package q756

type Bottom [2]byte

func canBuild(buffer []byte, layer, offset int, allowed map[Bottom][]byte) bool {
	layerOffset := len(buffer) - (layer+1)*layer/2
	current := buffer[layerOffset : layerOffset+layer]
	bottom := buffer[layerOffset-layer-1 : layerOffset]

	nextLayer, nextOffset := layer, offset+1
	if nextOffset == layer {
		nextLayer, nextOffset = layer-1, 0
	}
	if current[offset] != 0 { // skip
		return canBuild(buffer, nextLayer, nextOffset, allowed)
	}

	choices := allowed[Bottom{bottom[offset], bottom[offset+1]}]
	if len(choices) == 0 {
		return false
	}
	if layer == 1 {
		return true
	}

	next := buffer[layerOffset+layer:]
	for _, c := range choices {
		current[offset] = c
		if offset > 0 {
			next[offset-1] = 0 // needs recalculate
		}
		if offset < layer-1 {
			next[offset] = 0
		}

		if canBuild(buffer, nextLayer, nextOffset, allowed) {
			return true
		}
	}
	return false
}

func pyramidTransition(bottom string, allowed []string) bool {
	allowedIdx := map[Bottom][]byte{}
	for i := range allowed {
		k := [2]byte{allowed[i][0], allowed[i][1]}
		allowedIdx[k] = append(allowedIdx[k], allowed[i][2])
	}
	buffer := make([]byte, (len(bottom)+1)*len(bottom)/2)
	copy(buffer, []byte(bottom))

	return canBuild(buffer, len(bottom)-1, 0, allowedIdx)
}

var _ = pyramidTransition