Day 17.

src/aoc2024/Day17.kt

@@ -0,0 +1,123 @@
+package aoc2024
+
+import println
+import readInput
+import splitInts
+
+/*
+--- Day 17: Chronospatial Computer ---
+https://adventofcode.com/2024/day/17
+*/
+fun main() {
+
+    val inlineTestInput = """
+Register A: 117440
+Register B: 0
+Register C: 0
+
+Program: 0,3,5,4,3,0
+"""
+    val regs = LongArray(3)
+
+    fun getComboOperand(op: Int): Long =
+        when (op) {
+            0, 1, 2, 3 -> op.toLong()
+            4, 5, 6 -> regs[op - 4]
+            else -> throw IllegalStateException()
+        }
+
+    fun part1(input: List<String>): String {
+        regs[0] = input[0].split(" ")[2].toLong()
+        regs[1] = input[1].split(" ")[2].toLong()
+        regs[2] = input[2].split(" ")[2].toLong()
+        val ops = input[4].split(" ")[1].splitInts(",")
+        val outs = ArrayList<Int>()
+        var pc = 0
+        while (pc < ops.size) {
+            when (ops[pc++]) {
+                // adv
+                0 -> regs[0] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // bdv
+                6 -> regs[1] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // cdv
+                7 -> regs[2] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // bxl
+                1 -> regs[1] = regs[1] xor ops[pc++].toLong()
+                // bst
+                2 -> regs[1] = getComboOperand(ops[pc++]) and 7
+                // jnz
+                3 -> if (regs[0] == 0L) pc++ else pc = ops[pc]
+                // bxc
+                4 -> {
+                    regs[1] = regs[1] xor regs[2]
+                    pc++
+                }
+                // out
+                5 -> outs.add((getComboOperand(ops[pc++]) and 7).toInt())
+            }
+        }
+        return outs.joinToString(",")
+    }
+
+    fun verify(rega: Long, ops: List<Int>, outs: List<Int>): Boolean {
+        regs[0] = rega
+        var pc = 0
+        var outpos = 0
+        while (pc < ops.size) {
+            when (ops[pc++]) {
+                // adv
+                0 -> regs[0] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // bdv
+                6 -> regs[1] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // cdv
+                7 -> regs[2] = regs[0] shr getComboOperand(ops[pc++]).toInt()
+                // bxl
+                1 -> regs[1] = regs[1] xor ops[pc++].toLong()
+                // bst
+                2 -> regs[1] = getComboOperand(ops[pc++]) and 7
+                // jnz
+                3 -> if (regs[0] == 0L) pc++ else pc = ops[pc]
+                // bxc
+                4 -> {
+                    regs[1] = regs[1] xor regs[2]
+                    pc++
+                }
+                // out
+                5 -> {
+                    val o = (getComboOperand(ops[pc++]) and 7).toInt()
+                    if (outs[outpos++] != o) return false
+                }
+            }
+        }
+        return true
+    }
+
+    fun rec(a: Long, p: Int, ops: List<Int>, outs: List<Int>): Long {
+        val partOutput = outs.take(p).reversed()
+        // assuming that all programs use 3 bit shifts in the last operation
+        for (i in 0L..7L) {
+            if (verify((a shl 3) or i, ops, partOutput)) {
+                if (p == outs.size) return (a shl 3) or i
+                val res = rec((a shl 3) or i, p + 1, ops, outs)
+                if (res != 0L) return res
+            }
+        }
+        return 0
+    }
+
+    fun part2(input: List<String>): Long {
+        val ops = input[4].split(" ")[1].splitInts(",")
+        val outs = ops.reversed()
+        return rec(0L, 1, ops, outs)
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day17_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+
+    val input = readInput("aoc2024/Day17")
+    part1(input).println()
+    part2(input).println()
+}