aoc-2024/cmd/day_7/main.go

package main

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

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

func main() {
	r := bufio.NewReader(os.Stdin)
	calibration := 0

	for {
		line, err := r.ReadString('\n')
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading input: %v", err)
		}
		line = trim(line)

		parts := strings.SplitN(line, ":", 2)
		test64, err := strconv.ParseInt(parts[0], 10, 32)
		if err != nil {
			log.Fatalf("Unable to parse test value %#v: %v", parts[0], err)
		}
		test := int(test64)

		fs := strings.Fields(parts[1])
		operands := make([]int, 0, 8)
		for _, f := range fs {
			o, err := strconv.ParseInt(f, 10, 32)
			if err != nil {
				log.Fatalf("Unable to parse operand %#v: %v", f, err)
			}
			operands = append(operands, int(o))
		}

		if bruteForce(test, operands) {
			calibration += test
		}
	}
	fmt.Printf("%d\n", calibration)
}

func bruteForce(test int, vars []int) bool {
	if len(vars) > 64 {
		log.Fatalf("Maximum expected operands exceeded: %#v", vars)
	}
	var ops uint64

	for ops < uint64(len(vars)) {
		candidate := vars[0]
		for i := 0; i < len(vars)-1; i++ {
			switch (ops & (uint64(1) << i)) >> i {
			case 0:
				candidate += vars[i+1]
			case 1:
				candidate *= vars[i+1]
			default:
				log.Fatal("BUG: unexpected bit value: neither 0 nor 1!")
			}
			if candidate > test {
				break
			}
		}
		if candidate == test {
			return true
		}
		ops++
	}
	return false
}