Day 22.

src/aoc2024/Day22.kt

@@ -0,0 +1,103 @@
+package aoc2024
+
+import println
+import readInput
+
+/*
+--- Day 22: Monkey Market ---
+https://adventofcode.com/2024/day/22
+*/
+fun main() {
+
+    val inlineTestInput = """
+1
+10
+100
+2024
+"""
+
+    val inlineTestInput2 = """
+1
+2
+3
+2024
+"""
+
+    fun next(number: Int): Int {
+        val r1 = (number xor (number shl 6)) and 0xff_ff_ff
+        val r2 = (r1 xor (r1 shr 5)) and 0xff_ff_ff
+        val r3 = (r2 xor (r2 shl 11)) and 0xff_ff_ff
+        return r3
+    }
+
+    fun part1(input: List<String>): Long {
+        var sum = 0L
+        for (i in input.map(String::toInt)) {
+            var v = i
+            for (p in 1..2000) {
+                v = next(v)
+            }
+            sum += v
+        }
+        return sum
+    }
+
+    fun part2(input: List<String>): Int {
+        val normalStrings = ArrayList<String>(2500)
+        val deltaStrings = ArrayList<String>(2500)
+        for (i in input.map(String::toInt)) {
+            val normalString = StringBuffer(2024)
+            val deltaString = StringBuffer(2024)
+            var v = i
+            var lastd = v % 10
+            normalString.append(lastd)
+            for (p in 1..2000) {
+                v = next(v)
+                val d = v % 10
+                val delta = d - lastd
+                normalString.append(d)
+                deltaString.append('j' + delta)
+                lastd = d
+            }
+            normalStrings.add(normalString.toString())
+            deltaStrings.add(deltaString.toString())
+        }
+
+        // it's not my fault that brute force is still working
+        val tokenBuffer = StringBuffer(5)
+        tokenBuffer.append("    ")
+        val maxIdx = deltaStrings[0].length - 3
+        var maxBananas = 0
+        for (code in 0 until 19 * 19 * 19 * 19) {
+            tokenBuffer.setCharAt(0, 'a' + (code / (19 * 19 * 19)))
+            tokenBuffer.setCharAt(1, 'a' + ((code / (19 * 19)) % 19))
+            tokenBuffer.setCharAt(2, 'a' + ((code / 19) % 19))
+            tokenBuffer.setCharAt(3, 'a' + ((code % 19)))
+            var bananas = 0
+            val token = tokenBuffer.toString()
+            for (i in deltaStrings.indices) {
+                val idx = deltaStrings[i].indexOf(token)
+                if (idx in 0..maxIdx) {
+                    bananas += normalStrings[i][idx + 4].digitToInt()
+                }
+            }
+            maxBananas = maxBananas.coerceAtLeast(bananas)
+        }
+        return maxBananas
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    val testInput2 = inlineTestInput2.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day22_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput2)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 37327623L)
+    check(testInputPart2Result == 23)
+
+    val input = readInput("aoc2024/Day22")
+    part1(input).println()
+    part2(input).println()
+}