bin_size / 100.0, 'f', 2),
+								       get_volume_unit());
+	}
+	QString format(const StatsBin &bin) const override {
+		int value = derived_bin(bin).value;
+		QLocale loc;
+		return StatsTranslations::tr("%1–%2").arg(loc.toString((value * bin_size) / 100.0, 'f', 2),
+							  loc.toString(((value + 1) * bin_size) / 100.0, 'f', 2));
+	}
+	QString unitSymbol() const override {
+		return get_volume_unit(false) + StatsTranslations::tr("/min");
+	}
+	QString formatLowerBound(const StatsBin &bin) const override {
+		int value = derived_bin(bin).value;
+		return QStringLiteral("%L1").arg((value * bin_size) / 100.0, 0, 'f', 2);
+	}
+	double lowerBoundToFloatBase(int value) const {
+		return static_cast<double>((value * bin_size) / 100.0);
+	}
+	int to_bin_value(const dive *d) const {
+		if (d->sac <= 0)
+			return invalid_value<int>();
+		return lrint(ml_to_cuft(d->sac) * 100.0) / bin_size;
+	}
+};
+
+MetricSACBinner metric_sac_binner2(2);
+MetricSACBinner metric_sac_binner5(5);
+MetricSACBinner metric_sac_binner10(10);
+ImperialSACBinner imperial_sac_binner1(0.1);
+ImperialSACBinner imperial_sac_binner2(0.2);
+ImperialSACBinner imperial_sac_binner4(0.4);
+ImperialSACBinner imperial_sac_binner8(0.8);
+
+struct SACVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+	QString name() const override {
+		return StatsTranslations::tr("SAC");
+	}
+	QString unitSymbol() const override {
+		return get_volume_unit() + StatsTranslations::tr("/min");
+	}
+	int decimals() const override {
+		return prefs.units.volume == units::LITER ? 0 : 2;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		if (prefs.units.volume == units::LITER)
+			return { &metric_sac_binner2, &metric_sac_binner5, &metric_sac_binner10 };
+		else
+			return { &imperial_sac_binner1, &imperial_sac_binner2, &imperial_sac_binner4, &imperial_sac_binner8 };
+	}
+	double toFloat(const dive *d) const override {
+		if (d->sac <= 0)
+			return invalid_value<double>();
+		return prefs.units.volume == units::LITER ? d->sac / 1000.0 :
+							    ml_to_cuft(d->sac);
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::TimeWeightedMean };
+	}
+};
+
+// ============ Water and air temperature, binned in 2, 5, 10, 20 °C/°F bins ============
+
+struct TemperatureBinner : public IntRangeBinner<TemperatureBinner, IntBin> {
+	bool air;
+	bool metric;
+	TemperatureBinner(int bin_size, bool air, bool metric) :
+		IntRangeBinner(bin_size),
+		air(air),
+		metric(metric)
+	{
+	}
+	QString name() const override {
+		QLocale loc;
+		return StatsTranslations::tr("in %1 %2 steps").arg(loc.toString(bin_size),
+								   get_temp_unit(metric));
+	}
+	QString unitSymbol() const override {
+		return get_temp_unit(metric);
+	}
+	int to_bin_value(const struct dive *d) const {
+		temperature_t t = air ? d->airtemp : d->watertemp;
+		if (t.mkelvin <= 0)
+			return invalid_value<int>();
+		int temp = metric ?  static_cast<int>(mkelvin_to_C(t.mkelvin))
+				  :  static_cast<int>(mkelvin_to_F(t.mkelvin));
+		return temp / bin_size;
+	}
+};
+
+struct TemperatureVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+	TemperatureBinner bin2C, bin5C, bin10C, bin20C;
+	TemperatureBinner bin2F, bin5F, bin10F, bin20F;
+	TemperatureVariable(bool air) :
+		bin2C(2, air, true), bin5C(5, air, true), bin10C(10, air, true), bin20C(20, air, true),
+		bin2F(2, air, false), bin5F(5, air, false), bin10F(10, air, false), bin20F(20, air, false)
+	{
+	}
+	QString unitSymbol() const override {
+		return get_temp_unit();
+	}
+	int decimals() const override {
+		return 1;
+	}
+	double tempToFloat(temperature_t t) const {
+		if (t.mkelvin <= 0)
+			return invalid_value<double>();
+		return prefs.units.temperature == units::CELSIUS ?
+				mkelvin_to_C(t.mkelvin) : mkelvin_to_F(t.mkelvin);
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		if (prefs.units.temperature == units::CELSIUS)
+			return { &bin2C, &bin5C, &bin10C, &bin20C };
+		else
+			return { &bin2F, &bin5F, &bin10F, &bin20F };
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::TimeWeightedMean };
+	}
+};
+
+struct WaterTemperatureVariable : TemperatureVariable {
+	WaterTemperatureVariable() : TemperatureVariable(false)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("Water temperature");
+	}
+	double toFloat(const dive *d) const override {
+		return tempToFloat(d->watertemp);
+	}
+};
+
+struct AirTemperatureVariable : TemperatureVariable {
+	AirTemperatureVariable() : TemperatureVariable(true)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("Air temperature");
+	}
+	double toFloat(const dive *d) const override {
+		return tempToFloat(d->airtemp);
+	}
+};
+
+// ============ Weight, binned in 1, 2, 5, 10 kg or 2, 4, 10 or 20 lbs bins ============
+
+struct WeightBinner : public IntRangeBinner<WeightBinner, IntBin> {
+	bool metric;
+	WeightBinner(int bin_size, bool metric) : IntRangeBinner(bin_size), metric(metric)
+	{
+	}
+	QString name() const override {
+		QLocale loc;
+		return StatsTranslations::tr("in %1 %2 steps").arg(loc.toString(bin_size),
+								   get_weight_unit(metric));
+	}
+	QString unitSymbol() const override {
+		return get_weight_unit(metric);
+	}
+	int to_bin_value(const dive *d) const {
+		return metric ? total_weight(d) / 1000 / bin_size
+			      : lrint(grams_to_lbs(total_weight(d))) / bin_size;
+	}
+};
+
+static WeightBinner weight_binner_1kg(1, true);
+static WeightBinner weight_binner_2kg(2, true);
+static WeightBinner weight_binner_5kg(5, true);
+static WeightBinner weight_binner_10kg(10, true);
+static WeightBinner weight_binner_2lbs(2, false);
+static WeightBinner weight_binner_5lbs(4, false);
+static WeightBinner weight_binner_10lbs(10, false);
+static WeightBinner weight_binner_20lbs(20, false);
+
+struct WeightVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+	QString name() const override {
+		return StatsTranslations::tr("Weight");
+	}
+	QString unitSymbol() const override {
+		return get_weight_unit();
+	}
+	int decimals() const override {
+		return 1;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		if (prefs.units.weight == units::KG)
+			return { &weight_binner_1kg, &weight_binner_2kg, &weight_binner_5kg, &weight_binner_10kg };
+		else
+			return { &weight_binner_2lbs, &weight_binner_5lbs, &weight_binner_10lbs, &weight_binner_20lbs };
+	}
+	double toFloat(const dive *d) const override {
+		return prefs.units.weight == units::KG ? total_weight(d) / 1000.0
+						       : grams_to_lbs(total_weight(d));
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::Sum };
+	}
+};
+
+// ============ Dive mode ============
+
+struct DiveModeBinner : public SimpleBinner<DiveModeBinner, IntBin> {
+	QString format(const StatsBin &bin) const override {
+		return QString(divemode_text_ui[derived_bin(bin).value]);
+	}
+	int to_bin_value(const dive *d) const {
+		int res = (int)d->dc.divemode;
+		return res >= 0 && res < NUM_DIVEMODE ? res : OC;
+	}
+};
+
+static DiveModeBinner dive_mode_binner;
+struct DiveModeVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Dive mode");
+	}
+	QString diveCategories(const dive *d) const override {
+		int mode = (int)d->dc.divemode;
+		return mode >= 0 && mode < NUM_DIVEMODE ?
+			QString(divemode_text_ui[mode]) : QString();
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &dive_mode_binner };
+	}
+};
+
+// ============ Buddy (including dive guides) ============
+
+struct BuddyBinner : public StringBinner<BuddyBinner, StringBin> {
+	std::vector<QString> to_bin_values(const dive *d) const {
+		std::vector<QString> dive_people;
+		for (const QString &s: QString(d->buddy).split(",", SKIP_EMPTY))
+			dive_people.push_back(s.trimmed());
+		for (const QString &s: QString(d->divemaster).split(",", SKIP_EMPTY))
+			dive_people.push_back(s.trimmed());
+		return dive_people;
+	}
+};
+
+static BuddyBinner buddy_binner;
+struct BuddyVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Buddies");
+	}
+	QString diveCategories(const dive *d) const override {
+		QString buddy = QString(d->buddy).trimmed();
+		QString divemaster = QString(d->divemaster).trimmed();
+		if (!buddy.isEmpty() && !divemaster.isEmpty())
+			buddy += ", ";
+		return buddy + divemaster;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &buddy_binner };
+	}
+};
+
+// ============ Gas type, in 2%, 5%, 10% and 20% steps  ============
+// This is a bit convoluted: We differentiate between four types: air, pure oxygen, EAN and trimix
+// The latter two are binned in x% steps. The problem is that we can't use the "simple binner",
+// because a dive can have more than one cylinder. Moreover, the string-binner might not be optimal
+// because we don't want to format a string for every gas types, when there are thousands of dives!
+// Note: when the same dive has multiple cylinders that fall inside a bin, that cylinder will
+// only be counted once. Thus, depending on the bin size, the number of entries may change!
+// In addition to a binner with percent-steps also provide a general-type binner (air, nitrox, etc.)
+
+// bin gasmix with size given in percent
+struct gas_bin_t bin_gasmix(struct gasmix mix, int size)
+{
+	if (gasmix_is_air(mix))
+		return gas_bin_t::air();
+	if (mix.o2.permille == 1000)
+		return gas_bin_t::oxygen();
+	return mix.he.permille == 0 ?
+		gas_bin_t::ean(mix.o2.permille / 10 / size * size) :
+		gas_bin_t::trimix(mix.o2.permille / 10 / size * size,
+				  mix.he.permille / 10 / size * size);
+}
+
+struct GasTypeBinner : public MultiBinner<GasTypeBinner, GasTypeBin> {
+	int bin_size;
+	GasTypeBinner(int size)
+		: bin_size(size)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("in %1% steps").arg(bin_size);
+	}
+	std::vector<gas_bin_t> to_bin_values(const dive *d) const {
+		std::vector<gas_bin_t> res;
+		res.reserve(d->cylinders.nr);
+		for (int i = 0; i < d->cylinders.nr; ++i) {
+			struct gasmix mix = d->cylinders.cylinders[i].gasmix;
+			if (gasmix_is_invalid(mix))
+				continue;
+			// Add dive to each bin only once.
+			add_to_vector_unique(res, bin_gasmix(mix, bin_size));
+		}
+		return res;
+	}
+	QString format(const StatsBin &bin) const override {
+		gas_bin_t type = derived_bin(bin).value;
+		QLocale loc;
+		switch (type.type) {
+		default:
+		case gas_bin_t::Type::Air:
+			return StatsTranslations::tr("Air");
+		case gas_bin_t::Type::Oxygen:
+			return StatsTranslations::tr("Oxygen");
+		case gas_bin_t::Type::EAN:
+			return StatsTranslations::tr("EAN%1–%2").arg(loc.toString(type.o2),
+								     loc.toString(type.o2 + bin_size - 1));
+		case gas_bin_t::Type::Trimix:
+			return StatsTranslations::tr("%1/%2–%3/%4").arg(loc.toString(type.o2),
+									loc.toString(type.he),
+									loc.toString(type.o2 + bin_size - 1),
+									loc.toString(type.he + bin_size - 1));
+		}
+	}
+};
+
+struct GasTypeGeneralBinner : public MultiBinner<GasTypeGeneralBinner, IntBin> {
+	using MultiBinner::MultiBinner;
+	QString name() const override {
+		return StatsTranslations::tr("General");
+	}
+	std::vector<int> to_bin_values(const dive *d) const {
+		std::vector<int> res;
+		res.reserve(d->cylinders.nr);
+		for (int i = 0; i < d->cylinders.nr; ++i) {
+			struct gasmix mix = d->cylinders.cylinders[i].gasmix;
+			if (gasmix_is_invalid(mix))
+				continue;
+			res.push_back(gasmix_to_type(mix));
+		}
+		return res;
+	}
+	QString format(const StatsBin &bin) const override {
+		int type = derived_bin(bin).value;
+		return gastype_name((gastype)type);
+	}
+};
+
+static GasTypeGeneralBinner gas_type_general_binner;
+static GasTypeBinner gas_type_binner2(2);
+static GasTypeBinner gas_type_binner5(5);
+static GasTypeBinner gas_type_binner10(10);
+static GasTypeBinner gas_type_binner20(20);
+
+struct GasTypeVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Gas type");
+	}
+	QString diveCategories(const dive *d) const override {
+		QString res;
+		std::vector<gasmix> mixes;	// List multiple cylinders only once
+		mixes.reserve(d->cylinders.nr);
+		for (int i = 0; i < d->cylinders.nr; ++i) {
+			struct gasmix mix = d->cylinders.cylinders[i].gasmix;
+			if (gasmix_is_invalid(mix))
+				continue;
+			if (std::find_if(mixes.begin(), mixes.end(),
+					 [mix] (gasmix mix2)
+					 { return same_gasmix(mix, mix2); }) != mixes.end())
+				continue;
+			mixes.push_back(mix);
+			if (!res.isEmpty())
+				res += ", ";
+			res += get_gas_string(mix);
+		}
+		return res;
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &gas_type_general_binner,
+			 &gas_type_binner2, &gas_type_binner5, &gas_type_binner10, &gas_type_binner20 };
+	}
+};
+
+// ============ O2 and H2 content, binned in 2, 5, 10, 20 % bins ============
+
+// Get the gas content in permille of a dive, based on two flags:
+//  - he: get he content, otherwise o2
+//  - max_he: get cylinder with maximum he content, otherwise with maximum o2 content
+static int get_gas_content(const struct dive *d, bool he, bool max_he)
+{
+	if (d->cylinders.nr <= 0)
+		return invalid_value<int>();
+	// If sorting be He, the second sort criterion is O2 descending, because
+	// we are interested in the "bottom gas": highest He and lowest O2.
+	auto comp = max_he ? [] (const cylinder_t &c1, const cylinder_t &c2)
+				{ return std::make_tuple(get_he(c1.gasmix), -get_o2(c1.gasmix)) <
+					 std::make_tuple(get_he(c2.gasmix), -get_o2(c2.gasmix)); }
+			   : [] (const cylinder_t &c1, const cylinder_t &c2)
+				{ return get_o2(c1.gasmix) < get_o2(c2.gasmix); };
+	auto it = std::max_element(d->cylinders.cylinders, d->cylinders.cylinders + d->cylinders.nr, comp);
+	return he ? get_he(it->gasmix) : get_o2(it->gasmix);
+}
+
+// We use the same binner for all gas contents
+struct GasContentBinner : public IntRangeBinner<GasContentBinner, IntBin> {
+	bool he; // true if this returns He content, otherwise O2
+	bool max_he; // true if this takes the gas with maximum helium, otherwise maximum O2
+	GasContentBinner(int bin_size, bool he, bool max_he) : IntRangeBinner(bin_size),
+		he(he), max_he(max_he)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("In %L1% steps").arg(bin_size);
+	}
+	QString unitSymbol() const override {
+		return "%";
+	}
+	int to_bin_value(const struct dive *d) const {
+		int res = get_gas_content(d, he, max_he);
+		// Convert to percent and then bin, but take care not to mangle the invalid value.
+		return is_invalid_value(res) ? res : res / 10 / bin_size;
+	}
+};
+
+struct GasContentVariable : public StatsVariableTemplate<StatsVariable::Type::Numeric> {
+
+	// In the constructor, generate binners with 2, 5, 10 and 20% bins.
+	GasContentBinner b1, b2, b3, b4;
+	bool he, max_he;
+	GasContentVariable(bool he, bool max_he) :
+		b1(2, he, max_he), b2(5, he, max_he),
+		b3(10, he, max_he), b4(20, he, max_he),
+		he(he), max_he(max_he)
+	{
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &b1, &b2, &b3, &b4 };
+	}
+	QString unitSymbol() const override {
+		return "%";
+	}
+	int decimals() const override {
+		return 1;
+	}
+	std::vector<StatsOperation> supportedOperations() const override {
+		return { StatsOperation::Median, StatsOperation::Mean, StatsOperation::TimeWeightedMean };
+	}
+	double toFloat(const dive *d) const override {
+		int res = get_gas_content(d, he, max_he);
+		// Attn: we have to turn invalid-int into invalid-float.
+		// Perhaps we should signal invalid with an std::optional kind of object?
+		if (is_invalid_value(res))
+			return invalid_value<double>();
+		return res / 10.0;
+	}
+};
+
+struct GasContentO2Variable : GasContentVariable {
+	GasContentO2Variable() : GasContentVariable(false, false)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("O₂ (max)");
+	}
+};
+
+struct GasContentO2HeMaxVariable : GasContentVariable {
+	GasContentO2HeMaxVariable() : GasContentVariable(false, true)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("O₂ (bottom gas)");
+	}
+};
+
+struct GasContentHeVariable : GasContentVariable {
+	GasContentHeVariable() : GasContentVariable(true, true)
+	{
+	}
+	QString name() const override {
+		return StatsTranslations::tr("He (max)");
+	}
+};
+
+// ============ Suit ============
+
+struct SuitBinner : public StringBinner<SuitBinner, StringBin> {
+	std::vector<QString> to_bin_values(const dive *d) const {
+		return { QString(d->suit) };
+	}
+};
+
+static SuitBinner suit_binner;
+struct SuitVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Suit type");
+	}
+	QString diveCategories(const dive *d) const override {
+		return QString(d->suit);
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &suit_binner };
+	}
+};
+
+// ============ Weightsystem ============
+
+static std::vector<QString> weightsystems(const dive *d)
+{
+	std::vector<QString> res;
+	res.reserve(d->weightsystems.nr);
+	for (int i = 0; i < d->weightsystems.nr; ++i)
+		add_to_vector_unique(res, QString(d->weightsystems.weightsystems[i].description).trimmed());
+	return res;
+}
+
+struct WeightsystemBinner : public StringBinner<WeightsystemBinner, StringBin> {
+	std::vector<QString> to_bin_values(const dive *d) const {
+		return weightsystems(d);
+	}
+};
+
+static WeightsystemBinner weightsystem_binner;
+struct WeightsystemVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Weightsystem");
+	}
+	QString diveCategories(const dive *d) const override {
+		return join_strings(weightsystems(d));
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &weightsystem_binner };
+	}
+};
+
+// ============ Cylinder types ============
+
+static std::vector<QString> cylinder_types(const dive *d)
+{
+	std::vector<QString> res;
+	res.reserve(d->cylinders.nr);
+	for (int i = 0; i < d->cylinders.nr; ++i)
+		add_to_vector_unique(res, QString(d->cylinders.cylinders[i].type.description).trimmed());
+	return res;
+}
+
+struct CylinderTypeBinner : public StringBinner<CylinderTypeBinner, StringBin> {
+	std::vector<QString> to_bin_values(const dive *d) const {
+		return cylinder_types(d);
+	}
+};
+
+static CylinderTypeBinner cylinder_type_binner;
+struct CylinderTypeVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Cylinder type");
+	}
+	QString diveCategories(const dive *d) const override {
+		return join_strings(cylinder_types(d));
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &cylinder_type_binner };
+	}
+};
+
+// ============ Location (including trip location) ============
+
+using LocationBin = SimpleBin<const dive_site *>;
+
+struct LocationBinner : public SimpleBinner<LocationBinner, LocationBin> {
+	QString format(const StatsBin &bin) const override {
+		const dive_site *ds = derived_bin(bin).value;
+		return QString(ds ? ds->name : "-");
+	}
+	const dive_site *to_bin_value(const dive *d) const {
+		return d->dive_site;
+	}
+};
+
+static LocationBinner location_binner;
+struct LocationVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Dive site");
+	}
+	QString diveCategories(const dive *d) const override {
+		return d->dive_site ? d->dive_site->name : "-";
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &location_binner };
+	}
+};
+
+// ============ Day of the week ============
+
+struct DayOfWeekBinner : public SimpleBinner<DayOfWeekBinner, IntBin> {
+	QString format(const StatsBin &bin) const override {
+		return formatDayOfWeek(derived_bin(bin).value);
+	}
+	int to_bin_value(const dive *d) const {
+		return utc_weekday(d->when);
+	}
+};
+
+static DayOfWeekBinner day_of_week_binner;
+struct DayOfWeekVariable : public StatsVariableTemplate<StatsVariable::Type::Discrete> {
+	QString name() const override {
+		return StatsTranslations::tr("Day of week");
+	}
+	QString diveCategories(const dive *d) const override {
+		return formatDayOfWeek(utc_weekday(d->when));
+	}
+	std::vector<const StatsBinner *> binners() const override {
+		return { &day_of_week_binner };
+	}
+};
+
+static DateVariable date_variable;
+static DepthVariable depth_variable;
+static DurationVariable duration_variable;
+static SACVariable sac_variable;
+static WaterTemperatureVariable water_temperature_variable;
+static AirTemperatureVariable air_temperature_variable;
+static WeightVariable weight_variable;
+static DiveModeVariable dive_mode_variable;
+static BuddyVariable buddy_variable;
+static GasTypeVariable gas_type_variable;
+static GasContentO2Variable gas_content_o2_variable;
+static GasContentO2HeMaxVariable gas_content_o2_he_max_variable;
+static GasContentHeVariable gas_content_he_variable;
+static SuitVariable suit_variable;
+static WeightsystemVariable weightsystem_variable;
+static CylinderTypeVariable cylinder_type_variable;
+static LocationVariable location_variable;
+static DayOfWeekVariable day_of_week_variable;
+const std::vector<const StatsVariable *> stats_variables = {
+	&date_variable, &depth_variable, &duration_variable, &sac_variable,
+	&water_temperature_variable, &air_temperature_variable, &weight_variable,
+	&gas_content_o2_variable, &gas_content_o2_he_max_variable, &gas_content_he_variable,
+	&dive_mode_variable, &buddy_variable, &gas_type_variable, &suit_variable,
+	&weightsystem_variable, &cylinder_type_variable, &location_variable, &day_of_week_variable
+};
diff --git a/stats/statsvariables.h b/stats/statsvariables.h
new file mode 100644
index 000000000..06fb90f2a
--- /dev/null
+++ b/stats/statsvariables.h
@@ -0,0 +1,149 @@
+// SPDX-License-Identifier: GPL-2.0
+// Variables displayed by the statistic widgets. There are three
+// kinds of variables:
+// 1) Discrete variables can only adopt discrete values.
+//    Examples are dive-type or dive buddy.
+//    Note that for example dive buddy means that a dive can have
+//    multiple values.
+// 2) Continuous variables have a notion of a linear distance and can be
+//    plotted on a linear axis.
+//    An Example is the dive-date.
+// 3) Numeric variables are continuous variables that support operations
+//    such as averaging.
+#ifndef STATS_TYPES_H
+#define STATS_TYPES_H
+
+#include <vector>
+#include <memory>
+#include <QString>
+#include <QObject>
+
+struct dive;
+
+// Operations that can be performed on numeric variables
+enum class StatsOperation : int {
+	Median = 0,
+	Mean,
+	TimeWeightedMean,
+	Sum,
+	Invalid
+};
+
+// Results of the above operations
+struct StatsOperationResults {
+	int count;
+	double median;
+	double mean;
+	double timeWeightedMean;
+	double sum;
+	StatsOperationResults(); // Initialize to invalid (e.g. no dives)
+	bool isValid() const;
+	double get(StatsOperation op) const;
+};
+
+// For median and quartiles.
+struct StatsQuartiles {
+	double min;
+	double q1, q2, q3;
+	double max;
+	bool isValid() const;
+};
+
+struct StatsBin {
+	virtual ~StatsBin();
+	virtual bool operator<(StatsBin &) const = 0;
+	virtual bool operator==(StatsBin &) const = 0;
+	bool operator!=(StatsBin &b) const { return !(*this == b); }
+};
+
+using StatsBinPtr = std::unique_ptr<StatsBin>;
+
+// A value and a dive
+struct StatsValue {
+	double v;
+	dive *d;
+};
+
+// A bin and an arbitrarily associated value, e.g. a count or a list of dives.
+template<typename T>
+struct StatsBinValue {
+	StatsBinPtr bin;
+	T value;
+};
+using StatsBinDives = StatsBinValue<std::vector<dive *>>;
+using StatsBinValues = StatsBinValue<std::vector<StatsValue>>;
+using StatsBinCount = StatsBinValue<int>;
+using StatsBinQuartiles = StatsBinValue<StatsQuartiles>;
+using StatsBinOp = StatsBinValue<StatsOperationResults>;
+
+struct StatsBinner {
+	virtual ~StatsBinner();
+	virtual QString name() const; // Only needed if there are multiple binners for a variable
+	virtual QString unitSymbol() const; // For numeric variables - by default returns empty string
+
+	// The binning functions have a parameter "fill_empty". If true, missing
+	// bins in the range will be filled with empty bins. This only works for continuous variables.
+	virtual std::vector<StatsBinDives> bin_dives(const std::vector<dive *> &dives, bool fill_empty) const = 0;
+	virtual std::vector<StatsBinCount> count_dives(const std::vector<dive *> &dives, bool fill_empty) const = 0;
+
+	// Note: these functions will crash with an exception if passed incompatible bins!
+	virtual QString format(const StatsBin &bin) const = 0;
+	QString formatWithUnit(const StatsBin &bin) const;
+	virtual QString formatLowerBound(const StatsBin &bin) const; // Only for continuous variables
+	virtual QString formatUpperBound(const StatsBin &bin) const; // Only for continuous variables
+	virtual double lowerBoundToFloat(const StatsBin &bin) const; // Only for continuous variables
+	virtual double upperBoundToFloat(const StatsBin &bin) const; // Only for continuous variables
+	virtual bool preferBin(const StatsBin &bin) const; // Prefer to show this bins tick if bins are omitted. Default to true.
+
+	// Only for continuous and numeric variables
+	// Note: this will crash with an exception if passed incompatible bins!
+	virtual std::vector<StatsBinPtr> bins_between(const StatsBin &bin1, const StatsBin &bin2) const;
+};
+
+// A scatter item is two values and a dive
+struct StatsScatterItem {
+	double x, y;
+	dive *d;
+};
+
+struct StatsVariable {
+	enum class Type {
+		Discrete,
+		Continuous,
+		Numeric
+	};
+
+	virtual ~StatsVariable();
+	virtual Type type() const = 0;
+	virtual QString name() const = 0;
+	virtual QString unitSymbol() const; // For numeric variables - by default returns empty string
+	virtual int decimals() const; // For numeric variables: numbers of decimals to display on axes. Defaults to 0.
+	virtual std::vector<const StatsBinner *> binners() const = 0; // Note: may depend on current locale!
+	virtual QString diveCategories(const dive *d) const; // Only for discrete variables
+	std::vector<StatsBinQuartiles> bin_quartiles(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const;
+	std::vector<StatsBinOp> bin_operations(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const;
+	std::vector<StatsBinValues> bin_values(const StatsBinner &binner, const std::vector<dive *> &dives, bool fill_empty) const;
+	const StatsBinner *getBinner(int idx) const; // Handles out of bounds gracefully (returns first binner)
+	QString nameWithUnit() const;
+	QString nameWithBinnerUnit(const StatsBinner &) const;
+	virtual std::vector<StatsOperation> supportedOperations() const; // Only for numeric variables
+	QStringList supportedOperationNames() const; // Only for numeric variables
+	StatsOperation idxToOperation(int idx) const;
+	static QString operationName(StatsOperation);
+	double mean(const std::vector<dive *> &dives) const; // Returns NaN for empty list
+	static StatsQuartiles quartiles(const std::vector<StatsValue> &values); // Returns invalid quartiles for empty list
+	StatsQuartiles quartiles(const std::vector<dive *> &dives) const; // Only for numeric variables
+	std::vector<StatsValue> values(const std::vector<dive *> &dives) const; // Only for numeric variables
+	QString valueWithUnit(const dive *d) const; // Only for numeric variables
+	std::vector<StatsScatterItem> scatter(const StatsVariable &t2, const std::vector<dive *> &dives) const;
+private:
+	virtual double toFloat(const struct dive *d) const; // For numeric variables - if dive doesn't have that value, returns NaN
+	StatsOperationResults applyOperations(const std::vector<dive *> &dives) const;
+};
+
+extern const std::vector<const StatsVariable *> stats_variables;
+
+// Helper function for date-based variables
+extern double date_to_double(int year, int month, int day);
+
+#endif