adventoc/2024/day02.go

package adventoc2024

import (
	"bufio"
	"fmt"
	"io"
	"strings"
)

type Day2 struct{}

func (d Day2) Part1(r io.Reader) string {
	s := bufio.NewScanner(r)
	var sum int

Scan:
	for s.Scan() {

		sz := strings.Count(s.Text(), " ") + 1

		level := make([]int, sz)
		levelprts := make([]any, sz)
		for i := range levelprts {
			levelprts[i] = &level[i]
		}

		fmt.Sscan(s.Text(), levelprts...)

		increasing := level[1]-level[0] > 0
		if increasing {
			for i := 0; i < len(level)-1; i++ {
				if diff := level[i+1] - level[i]; diff > 3 || diff < 1 {
					continue Scan
				}
			}
			sum++
		} else {
			for i := 0; i < len(level)-1; i++ {
				if diff := level[i] - level[i+1]; diff > 3 || diff < 1 {
					continue Scan
				}
			}
			sum++
		}
	}

	return printi(sum)
}

// part 2 does not yet work because this code makes a false assumption.
// that assumption is that in order to fix a broken level, you can only skip the
// one that made it broken.
//
// this is not true because of the following level
// 5 6 3 2
// 5 and 6 would be OK, so the algorithm will blame the 3 and attempt to skip it.
// the real solution however, is to skip the 6 because it would be fine if all descending
func (d Day2) Part2(r io.Reader) string {
	s := bufio.NewScanner(r)
	var sum int

Scan:
	for s.Scan() {

		sz := strings.Count(s.Text(), " ") + 1

		level := make([]int, sz)
		levelprts := make([]any, sz)
		for i := range levelprts {
			levelprts[i] = &level[i]
		}

		fmt.Sscan(s.Text(), levelprts...)

		increasing := level[1]-level[0] > 0
		var dampened bool
		if increasing {
			for i := 0; i < len(level)-1; i++ {
				if diff := level[i+1] - level[i]; !inrange(diff) {
					if dampened {
						continue Scan
					}

					if i+2 > len(level)-1 {
						continue Scan
					}

					d2 := level[i+2] - level[i]
					if !inrange(d2) {
						continue Scan
					}

					dampened = true
					i++
				}
			}
			sum++
		} else {
			for i := 0; i < len(level)-1; i++ {
				if diff := level[i] - level[i+1]; !inrange(diff) {
					if dampened {
						continue Scan
					}

					if i+2 > len(level)-1 {
						continue Scan
					}

					d2 := level[i] - level[i+2]
					if !inrange(d2) {
						continue Scan
					}

					dampened = true
					i++
				}
			}
			sum++
		}
	}
	return printi(sum)
}

func inrange(i int) bool {
	return i < 4 && i > 0
}