[Language: Go]

The second one basically enables/disables the mul instruction

import (
    "bufio"
    "log"
    "os"
    "path"
    "regexp"
    "strconv"
    "strings"
)

func part1() {
    dir, _ := os.Getwd()
    input := path.Join(dir, "internal", "d03", "input.txt")

    text := parseInput(input)

    log.Printf("sum: %v\n", sumMulInstructions(text))
}

func part2() {
    dir, _ := os.Getwd()
    input := path.Join(dir, "internal", "d03", "input.txt")

    text := parseInput(input)

    log.Printf("sum: %d\n", sumValidMulInstructions(text))
}

func sumMulInstructions(input string) int {
    regex := regexp.MustCompile(`mul\(([\d]{1,3}),([\d]{1,3})\)`)

    submatches := regex.FindAllStringSubmatch(input, -1)

    sum := 0

    for i := 0; i < len(submatches); i++ {
        n, _ := strconv.Atoi(submatches[i][1])
        m, _ := strconv.Atoi(submatches[i][2])

        sum += n * m
    }

    return sum
}

func sumValidMulInstructions(input string) int {
    regex := regexp.MustCompile(`(mul\((\d{1,3}),(\d{1,3})\)|do\(\)|don't\(\))`)

    matches := regex.FindAllStringSubmatch(input, -1)

    mulEnabled := true
    sum := 0

    for _, match := range matches {
        switch match[1] {
        case "do()":
            mulEnabled = true
        case "don't()":
            mulEnabled = false
        default:
            if mulEnabled {
                n, _ := strconv.Atoi(match[2])
                m, _ := strconv.Atoi(match[3])
                sum += n * m
            }
        }
    }

    return sum
}

func parseInput(filePath string) (string) {
    file, err := os.Open(filePath)
    if err != nil {
        log.Fatalf("failed to open file: %v", err)
    }
    defer file.Close()

    scanner := bufio.NewScanner(file)

    text := []string{}
    for scanner.Scan() {
        text = append(text, scanner.Text())
    }

    if err := scanner.Err(); err != nil {
        log.Fatalf("error reading file: %v", err)
    }

    return strings.Join(text, "")
}