Limit gas compressibility argument range to halfway sane values

The curve fitting for our gas compressibility was only done in the sane
range of 0-500 bar, which is what a scuba cylinder can reasonably be
expected to perhaps have.

But the planner ends up happily using negative cylinder pressures when
you run out of gas, and then the compressibility gives nonsensical
results.

That's clearly a planner bug, but the nonsensical gas compressibility
values made it harder to see what could be wrong.

So we just clamp the inpot range to the range we have verified against
experimental data.  If you try to get compressibility for negative
pressures, you get the compressibility for an ideal and imaginary gas.
And if you try to get compressibility for pressures over 500 bar, we'll
just assume that it's 500 bar.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/core/gas-model.c b/core/gas-model.c
index 79dbd2ded..4dccdf6b3 100644
--- a/core/gas-model.c
+++ b/core/gas-model.c
@@ -47,6 +47,14 @@ double gas_compressibility_factor(struct gasmix gas, double bar)
 	double x1, x2, x3;
 	double Z;
 
+	/*
+	 * The curve fitting range is only [0,500] bar.
+	 * Anything else is way out of range for cylinder
+	 * pressures.
+	 */
+	if (bar < 0) bar = 0;
+	if (bar > 500) bar = 500;
+
 	o2 = get_o2(gas);
 	he = get_he(gas);