Shearwater: Add guessing of the calibration for DiveCAN controllers.

[mikeller]

If the Shearwater default calibration factor (2100) is returned for the
ppO2 calibration factor, we know that the dive computer might be a DiveCAN
controller, where the calibration is not available on the dive computer.
In this case, calculate the scaling factor that is required to make the
averaged sensor reading match the dive computer reported ppO2, and use
this instead of the returned calibration factor.
This will give us:

• individual sensor graphs that show the sensor behaviour, and have the
  approximately correct ppO2 reading;
• a calculated ppO2 used by Subsurface that is close to what the dive
  computer would have used, without the smoothing seen in the calculated
  ppO2 returned by the dive computer.

Signed-off-by: Subsurface CI mikeller@042.ch

[Copilot]

Pull Request Overview

This PR introduces logic to detect when a DiveCAN controller returns the default calibration value and then estimate a more accurate calibration scaling factor.

• Added a flag and related logic to determine when sensor calibration values are default.
• Calculated an estimated calibration scaling factor to adjust sensor readings when necessary.

[Copilot]

There's a potential division by zero when ppo2_count is zero. Please add a condition to ensure that ppo2_count is greater than zero before performing the division.

Suggested change

	double calibration_scaling_factor = calculated_ppo2 / (ppo2_sum / ppo2_count);
	if (calibration_scaling_factor < 0.95 || calibration_scaling_factor > 1.05) {
	// The calibration scaling is significant, use it.
	calibration *= calibration_scaling_factor;
	if (ppo2_count > 0) {
	double calibration_scaling_factor = calculated_ppo2 / (ppo2_sum / ppo2_count);
	if (calibration_scaling_factor < 0.95 || calibration_scaling_factor > 1.05) {
	// The calibration scaling is significant, use it.
	calibration *= calibration_scaling_factor;
	}

[Copilot]

Pull Request Overview

Adds logic to detect when a Shearwater dive computer returns its default sensor calibration (2100) and treats it as a DiveCAN controller. In that case, the parser walks the sample data to estimate a more accurate per-sensor calibration factor and applies it to each PP02 reading.

• Introduces SENSOR_CALIBRATION_DEFAULT and a needs_divecan_calibration_estimate flag
• Captures and sums raw vs. calculated ppO₂ in a temporary struct via samples_foreach
• Adjusts both the cached calibration prints and the per-sample PP02 callback to use the estimated factor

Comments suppressed due to low confidence (2)

src/shearwater_predator_parser.c:899

• [nitpick] The local variable data shadows the parser's raw-data pointer and is confusing; consider renaming this to calib_data or similar for clarity.

struct dc_parser_sensor_calibration_t data = { 0 };

src/shearwater_predator_parser.c:898

• The new DiveCAN calibration-estimation branch isn’t covered by existing tests; add unit or integration tests that simulate default calibration values and verify both estimation and fallback paths.

if (parser->needs_divecan_calibration_estimate) {

[Copilot]

Guard against division by zero on data.sum_ppo2 before computing the estimated calibration factor, otherwise you may produce NaN or infinities when no samples are accumulated.

Suggested change

	double calibration = data.sum_calculated_ppo2 / data.sum_ppo2;
	if (calibration < 0.95 || calibration > 1.05) {
	// The calibration scaling is significant, use it.
	calibration *= SENSOR_CALIBRATION_DEFAULT / 100000.0;
	parser->calibration[0] = calibration;
	parser->calibration[1] = calibration;
	parser->calibration[2] = calibration;
	dc_field_add_string_fmt(&parser->cache, "Estimated (DiveCAN?) sensor calibration [bar / V]", "%.2f", calibration * 1000);
	} else {
	print_calibration(parser);
	double calibration = 0.0;
	if (data.sum_ppo2 != 0) {
	calibration = data.sum_calculated_ppo2 / data.sum_ppo2;
	if (calibration < 0.95 || calibration > 1.05) {
	// The calibration scaling is significant, use it.
	calibration *= SENSOR_CALIBRATION_DEFAULT / 100000.0;
	parser->calibration[0] = calibration;
	parser->calibration[1] = calibration;
	parser->calibration[2] = calibration;
	dc_field_add_string_fmt(&parser->cache, "Estimated (DiveCAN?) sensor calibration [bar / V]", "%.2f", calibration * 1000);
	} else {
	print_calibration(parser);
	}
	} else {
	// Handle the case where no samples are accumulated.
	// Optionally log a warning or assign a default calibration value.
	calibration = SENSOR_CALIBRATION_DEFAULT / 1000.0; // Default calibration value
	parser->calibration[0] = calibration;
	parser->calibration[1] = calibration;
	parser->calibration[2] = calibration;

[Copilot]

[nitpick] The ppo2_sample_count field in dc_parser_sensor_calibration_t is incremented but never used after collection; consider removing it or adding logic to use it (e.g., logging or validation).

[Copilot]

[nitpick] The thresholds 0.95 and 1.05 are magic numbers—consider defining named constants (e.g., DIVECAN_CALIBRATION_MIN_RATIO) to clarify their significance.

Suggested change

	if (calibration < 0.95 || calibration > 1.05) {
	if (calibration < DIVECAN_CALIBRATION_MIN_RATIO || calibration > DIVECAN_CALIBRATION_MAX_RATIO) {