aoc-2024/cmd/day_5/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
	"strings"
)

const MAX_LOOP = 1_000_000

var (
	MAX_INT int = int(^uint(0) >> 1)
	INT_SZ  int = 0
)

func init() {
	tmp := MAX_INT
	for tmp > 0 {
		INT_SZ++
		tmp >>= 1
	}
}

func trim(bs []byte) []byte {
	last := len(bs) - 1
	if last >= 0 && bs[last] == '\n' {
		return bs[:last]
	}
	return bs
}

func main() {
	rules := make(map[string][]string)
	validUpdates := make([][]string, 0, 16)

	r := bufio.NewReader(os.Stdin)
	var fs []string

	// Read rules
	for i := 0; i < MAX_LOOP; i++ {
		line, err := r.ReadString('\n')
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading rule %#v: %v", line, err)
		}
		line = string(trim([]byte(line)))
		if line == "" {
			break
		}
		fs = strings.SplitN(line, "|", 2)
		if len(fs) != 2 {
			log.Fatalf("Unexpected format for rule %#v", line)
		}
		if _, ok := rules[fs[1]]; !ok {
			rules[fs[1]] = make([]string, 0, 8)
		}
		rules[fs[1]] = append(rules[fs[1]], fs[0])
	}

	// Read updates
	for i := 0; i < MAX_LOOP; i++ {
		line, err := r.ReadString('\n')
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading update %#v: %v", line, err)
		}
		line = string(trim([]byte(line)))
		if line == "" {
			continue
		}
		fs = strings.Split(line, ",")
		if len(fs) == 0 {
			log.Fatalf("Unexpected format for update %#v", line)
		}
		forbidden := make(map[string]struct{})
		var invalid bool
		for _, f := range fs {
			if _, ok := forbidden[f]; ok {
				invalid = true
				break
			}
			for _, r := range rules[f] {
				forbidden[r] = struct{}{}
			}
		}
		if invalid {
			continue
		}
		validUpdates = append(validUpdates, fs)
	}

	// Sum middle numbers of valid updates
	sum := 0
	for _, u := range validUpdates {
		i := len(u) >> 1
		u_i, err := strconv.ParseInt(u[i], 10, INT_SZ)
		if err != nil {
			log.Fatalf("Unable to parse page number %#v in update %s", u[i], strings.Join(u, ","))
		}
		sum += int(u_i)
	}
	fmt.Printf("%d\n", sum)
}