Golang Advent of Code 2024

Learning go by doing the Advent of Code 2024.

Usage

go run day<day_number>/main.go --part <1 or 2> --input <input_file>

Testing

go test ./day<day_number>  # consider the dot

Learnings

Day 1

Flags

The flag package can be used to parse command line arguments.

var part int
var inputpath string
flag.IntVar(&part, "part", 1, "part 1 or 2")
flag.StringVar(&inputpath, "input", "example.txt", "file path of input")
flag.Parse()

Reading files

To read a whole file, use os.ReadFile.

content, err := os.ReadFile(inputpath)

Splitting strings

To split a string, use strings.Split.

lines := strings.Split(string(content), "\n")

To decompose a string with space separated values, use fmt.Sscanf.

var a, b int
_, err := fmt.Sscanf(line, "%d %d", &a, &b)

Day 2

Parsing strings to int

To parse a string to an int, use strconv.Atoi.

strconv.Atoi("123")

Popping elements from a slice

To pop an element from a slice, use the following pattern. Without copying the original slice, it the original slice is modified.

c := make([]string, len(original))
copy(c, levels)
removedOne := append(c[:i], c[i+1:]...)

Day 3

Regex

To use regex, use the regexp package.

r := regexp.MustCompile(`(?s)(?:do\(\)|^)(.*?)(?:don't\(\)|$)`)
matches := r.FindAllStringSubmatch(input, -1)

The (?s) flag is used to make . match newlines. All in FindAllStringSubmatch is used to find all matches. Submatch is used to get the matched groups.

Day 4

Slicing strings

If the string is containing ASCII only, it can be treated as a slice of bytes.

reversed := make([]byte, len(subString))
copy(reversed, subString)
slices.Reverse(reversed)

In case it would contain unicode, one can convert to []rune first.

runes := []rune(someString)

Double-check off-by-one errors

Always make sure to double-check your lengths and limits.

Constant array

An array can not be defined as a constant.

References

Project structure based on alexchao26/advent-of-code-go.