fix #4150: display 0°C instead of -0°C for near-zero negative temperatures

[lohwasser]

Summary

Fixes the negative zero temperature display bug, where values between -0.4°C and -0.1°C would incorrectly show as -0°C.

Fixes #4150

Changes

Modified files:

• core/model/src/main/kotlin/org/meshtastic/core/model/util/UnitConversions.kt - Updated toTempString() to use absolute value rounding with separate sign handling
• core/model/src/test/kotlin/org/meshtastic/core/model/util/UnitConversionsTest.kt - Added unit tests for temperature conversion

Implementation details:

The toTempString() function now:

1. Rounds the absolute temperature value
2. Determines sign separately
3. Ensures zero is always displayed without a negative sign

This approach handles the edge case where %.0f formatting produces -0 for values between -0.4 and -0.1.

Testing

Added unit tests including:

• Data-driven test cases covering the reported bug scenario
• Boundary tests around the -0.5°C rounding threshold
• Fahrenheit conversion tests
• Dew point calculation tests

Run tests:

./gradlew :core:model:testGoogleDebugUnitTest --tests "org.meshtastic.core.model.util.UnitConversionsTest"

Note: I don't have a node with a temperature sensor to verify on-device, but the fix is in a pure function with full unit test coverage.

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR fixes issue #4150 where temperatures between -0.4°C and -0.1°C would incorrectly display as -0°C instead of 0°C. The fix implements a more careful rounding approach by handling the absolute value and sign separately.

Changes:

• Updated toTempString() function to round absolute temperature values and handle the sign separately to avoid negative zero display
• Added comprehensive unit tests covering the bug scenario, boundary cases, and Fahrenheit conversions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
core/model/src/main/kotlin/org/meshtastic/core/model/util/UnitConversions.kt	Refactored toTempString() to use absolute value rounding with separate sign determination to prevent -0°C display
core/model/src/test/kotlin/org/meshtastic/core/model/util/UnitConversionsTest.kt	Added comprehensive unit tests for temperature formatting including edge cases, boundary conditions, and Fahrenheit conversions

[Copilot]

The same typo appears here: roundedAbsoluteTempt should be roundedAbsoluteTemp to match the intended variable name and maintain consistency.

Suggested change

	val roundedAbsoluteTempt: Int = absoluteTemp.roundToInt()
	val isZero = roundedAbsoluteTempt == 0
	val isPositive = kotlin.math.sign(temp) > 0
	val sign: String = if (isPositive || isZero) "" else "-"
	return "$sign$roundedAbsoluteTempt°$unit"
	val roundedAbsoluteTemp: Int = absoluteTemp.roundToInt()
	val isZero = roundedAbsoluteTemp == 0
	val isPositive = kotlin.math.sign(temp) > 0
	val sign: String = if (isPositive || isZero) "" else "-"
	return "$sign$roundedAbsoluteTemp°$unit"

[Copilot]

The comment mentions "Rounding -0.5 gives: 0, since we're rounding up", but this is inaccurate. Kotlin's roundToInt() uses "round half to even" (banker's rounding) by default, and for -0.5, the absolute value is 0.5, which would round to 0. However, the statement "since we're rounding up" is misleading because the rounding behavior being described is for the absolute value, not the original negative number. Consider clarifying that you're working with the absolute value to avoid confusion about whether -0.5 rounds to 0 or -1.

Suggested change

	// Rounding `-0.5` gives: `0`, since we're rounding `up`. We want `-0.5` to be rounded to `-1`.
	// We round the absolute value using roundToInt() (banker's rounding), then reapply the sign so values

[Copilot]

There is a typo in the variable name: roundedAbsoluteTempt should be roundedAbsoluteTemp (missing the final 'p'). This typo makes the variable name inconsistent and potentially confusing.

Suggested change

	val roundedAbsoluteTempt: Int = absoluteTemp.roundToInt()
	val isZero = roundedAbsoluteTempt == 0
	val isPositive = kotlin.math.sign(temp) > 0
	val sign: String = if (isPositive || isZero) "" else "-"
	return "$sign$roundedAbsoluteTempt°$unit"
	val roundedAbsoluteTemp: Int = absoluteTemp.roundToInt()
	val isZero = roundedAbsoluteTemp == 0
	val isPositive = kotlin.math.sign(temp) > 0
	val sign: String = if (isPositive || isZero) "" else "-"
	return "$sign$roundedAbsoluteTemp°$unit"

[DaneEvans]

Bot's comments look reasonable, please make the changes.

And I definitely appreciate the added tests !