aoc_2024/2025/two/two.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"log"
	"os"
	"strconv"
	"strings"
)

func scanComma(data []byte, atEOF bool) (advance int, token []byte, err error) {
	if atEOF && len(data) == 0 {
		return 0, nil, nil
	}
	if i := bytes.IndexByte(data, ','); i > 0 {
		return i + 1, data[0:i], nil
	}
	if atEOF {
		if len(data) > 0 && data[len(data)-1] == '\n' {
			return len(data), data[0 : len(data)-1], nil
		}
		return len(data), data, nil
	}
	return 0, nil, nil

}

func check(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func intPow(base int, exponent int) int {

	if exponent == 0 {
		return 1
	} else if exponent == 1 {
		return base
	}

	val := base
	remaining_mults := exponent - 1
	for ; remaining_mults > 0; remaining_mults-- {
		val = val * base
	}
	return val
}

func makeCandidateInt(prefix int, n_repeats int) (int, error) {
	prefixStr := strconv.Itoa(prefix)

	var candidateBuilder strings.Builder

	for ; n_repeats > 0; n_repeats-- {
		candidateBuilder.WriteString(prefixStr)
	}

	return strconv.Atoi(candidateBuilder.String())

}

func largestPrefix(rangeEnd string) (int, error) {
	if len(rangeEnd)%2 == 0 {
		return strconv.Atoi(rangeEnd[0 : len(rangeEnd)/2])
	} else {
		prefixLen := (len(rangeEnd) - 1) / 2
		return intPow(10, prefixLen) - 1, nil
	}
}

func sumInt(seq []int) int {
	sum := 0

	for _, val := range seq {
		sum += val
	}
	return sum
}

func getInvalidsForRange(start int, end int) []int {
	maxPrefix, err := largestPrefix(strconv.Itoa(end))
	check(err)

	invalidIds := make(map[int]struct{})

	fmt.Printf("Considering range %d - %d, max prefix %d\n", start, end, maxPrefix)
	for candidatePrefix := 1; candidatePrefix <= maxPrefix; candidatePrefix++ {
		for n_repeats := 2; ; n_repeats++ {
			candidateInt, err := makeCandidateInt(candidatePrefix, n_repeats)
			check(err)

			if candidateInt > end {
				break
			}

			if (start <= candidateInt) && (candidateInt <= end) {
				fmt.Printf("Can use %d\n", candidateInt)
				invalidIds[candidateInt] = struct{}{}
			}
		}
	}

	invalidIdList := make([]int, 0, len(invalidIds))

	for invalidId := range invalidIds {
		invalidIdList = append(invalidIdList, invalidId)

	}
	return invalidIdList
}

func main() {
	file, err := os.Open("two.txt")
	check(err)
	defer file.Close()

	invalidIdSum := 0

	scanner := bufio.NewScanner(file)
	scanner.Split(scanComma)

	for scanner.Scan() {
		idRange := scanner.Text()

		idRangeSplit := strings.SplitN(idRange, "-", 2)

		rangeStart, err := strconv.Atoi(idRangeSplit[0])
		check(err)
		rangeEnd, err := strconv.Atoi(idRangeSplit[1])
		check(err)

		invalidIdSum += sumInt(getInvalidsForRange(rangeStart, rangeEnd))
	}
	fmt.Printf("%d\n", invalidIdSum)
}