aoc2023/day-14/two.go

package main

const CYCLES = 1000000000

// cycleOnce transposes and rolls a single image once
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 also caches the index of the interation for each image
func cycleMany(image img, iterations int) {
	cache := make(map[string]int)
	i, start := 0, 0
	for ; i < iterations; i++ {
		cycleOnce(image)

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

		cache[image.String()] = i + 1
	}
	period := i - start                        // length of a full period
	remaining := (iterations - i - 1) % period // number of cycles left in current period
	for j := 0; j < remaining; j++ {
		cycleOnce(image) // rotate to the final image in the period
	}
}

// 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())
}