aoc2023/day15/day15.go

package day15

import "strconv"

func hash(bytes []byte) int {
	val := 0
	for _, b := range bytes {
		val += int(b)
		val *= 17
		val %= 256
	}
	return val
}

func SumHashes(line string) int {
	result := 0
	l := []byte(line)

	step := []byte{}

	for _, b := range l {

		if b == '\n' {
			break
		}

		if b == ',' {
			hashValue := hash(step)
			result += hashValue
			step = []byte{}
		} else {
			step = append(step, b)
		}
	}

	return result
}

type Lense struct {
	label string
	focal int
}
type Lenses []*Lense

type LensePresences map[string]int

type Box struct {
	lenses         Lenses
	lensePresences LensePresences
}

func parseStep(bytes []byte) (string, byte, int) {
	label := []byte{}
	focal := []byte{}
	parseFocal := false

	for _, b := range bytes {
		if b == '-' {
			return string(label), '-', -1
		}

		if b == '=' {
			parseFocal = true
			continue
		}

		if parseFocal {
			focal = append(focal, b)
		} else {
			label = append(label, b)
		}

	}

	focalValue, _ := strconv.Atoi(string(focal))

	return string(label), '=', focalValue
}

func removeLense(box *Box, label string) {
	idx, ok := box.lensePresences[label]
	if ok {
		delete(box.lensePresences, label)

		/*
			idx := 0
			for i, lense := range box.lenses {
				if lense.label == label {
					idx = i
					break
				}
			}
		*/

		for i := idx; i < len(box.lenses)-1; i++ {
			box.lenses[i] = box.lenses[i+1]
			box.lensePresences[box.lenses[i].label] = i
		}

		box.lenses = box.lenses[0 : len(box.lenses)-1]
	}
}

func addOrChangeLense(box *Box, label string, focal int) {
	newLense := &Lense{label, focal}

	idx, ok := box.lensePresences[label]
	if ok { // change lense
		box.lenses[idx] = newLense
	} else {
		// add lense
		box.lenses = append(box.lenses, newLense)
		box.lensePresences[label] = len(box.lenses) - 1
	}
}

func SumBoxes(line string) int {
	l := []byte(line)
	boxes := make([]*Box, 256)

	step := []byte{}

	for _, b := range l {

		if b == '\n' {
			break
		}

		if b == ',' {
			label, operator, focal := parseStep(step)
			labelHash := hash([]byte(label))

			box := boxes[labelHash]
			if box == nil {
				box = &Box{Lenses{}, LensePresences{}}
				boxes[labelHash] = box
			}

			if operator == '-' {
				removeLense(box, label)
				if len(box.lenses) == 0 {
					box = nil
					boxes[labelHash] = nil
				}
			} else {
				addOrChangeLense(box, label, focal)
			}

			step = []byte{}
		} else {
			step = append(step, b)
		}
	}

	sum := 0
	for i, box := range boxes {
		if box != nil {
			boxNumber := i + 1

			for j, lense := range box.lenses {
				slotNumber := j + 1

				focusingPower := boxNumber * slotNumber * lense.focal
				sum += focusingPower
			}

		}
	}

	return sum
}