Day21

src/Day21.kt

@@ -0,0 +1,131 @@
+fun main() {
+    day21Part(1)
+    day21Part(2)
+}
+
+private const val ROOT = "root"
+private const val HUMN = "humn"
+
+fun day21Part(part: Int) {
+    val dayId = "21"
+    val input = readInput("Day${dayId}")
+
+    data class Monkey(val name: String, var x: Long?, val a: String, var op: String, val b: String)
+
+    val ms = input.map { s ->
+        val (name, os) = s.split(": ")
+        val x = os.toLongOrNull()
+        if (x != null) {
+            Monkey(name, x, "", "", "")
+        } else {
+            val (a, op, b) = os.split(" ")
+            Monkey(name, null, a, op, b)
+        }
+    }.associateBy { it.name }
+
+    if (part == 2) {
+        ms[HUMN]!!.x = null
+        ms[ROOT]!!.op = "="
+    }
+
+    fun compute(name: String): Long? {
+        val m = ms[name]!!
+        if (m.name == HUMN || m.x != null) return m.x
+        val av = compute(m.a)
+        val bv = compute(m.b)
+        if (av == null || bv == null) return null
+        val x = when (m.op) {
+            "+" -> av + bv
+            "-" -> av - bv
+            "*" -> av * bv
+            "/" -> av / bv
+            else -> error(m.op)
+        }
+        m.x = x
+        return x
+    }
+
+    val ans1 = compute(ROOT)
+
+    if (part == 1) {
+        println("part1 = $ans1")
+        return
+    }
+
+    var r = ROOT
+    var c = 0L
+    while (r != HUMN) {
+        val m = ms[r]!!
+        val a = ms[m.a]!!
+        val b = ms[m.b]!!
+        when (m.op) {
+            "=" -> when {
+                a.x != null -> {
+                    check(b.x == null)
+                    r = m.b
+                    c = a.x!!
+                }
+                b.x != null -> {
+                    check(a.x == null)
+                    r = m.a
+                    c = b.x!!
+                }
+                else -> error(m)
+            }
+            "+" -> when {
+                a.x != null -> {
+                    check(b.x == null)
+                    r = m.b
+                    c -= a.x!!
+                }
+                b.x != null -> {
+                    check(a.x == null)
+                    r = m.a
+                    c -= b.x!!
+                }
+                else -> error(m)
+            }
+            "-" -> when {
+                a.x != null -> {
+                    check(b.x == null) { "$m" }
+                    r = m.b
+                    c = a.x!! - c
+                }
+                b.x != null -> {
+                    check(a.x == null)
+                    r = m.a
+                    c += b.x!!
+                }
+                else -> error(m)
+            }
+            "*" -> when {
+                a.x != null -> {
+                    check(b.x == null)
+                    r = m.b
+                    c /= a.x!!
+                }
+                b.x != null -> {
+                    check(a.x == null)
+                    r = m.a
+                    c /= b.x!!
+                }
+                else -> error(m)
+            }
+            "/" -> when {
+                a.x != null -> {
+                    check(b.x == null)
+                    r = m.b
+                    c = a.x!! / c
+                }
+                b.x != null -> {
+                    check(a.x == null)
+                    r = m.a
+                    c *= b.x!!
+                }
+                else -> error(m)
+            }
+            else -> error(m)
+        }
+    }
+    println("part2 = $c")
+}