adventofcode2023/day04.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"math"
	"sort"
	"strconv"
	"strings"
)

func countwinings(line string) int {
	var matches int

	before, after, _ := strings.Cut(line, "|")
	w := strings.Split(before, " ")
	o := strings.Split(after, " ")

	winnings := make([]int, 0, len(w))
	obtained := make([]int, 0, len(o))

	for i := range w {
		val, err := strconv.Atoi(strings.TrimSpace(w[i]))
		if err == nil {
			winnings = append(winnings, val)
		}
	}

	for i := range o {
		val, err := strconv.Atoi(strings.TrimSpace(o[i]))
		if err == nil {
			obtained = append(obtained, val)
		}
	}

	sort.Ints(winnings)
	sort.Ints(obtained)

	for i, j := 0, 0; i < len(winnings) && j < len(obtained); {
		if winnings[i] == obtained[j] {
			matches++
			i++
			j++
		} else if winnings[i] > obtained[j] {
			j++
		} else {
			i++
		}
	}
	return matches
}

type day04 struct{}

func (d day04) solve1(r io.Reader) string {
	var (
		sum     int
		scanner = bufio.NewScanner(r)
	)

	for scanner.Scan() {
		_, line, _ := strings.Cut(scanner.Text(), ": ")
		matches := countwinings(line)
		if matches > 0 {
			sum += int(math.Pow(2, float64(matches)-1))
		}
	}
	return fmt.Sprintf("%d", sum)
}

func (d day04) solve2(r io.Reader) string {
	var (
		current int
		sum     = 1
		cards   = make([]int, 185)
		scanner = bufio.NewScanner(r)
	)

	for matches := -1; scanner.Scan() && matches != 0; current++ {
		_, line, _ := strings.Cut(scanner.Text(), ": ")
		sum++
		matches = countwinings(line)
		for i := 1; i <= matches; i++ {
			cards[current+i] = cards[current+i] + 1 + cards[current]
		}
	}

	for _, c := range cards {
		sum += c
	}
	return fmt.Sprintf("%d", sum)
}