adventofcode2021/6-1/main.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"os"
	"strconv"
)

func main() {
	fishies, err := readInput("input")
	if err != nil {
		fmt.Printf("Error reading or something\n%s", err)
		return
	}
	fishiesptr := &fishies
	// Run for i days
	for i := 0; i < 80; i++ {
		newfishies, _ := nextDay(*fishiesptr)
		fishiesptr = &newfishies
		fmt.Printf("On day %d: there are %d fishies\n", i+1, len(newfishies))
	}
}

// Read the fishies slice and returns the slice of new fishies
func nextDay(fishies []int) ([]int, error) {
	// For every fish (integer) in the slice, decrement by 1 if >= 1, reset to 6 if it was 0
	// Also, if the number was 0, add a new fish with counter 8 to a second slice
	// After everything, append the slices and return the result
	newfishies := make([]int, 0)
	for i := 0; i < len(fishies); i++ {
		if fishies[i] >= 1 {
			fishies[i] -= 1
			continue
		}
		if fishies[i] == 0 {
			fishies[i] = 6
			newfishies = append(newfishies, 8)
			continue
		}
	}

	fishies = append(fishies, newfishies...)
	return fishies, nil
}

func readInput(inputfile string) ([]int, error) {
	file, err := os.OpenFile(inputfile, 0, 0)
	if err != nil {
		return nil, err
	}
	defer file.Close()

	splitfunc := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
		nextIndex := bytes.IndexByte(data, ',')
		if nextIndex > 0 {
			// Returning the next position aka taking only the stuff up to nextIndex and slicing off everything else
			buffer := data[:nextIndex]
			// nextIndex plus 1 because index is at the comma, we want the next number
			return nextIndex + 1, bytes.TrimSpace(buffer), nil
		}

		// If we are at the end of the buffer, then return the entire buffer, but only if there is data
		if atEOF {
			if len(data) > 0 {
				return len(data), bytes.TrimSpace(data), nil
			}
		}

		// https://github.com/kgrz/reading-files-in-go/blob/master/comma-separated-string.go
		// For more info
		return 0, nil, nil
	}

	scanner := bufio.NewScanner(file)
	scanner.Split(splitfunc)

	fishies := make([]int, 0)
	for scanner.Scan() {
		fish, err := strconv.Atoi(scanner.Text())
		if err != nil {
			fmt.Printf("Error converting string to integer\n")
			return nil, err
		}
		fishies = append(fishies, fish)
	}
	return fishies, nil
}