AdventOfCode2021/03/main.go

package main

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

func main() {
	start := time.Now()
	partOne()
	duration := time.Since(start)
	partTwo()
	duration2 := time.Since(start)
	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
}

func makeScanner(test bool) *bufio.Scanner {
	var f *os.File
	if test {
		f, _ = os.Open("inputs/testinput")
	} else {
		f, _ = os.Open("inputs/input")
	}
	reader := bufio.NewReader(f)
	return bufio.NewScanner(reader)
}

func partOne() {
	scanner := makeScanner(false)

	r := map[int]int64{} // index => count of 1s
	var lines int64

	for scanner.Scan() {
		line := scanner.Text()
		lines++
		n, _ := strconv.ParseInt(line, 2, 64)
		for i := 0; i < len(line); i++ {
			// check if the number in the i'th column is 1, then bit-shift back to the 1s place to add it if it is
			r[i] += (n & (1 << i)) >> i
		}
	}
	var epsilon, gamma int64
	for i, b := range r {
		// if more than half the bits were ones, gamma gets the 1, otherwise epsilon gets the 1
		// alternatively, this could have been an XOR operation, I suppose, once one or the other had been computed
		if b > lines/2 {
			gamma += 1 << i
		} else {
			epsilon += 1 << i
		}
	}
	fmt.Println(epsilon * gamma)
}

func partTwo() {
	scanner := makeScanner(false)

	inputs := []int64{}
	var oxgen, co2scrub int64

	var bitlen int64
	for scanner.Scan() {
		line := scanner.Text()
		n, _ := strconv.ParseInt(line, 2, 64)
		inputs = append(inputs, n)
		bitlen = int64(len(line))
	}

	// on initial loop, oxgen>>i == n>>i etc always come out to 0 == 0
	for i := bitlen; i > 0; i-- {
		var oxOnes, oxZeroes, coOnes, coZeroes int64
		for _, n := range inputs {
			if oxgen>>i == n>>i {
				if oxgen>>(i-1)^n>>(i-1) == 1 {
					oxOnes++
				} else {
					oxZeroes++
				}
			}
			if co2scrub>>i == n>>i {
				if co2scrub>>(i-1)^n>>(i-1) == 1 {
					coOnes++
				} else {
					coZeroes++
				}
			}
		}
		if oxOnes >= oxZeroes {
			oxgen += (1 << (i - 1))
		}
		if (coZeroes == 0 && coOnes != 0) || (coOnes < coZeroes && coOnes != 0) {
			co2scrub += (1 << (i - 1))
		}
	}
	fmt.Println(oxgen * co2scrub)
}