package helper

import (
	"image/color"
	"iter"
)

// Enum iterates over a sparse sample of the RGBA colour space.
//
// If alpha is true, the colours will include transparency,
// otherwise the returned colours will be fully opaque.
//
// If slow is false, an even smaller number of samples will be returned
// making this suitable for use in a nested loop.
//
// alpha=true slow=true: 87481 samples.
// alpha=false slow=true: 140608 samples.
// alpha=true slow=false: 649 samples.
// alpha=false slow=false: 216 samples.
func Enum(alpha, slow bool) iter.Seq[color.RGBA64] {
	var aStart, aStep, cDiv uint32

	switch {
	case alpha && slow:
		aStart, aStep, cDiv = 0, 0xffff/15, 17
	case alpha: // alpha && !slow
		aStart, aStep, cDiv = 0, 0xffff/3, 5
	case slow: // !alpha && slow
		aStart, aStep, cDiv = 0xffff, 1, 51
	default: // !alpha && !slow
		aStart, aStep, cDiv = 0xffff, 1, 5
	}

	return func(yield func(color.RGBA64) bool) {
		for a := aStart; a <= 0xffff; a += aStep {
			cStep := max(1, a/cDiv)

			for b := uint32(0); b <= a; b += cStep {
				for g := uint32(0); g <= a; g += cStep {
					for r := uint32(0); r <= a; r += cStep {
						if !yield(color.RGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}) {
							return
						}
					}
				}
			}
		}
	}
}

// EnumColor is identical to [Enum], but invokes a [color.Model] to return a concrete colour type.
func EnumColor[C color.Color](alpha, slow bool, m color.Model) iter.Seq[C] {
	return func(yield func(C) bool) {
		for rgba := range Enum(alpha, slow) {
			if !yield(m.Convert(rgba).(C)) {
				return
			}
		}
	}
}