aoc2025/day6/main.go

package main

import (
	"fmt"
	"os"
	"regexp"
	"strconv"
	"strings"
)

func readLines(filename string) []string {
	raw, err := os.ReadFile(filename)
	if err != nil {
		panic(err)
	}
	return strings.Split(string(raw), "\n")
}

func main() {
	filename := "input0.txt"
	if len(os.Args) > 1 {
		filename = os.Args[1]
	}
	lines := readLines(filename)
	hw, err := parseHomework(lines)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%v\n", hw.do())
	hw2, err := parseWeirdHomework(lines)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%v\n", hw2.do())
}

type op uint8

const (
	STAR op = iota
	PLUS
)

type homework struct {
	cols [][]uint64
	ops  []op
}

var spaces = regexp.MustCompile(" +")

func parseHomework(lines []string) (homework, error) {
	var hw homework
	var err error
	for i := 0; i < len(lines)-1; i++ {
		parts := spaces.Split(strings.TrimSpace(lines[i]), -1)
		if i == 0 {
			hw.ops = make([]op, len(parts))
			hw.cols = make([][]uint64, len(parts))
			for j := range hw.cols {
				hw.cols[j] = make([]uint64, len(lines)-1)
			}
		}
		for j, part := range parts {
			hw.cols[j][i], err = strconv.ParseUint(part, 10, 64)
			if err != nil {
				return hw, err
			}
		}
	}
	parts := spaces.Split(strings.TrimSpace(lines[len(lines)-1]), -1)
	for j, part := range parts {
		var op op
		switch part {
		case "*":
			op = STAR
		case "+":
			op = PLUS
		default:
			return hw, fmt.Errorf("invalid op")
		}
		hw.ops[j] = op
	}
	return hw, nil
}

func parseWeirdHomework(lines []string) (homework, error) {
	var hw homework
	hw.ops = []op{}
	hw.cols = [][]uint64{}
	c := 0
	hw.cols = append(hw.cols, []uint64{})
	for i := len(lines[0]) - 1; i >= 0; i-- {
		value := uint64(0)
		foundValue := false
		for j := 0; j < len(lines)-1; j++ {
			line := lines[j]
			char := line[i]
			switch char {
			case ' ':
				continue
			default:
				digit, err := strconv.ParseUint(string(char), 10, 4)
				if err != nil {
					return hw, err
				}
				foundValue = true
				value = value*10 + digit
			}
		}
		if foundValue {
			hw.cols[c] = append(hw.cols[c], value)
		} else {
			c++
			hw.cols = append(hw.cols, []uint64{})
		}
	}
	parts := spaces.Split(strings.TrimSpace(lines[len(lines)-1]), -1)
	for i := len(parts) - 1; i >= 0; i-- {
		var op op
		switch parts[i] {
		case "*":
			op = STAR
		case "+":
			op = PLUS
		default:
			return hw, fmt.Errorf("invalid op")
		}
		hw.ops = append(hw.ops, op)
	}
	return hw, nil
}

func (hw homework) do() uint64 {
	total := uint64(0)
	for i, col := range hw.cols {
		op := hw.ops[i]
		value := uint64(0)
		switch op {
		case PLUS:
			for _, x := range col {
				value += x
			}
		case STAR:
			value = 1
			for _, x := range col {
				value *= x
			}
		}
		total += value
	}
	return total
}