main.go

package main

import (
	"fmt"
	"io"
	"strings"

	lib "github.com/teivah/advent-of-code"
)

func fs1(input io.Reader, phases int) string {
	s := lib.ReaderToString(input)

	pattern := []int{0, 1, 0, -1}
	for phase := 0; phase < phases; phase++ {
		sb := strings.Builder{}
		for i := 0; i < len(s); i++ {
			fmt.Println(i, len(s))
			r := to(pattern, i, s)
			sb.WriteRune(r)
		}
		s = sb.String()
		_ = s
	}
	return s[:8]
}

func to(pattern []int, i int, s string) rune {
	sum := 0
	mult := pattern[0]
	patternIdx := 0
	if i == 0 {
		mult = pattern[1]
		patternIdx = 1
	}
	repeat := i
	for j := 0; j < len(s); j++ {
		v := lib.RuneToInt(rune(s[j])) * mult
		sum += v
		repeat--
		if repeat <= 0 {
			patternIdx = (patternIdx + 1) % len(pattern)
			mult = pattern[patternIdx]
			repeat = i + 1
		}
	}
	return lib.IntToRune(lib.Abs(sum % 10))
}

func fs2(input io.Reader, phases, repeat int) []int {
	s := lib.ReaderToString(input)
	offset := lib.StringToInt(s[:7])
	d := 8
	s = strings.Repeat(s, repeat)

	v := toInts(s)
	for i := 0; i < phases; i++ {
		do(v, offset)
	}
	return v[offset : offset+d]
}

func toInts(s string) []int {
	res := make([]int, len(s))
	for i := 0; i < len(s); i++ {
		res[i] = lib.RuneToInt(rune(s[i]))
	}
	return res
}

func do(v []int, offset int) {
	sum := 0
	for i := len(v) - 1; i >= offset; i-- {
		sum += v[i]
		v[i] = (sum) % 10
	}
}