Solution for 2024, day 17

Author: lucianoq

2024/17/Makefile

@@ -0,0 +1,19 @@
+input:
+	http "https://adventofcode.com/2024/day/17/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input

2024/17/common.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func parseInput() (int, int, int, []int) {
+	scanner := bufio.NewScanner(os.Stdin)
+
+	scanner.Scan()
+	a, _ := strconv.Atoi(strings.Fields(scanner.Text())[2])
+
+	scanner.Scan()
+	b, _ := strconv.Atoi(strings.Fields(scanner.Text())[2])
+
+	scanner.Scan()
+	c, _ := strconv.Atoi(strings.Fields(scanner.Text())[2])
+
+	scanner.Scan()
+
+	scanner.Scan()
+	var code []int
+	for _, s := range strings.Split(strings.Fields(scanner.Text())[1], ",") {
+		n, _ := strconv.Atoi(s)
+		code = append(code, n)
+	}
+	return a, b, c, code
+}
+
+func run(a, b, c int, code []int) []int {
+	output := make([]int, 0)
+
+	for ip := 0; ip < len(code); {
+		var (
+			op    = code[ip]
+			arg   = code[ip+1]
+			combo = 0
+		)
+
+		switch arg {
+		case 1, 2, 3:
+			combo = arg
+		case 4:
+			combo = a
+		case 5:
+			combo = b
+		case 6:
+			combo = c
+		}
+
+		switch op {
+		case 0: // adv
+			a >>= combo
+		case 1: // bxl
+			b ^= arg
+		case 2: // bst
+			b = combo % 8
+		case 3: // jnz
+			if a != 0 {
+				ip = arg
+				continue
+			}
+		case 4: // bxc
+			b ^= c
+		case 5: // out
+			output = append(output, combo%8)
+		case 6: // bdv
+			b = a >> combo
+		case 7: // cdv
+			c = a >> combo
+		}
+
+		ip += 2
+	}
+
+	return output
+}

2024/17/main1.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	a, b, c, code := parseInput()
+
+	output := run(a, b, c, code)
+
+	fmt.Println(strings.Join(toStr(output), ","))
+}
+
+func toStr(x []int) []string {
+	s := make([]string, len(x))
+	for i := 0; i < len(x); i++ {
+		s[i] = strconv.Itoa(x[i])
+	}
+	return s
+}

2024/17/main2.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"slices"
+)
+
+func main() {
+	_, b, c, code := parseInput()
+
+	a := 0
+	for i := len(code) - 1; i >= 0; i-- {
+		// make room for 3 bits by shifting to the left
+		a <<= 3
+
+		// check incrementally only the latest bits,
+		// until we find the right value
+		for !slices.Equal(run(a, b, c, code), code[i:]) {
+			a++
+		}
+	}
+	fmt.Println(a)
+}