Handle a palette with fewer than two colors in Palette.At

[youdie006]

Palette.At is documented as taking a value in range 0-1, and it guards t <= 0, t >= 1 and NaN, so it looks total over that range. It is not, for a palette with fewer than two colors.

p := gfx.Palette{color.NRGBA{R: 255, A: 255}}
p.At(0.5)

At(0) = {255 0 0 255}
At(1) = {255 0 0 255}
panic: runtime error: index out of range [1] with length 1
gfx.Palette.At(...)
	palette.go:102

Both endpoints return correctly and only the interior of the documented range panics, which is why it survives a smoke test. The cause is at palette.go:99-102: n-1 is the divisor, so with n == 1 it is 0, s becomes +Inf, i stays 0, and the last line reads p[i+1]. An empty palette is worse -- it panics at p[0] for every t.

Why I picked these return values

Rather than invent a convention I followed the two sibling methods on the same type:

• Color(n) (palette.go:15) bounds-checks and returns color.NRGBA{}
• Convert (palette.go:48) guards len(p) == 0 and returns color.RGBA{}

So the empty case returns the same transparent zero value the rest of the type already returns for "nothing to give", and the single-color case returns that color, which is what both existing endpoint guards already do for t <= 0 and t >= 1.

Convert being guarded is not hypothetical -- palette_test.go:65 constructs Palette{} and passes it to Convert, so an empty palette is already an expected input to this type. CmplxPhaseAt (cmplx.go:56) forwards a [0,1] value straight into At.

Behaviour change

Only on inputs that previously panicked. No existing test row changes.

Verification

go test -count=1 ./... passes both packages; gofmt -l . empty; go vet ./... clean. The workflow runs go test -v ./... on Go 1.25.x/1.26.x; I ran go1.26.3.

I checked the boundary from both sides rather than only confirming the new rows go green:

• revert the guard -- TestPaletteAt fails with the panic above, so the test reaches the bug
• over-correct n == 1 to n <= 2 -- the new {Palette{ColorBlack, ColorWhite}, 0.5, ...} row fails with r = 0, want 32767

The two-color row is there specifically to pin that side, so the boundary is exactly 1 rather than merely "not the old behaviour".

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Disclosure: AI-assisted. I found and prepared this with an AI assistant, and I ran and verified everything above myself.