1 files changed, 102 insertions, 0 deletions
diff --git a/day3.go b/day3.go
new file mode 100644
index 0000000..98357dc
--- /dev/null
+++ b/day3.go
@@ -0,0 +1,102 @@
+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)
+}