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package main
import (
"bufio"
"fmt"
"log"
"os"
)
// Returns two slices of the input array representing both backpacks
func ruckSacks(inputline []byte) ([]byte, []byte) {
split := len(inputline) / 2
return inputline[:split], inputline[split:]
}
// Gets the priority based on the character byte
func itemPriority(item byte) byte {
if 'a' <= item && item <= 'z' {
return (item - 'a')
}
if 'A' <= item && item <= 'Z' {
return (item - 'A' + 26)
}
return 0
}
// Makes an array of 52 flags indicating with 0 or 1 if an item is in a rucksack
func insideFlags(ruckSack []byte) []byte {
flags := make([]byte, 52)
for _, item := range ruckSack {
if flags[item] != 1 {
flags[item] = 1
}
}
return flags
}
// Checks if one of the items in a rucksacks is flagged in another rucksack
func inBoth(ruckSack []byte, insideFlags []byte) byte {
for _, item := range ruckSack {
if insideFlags[item] == 1 {
return item
}
}
return 0
}
// Finds the item that is in all rucksacks
func inAll(elfGroup []byte, groupSize int) int {
for i, itemCount := range elfGroup {
if int(itemCount) == groupSize {
return i + 1
}
}
return 0
}
// Solves the rucksack problem in linear time (since we don't have nested loops to check for the presence in the rucksacks),
// could be more efficient with memory i think? But that would involve more bookkeeping
func main() {
f, err := os.Open("day3.txt")
if err != nil {
log.Fatal("Could not open inputfile")
}
// Part 1
runningSum := 0
scanner := bufio.NewScanner(f)
for scanner.Scan() {
priorities := make([]byte, len(scanner.Bytes()))
for i, item := range scanner.Bytes() {
priorities[i] = itemPriority(item)
}
r1, r2 := ruckSacks(priorities)
insideR2 := insideFlags(r2)
duplicate := inBoth(r1, insideR2)
runningSum += int(duplicate) + 1
}
fmt.Println("part 1:", runningSum)
// Part 2
runningSum = 0
elfGroup := make([]byte, 52)
groupSize := 3
elf := 0
f.Seek(0, 0)
scanner = bufio.NewScanner(f)
for scanner.Scan() {
priorities := make([]byte, len(scanner.Bytes()))
for i, item := range scanner.Bytes() {
priorities[i] = itemPriority(item)
}
for i, flag := range insideFlags(priorities) {
elfGroup[i] += flag
}
if elf == (groupSize - 1) {
runningSum += inAll(elfGroup, groupSize)
elfGroup = make([]byte, 52)
}
elf = (elf + 1) % groupSize
}
fmt.Println("part 2:", runningSum)
}
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