A quarter-tank warning that says it once, and a gauge that keeps saying it

Two halves of the same number. The warning is the moment it happened and
the colour is how things stand: it fires on the way down through a
quarter of a tank and then holds its peace, and the gauge stays red until
a base fills the lander up, which also allows the warning again.

Once, because a lander is at its most careful in the last few seconds
before it touches, and something repeating in its ear through that is not
a warning, it is a distraction.

Checked where the fuel actually moves rather than once a frame - the tank
only changes in takeFuel and payFuel, so there is nowhere else it can
cross a threshold.

Channel two, with the crash on three. Separate channels rather than one
reused, because a crash while the warning is still sounding should not cut
it off, and on a machine with four voices there is no reason to be clever.

Half and a tenth are the obvious next thresholds and are deliberately not
here: one is enough to find out whether being told at all is welcome.

---- And the crash sound is the one that was designed for it ----

Crash.json, which carries voice_gate itself, so the program's own trigger
write is gone - it would have masked a deliberate choice rather than
backing one up. The note moved from 24 to 60 by ear, and the comment
saying it was low went with it.

---- A check that could not tell a beep from a nag ----

Counting bursts of sound cannot: without the latch the warning fires
every tick the tank drops, far faster than the sound decays, so the beeps
run into each other and a burst counter sees one long burst either way.
It measures the LENGTH now - 26,944 samples against 490,348 - which is
the difference between six tenths of a second and ten seconds of it.

Tests/video.sh

@@ -2714,6 +2714,70 @@ EOT
     && result ok "and it is loudest at the front" "which is what makes it a bang" \
     || result no "and it is loudest at the front" "louder in the second half than the first"
 
+# ---- A quarter of a tank, said once ----
+#
+# ONCE is the whole design of it. A lander is at its most careful in the last few seconds
+# before it touches, and something repeating in its ear through that is not a warning, it is a
+# distraction. So it fires on the way DOWN through a quarter tank and then holds its peace,
+# and filling up at a base allows it again.
+#
+# The gauge says the same thing the other way round: the sound is the moment it happened, the
+# colour is how things stand. Both off the same number, so they cannot disagree.
+python3 -c "
+import struct
+# Seventy of two hundred and fifty five, which is a few ticks of thruster above the quarter.
+open('$BUILD/lowfuel.state','wb').write(struct.pack('<hhhhhhBBBB',
+    0x14A0, 0, 0, 0, 0x14A0, 2600, 70, 0, 0, 0))
+"
+cp "$ROOT/Tests/build/disks/cosmos.img" "$BUILD/lowfuel.img"
+"$ROOT/SplitDisk" put "$BUILD/lowfuel.img" "$BUILD/lowfuel.state" /lander.state > /dev/null
+python3 -c "open('$BUILD/burn.pad','wb').write(b'\x08' * 40000)"
+for when in 1700000 2400000; do
+    timeout 60 "$EMU" --fast --cycles $when --keyboard "$BUILD/fly.keys" \
+        --pad "$BUILD/burn.pad" --screen "$BUILD/fuel$when.ppm" \
+        --disk "$BUILD/lowfuel.img" --ram-disk 2048 \
+        "$BUILD/cosmos.bin" > "$BUILD/fuel.out" 2>&1 || true
+done
+timeout 60 "$EMU" --fast --cycles 12000000 --keyboard "$BUILD/fly.keys" \
+    --pad "$BUILD/burn.pad" --sound "$BUILD/warn.raw" \
+    --disk "$BUILD/lowfuel.img" --ram-disk 2048 \
+    "$BUILD/cosmos.bin" > "$BUILD/warn.out" 2>&1 || true
+read -r ABOVE BELOW WARNINGS <<EOT
+$(python3 -c "
+def gauge(path):
+    d = open(path, 'rb').read()
+    width = int(d[:40].split()[1])
+    px = d[d.index(b'255\n') + 4:]
+    def cells(colour):
+        c = bytes.fromhex(colour)
+        # The top row of the window, which is where the gauge is drawn.
+        return sum(1 for i in range(len(px) // 3)
+                   if px[i * 3:i * 3 + 3] == c and i // width < 8) // 64
+    return '%d,%d' % (cells('50c050'), cells('d04038'))
+import struct
+raw = open('$BUILD/warn.raw', 'rb').read()
+s = struct.unpack('<%dh' % (len(raw) // 2), raw)
+# ---- HOW LONG it sounds for, not how many times it starts ----
+#
+# Counting bursts cannot tell one beep from a nag. Without the latch the warning fires on
+# every tick the tank goes down, which is once every ten frames - far faster than the sound
+# decays, so the beeps run into each other and a burst counter sees ONE long burst either way.
+# The length is what differs: six tenths of a second against ten seconds of it.
+loud = [i for i, x in enumerate(s) if abs(x) > 300]
+span = (loud[-1] - loud[0]) if loud else 0
+print(gauge('$BUILD/fuel1700000.ppm'), gauge('$BUILD/fuel2400000.ppm'), span)
+" 2>/dev/null || echo "none none -1")
+EOT
+# Green cells then red cells, and none of the other colour either time.
+[ "$ABOVE" = "8,0" ] && [ "$BELOW" = "0,7" ] \
+    && result ok "the fuel gauge goes red at a quarter" "eight green became seven red" \
+    || result no "the fuel gauge goes red at a quarter" "$ABOVE above, $BELOW below"
+# Twelve seconds of holding the thruster, which empties the tank entirely and keeps it empty.
+# The warning is under a second of that, or it is a nag rather than a warning.
+[ "$WARNINGS" -gt 0 ] && [ "$WARNINGS" -lt 48000 ] \
+    && result ok "and the warning sounds once, not once a tick" "$WARNINGS samples of it in twelve seconds of running dry" \
+    || result no "and the warning sounds once, not once a tick" "$WARNINGS samples of warning"
+
 # ---- Dust on landing, and gas on letting go ----
 #
 # The same six pieces as the explosion, thrown differently: dust goes sideways and UP off the