Day 17

3 files changed, 119 insertions, 0 deletions

diff --git a/day17/go.mod b/day17/go.mod
new file mode 100644
index 0000000..cc9fe5f
--- /dev/null
+++ b/day17/go.mod
@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day17
+
+go 1.23.3
diff --git a/day17/main.go b/day17/main.go
new file mode 100644
index 0000000..7e45e94
--- /dev/null
+++ b/day17/main.go
@@ -0,0 +1,115 @@
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println("4,6,3,5,6,3,5,2,1,0,")
+	execute(729, 0, 0, []byte{0, 1, 5, 4, 3, 0})
+
+	fmt.Println("7,1,3,4,1,2,6,7,1,")
+	execute(46187030, 0, 0, []byte{2, 4, 1, 5, 7, 5, 0, 3, 4, 0, 1, 6, 5, 5, 3, 0})
+
+	// Example
+	// while (a != 0) {
+	//   a = a >> 3
+	//   print(a%8)
+	// }
+
+	// Data
+	// while (a != 0) {
+	//   b = a % 8
+	//   b = b ^ 5
+	//   c = a >> b
+	//   a = a >> 3
+	//   b = b ^ c
+	//   b = b ^ 6
+	//   print(b % 8)
+	// }
+	part2([]byte{2, 4, 1, 5, 7, 5, 0, 3, 4, 0, 1, 6, 5, 5, 3, 0})
+}
+
+func solve(ops []byte, forOp int, startingA int64) bool {
+	if forOp < 0 {
+		fmt.Println(startingA)
+		return true
+	}
+
+	// For the last iteration, only the last 3 can be non 0.
+	// So we only have 8 values to test.
+	// For the other operands to invert, the higher bits are fixed
+	// so we only check the last 3 bits.
+	for n := range 8 {
+		a := (startingA << 3) | int64(n)
+		b := a % 8
+		b = b ^ 5
+		c := a >> b
+		b = b ^ c
+		b = b ^ 6
+		if byte(b%8) == ops[forOp] && solve(ops, forOp-1, a) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func part2(ops []byte) (a int64) {
+	solve(ops, len(ops)-1, 0)
+	return
+}
+
+func execute(a, b, c int64, ops []byte) {
+	pc := 0
+
+	readOp := func() (op byte) {
+		op = ops[pc]
+		pc++
+		return
+	}
+
+	getCombo := func(op byte) int64 {
+		if op <= 3 {
+			return int64(op)
+		}
+
+		switch op {
+		case 4:
+			return a
+		case 5:
+			return b
+		case 6:
+			return c
+		}
+
+		panic("")
+	}
+
+	for pc < len(ops)-1 {
+		switch readOp() {
+		case 0:
+			a = a >> getCombo(readOp())
+		case 1:
+			b = b ^ int64(readOp())
+		case 2:
+			b = getCombo(readOp()) % 8
+		case 3:
+			if a != 0 {
+				pc = int(readOp())
+			}
+		case 4:
+			b = b ^ c
+			pc++
+		case 5:
+			fmt.Print(getCombo(readOp()) % 8)
+			fmt.Print(",")
+		case 6:
+			b = a >> getCombo(readOp())
+		case 7:
+			c = a >> getCombo(readOp())
+		}
+	}
+
+	fmt.Println("")
+}
diff --git a/go.work b/go.work
index 60d840c..ad53500 100644
--- a/go.work
+++ b/go.work
@@ -9,6 +9,7 @@ use (
 	./day14
 	./day15
 	./day16
+	./day17
 	./day2
 	./day3
 	./day4