main.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
	"regexp"
	"strconv"
	"time"
)

// DayFunc takes the input filename as input and returns the answer and error.
type DayFunc = func(string) (string, error)

func main() {
	if len(os.Args) < 3 {
		log.Fatal("Pass the day and input file name as arguments.")
	}

	var dayFn DayFunc
	switch os.Args[1] {
	case "11":
		dayFn = day1part1
	case "12":
		dayFn = day1part2
	case "21":
		dayFn = day2part1
	case "22":
		dayFn = day2part2
	case "31":
		dayFn = day3part1
	case "32":
		dayFn = day3part2
	case "41":
		dayFn = day4part1
	case "42":
		dayFn = day4part2
	case "51":
		dayFn = day5part1
	case "52":
		dayFn = day5part2
	case "61":
		dayFn = day6part1
	case "62":
		dayFn = day6part2
	case "71":
		dayFn = day7part1
	case "72":
		dayFn = day7part2
	case "91":
		dayFn = day9part1
	case "92":
		dayFn = day9part2
	case "101":
		dayFn = day10part1
	case "102":
		dayFn = day10part2
	case "111":
		dayFn = day11part1
	case "112":
		dayFn = day11part2
	case "131":
		dayFn = day13part1
	case "132":
		dayFn = day13part2
	case "141":
		dayFn = day14part1
	case "142":
		dayFn = day14part2
	case "151":
		dayFn = day15part1
	case "152":
		dayFn = day15part2
	case "161":
		dayFn = day16part1
	case "162":
		dayFn = day16part2
	case "171":
		dayFn = day17part1
	case "172":
		dayFn = day17part2
	case "181":
		dayFn = day18part1
	case "182":
		dayFn = day18part2
	case "191":
		dayFn = day19part1
	case "192":
		dayFn = day19part2
	case "201":
		dayFn = day20part1
	case "202":
		dayFn = day20part2
	case "221":
		dayFn = day22part1
	case "222":
		dayFn = day22part2
	case "231":
		dayFn = day23part1
	case "232":
		dayFn = day23part2
	case "241":
		dayFn = day24part1
	case "242":
		dayFn = day24part2
	case "251":
		dayFn = day25part1
	default:
		log.Fatalf("Invalid day identifier '%s'", os.Args[1])
	}

	startTime := time.Now()
	res, err := dayFn(os.Args[2])
	if err != nil {
		log.Fatal(err)
	}
	endTime := time.Now()

	log.Printf("Answer: %s", res)
	log.Printf("Run time: %v", endTime.Sub(startTime))
}

func forLine(filename string, run func(string)) error {
	f, err := os.Open(filename)
	if err != nil {
		return fmt.Errorf("could not open file: %s", filename)
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	scanner.Split(bufio.ScanLines)
	for scanner.Scan() {
		run(scanner.Text())
	}

	return nil
}

func forLineError(filename string, run func(string) error) error {
	f, err := os.Open(filename)
	if err != nil {
		return fmt.Errorf("could not open file: %s", filename)
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	scanner.Split(bufio.ScanLines)
	for scanner.Scan() {
		if err := run(scanner.Text()); err != nil {
			return err
		}
	}

	return nil
}

func allLines(filename string) ([]string, error) {
	lines := make([]string, 0)

	if err := forLine(filename, func(line string) {
		lines = append(lines, line)
	}); err != nil {
		return nil, err
	}

	return lines, nil
}

var numberLineRe = regexp.MustCompile(`-?\d+`)

func parseNumberLine(str string) []int {
	nums := make([]int, 0)
	for _, s := range numberLineRe.FindAllString(str, -1) {
		n, err := strconv.Atoi(s)
		if err != nil {
			log.Fatalf("could not convert to number: '%s'", s)
		}
		nums = append(nums, n)
	}
	return nums
}