AoC 2019 day 2

day2.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"strconv"
	"strings"
)

func main() {
	b := readFile("day2")
	strProgram := strings.Split(string(b), ",")
	program := make([]int, len(strProgram))

	for i, s := range strProgram {
		program[i] = toInt(s)
	}

	// part 1
	program[1] = 12
	program[2] = 2
	output := runIntcode(program)
	fmt.Println(output[0])

	// part 2
	for i := 0; i < 100; i++ {
		for j := 0; j < 100; j++ {
			program[1] = i
			program[2] = j
			output := runIntcode(program)
			if output[0] == 19690720 {
				fmt.Println(i, "and", j, "=>", 100*i+j)
			}
		}
	}

}

func runIntcode(programOrig []int) []int {
	program := make([]int, len(programOrig))
	copy(program, programOrig)
	cursor := 0
	for {
		opcode := program[cursor]
		switch opcode {
		case 1:
			i1Pos := program[cursor+1]
			i2Pos := program[cursor+2]
			outputPos := program[cursor+3]
			program[outputPos] = program[i1Pos] + program[i2Pos]
		case 2:
			i1Pos := program[cursor+1]
			i2Pos := program[cursor+2]
			outputPos := program[cursor+3]
			program[outputPos] = program[i1Pos] * program[i2Pos]
		case 99:
			return program
		default:
			log.Fatal("Received bad opcode: ", opcode)
		}
		cursor += 4
	}
}

func readFile(filename string) string {
	bytes, err := ioutil.ReadFile(filename)
	check(err)
	return strings.TrimSpace(string(bytes))
}

func toInt(s string) int {
	result, err := strconv.Atoi(s)
	check(err)
	return result
}

func check(err error) {
	if err != nil {
		panic(err)
	}
}