aoc2023/day-14/two.go

package main

const CYCLES = 1000000000

// cycleOnce transposes and rolls a single image four times (N,W,S,E)
func cycleOnce(image img) {
	for i := 0; i < 4; i++ {
		for i, line := range image.transposed() {
			image.raw[i] = rollRight(line)
		}
	}
}

// cycleMany cycles a single image for a given number of iterations
// it caches seen images as well as their location in the period
// finally it copies the cached image with idx matching the interval to image.raw
func cycleMany(image img, iterations int) {
	cachedIndexes := make(map[string]int)
	cachedImages := make(map[int][]string)
	i, start := 0, 0
	for ; i < iterations; i++ {
		cycleOnce(image)

		if idx, ok := cachedIndexes[image.String()]; ok { // we found first repeated image
			start = idx
			i++
			break
		}

		cachedIndexes[image.String()] = i + 1
		cachedImages[i+1] = make([]string, len(image.raw))
		copy(cachedImages[i+1], image.raw)
	}

	period := i - start                                            // length of a full period
	copy(image.raw, cachedImages[start+((iterations-i-1)%period)]) // copy cachedImage into image.raw
}

// two returns the load of all boulders after 1000000000 cycles
func two(lines []string) int {
	image := newImg(len(lines))
	copy(image.raw, lines)

	cycleMany(image, CYCLES)

	return getload(image.transposed())
}