palette/moreland: fix floating-point arithmetic error in Palette

[Juneezee]

Fixes #798.

This PR fixes a panic in the Palette method caused by a floating-point precision error, where the computed value v slightly exceeds the upper bound l.max.

Debugger screenshot

Problem

As shown in the above screenshot, we have the following values:

• l.min = 0.3402859786606234
• l.max = 15.322841335211892
• n = 15
• delta = 1.0701825254679478
• i = 14
• v = 15.322841335211894 (note here v is 0.000000000000002 greater than l.max)

The expected calculation of delta:

delta = (l.max - l.min) / (n - 1)
      = (15.322841335211892 - 0.3402859786606234) / (15 - 1)
      = 1.0701825254679478

This confirms that delta is correct and has no floating-point error.

Next, the expected calculation of v:

v = l.min + (delta  * i)
  = 0.3402859786606234 + (1.0701825254679478 * 14)
  = 15.322841335211892

Ideally, v should be exactly equal to l.max. However, due to a small floating-point precision error of 0.000000000000002, v becomes slightly greater than l.max, triggering the panic in checkRange function:

plot/palette/moreland/luminance.go

Lines 116 to 118 in 2815ea1

	if val > max {
	return palette.ErrOverflow
	}

Solution

Use the built-in min function to ensure v does not exceed l.max. If v becomes greater than l.max due to floating-point precision errors, it will be capped at l.max.

[sbinet]

Thanks for the PR.
I just have minor requests.

See below:

[sbinet]

LGTM

thanks

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.26%. Comparing base (c96920b) to head (1434e66).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
+ Coverage   70.78%   72.26%   +1.48%     
==========================================
  Files          60       59       -1     
  Lines        5291     7353    +2062     
==========================================
+ Hits         3745     5314    +1569     
- Misses       1350     1852     +502     
+ Partials      196      187       -9     

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[kortschak]

Thanks.

Please send a change to gonum/gonum with the commit message "A+C: add Eng Zer Jun" (please correct this if I got your name wrong) adding yourself to AUTHORS and CONTRIBUTORS.

[Juneezee]

Thanks.

Please send a change to gonum/gonum with the commit message "A+C: add Eng Zer Jun" (please correct this if I got your name wrong) adding yourself to AUTHORS and CONTRIBUTORS.

@kortschak My name is already in the AUTHORS and CONTRIBUTORS file 😊

• https://github.com/gonum/gonum/blob/7826ba4358a76646a8bd1f852201c4fed2c81c38/AUTHORS#L43
• https://github.com/gonum/gonum/blob/7826ba4358a76646a8bd1f852201c4fed2c81c38/CONTRIBUTORS#L50

[kortschak]

@Juneezee Ah, thanks. You show up as first time contributor here, but this is because the repo is different. Thanks for the change.