day(05): solve part 2

Signed-off-by: Matej Focko <me@mfocko.xyz>

src/Day05.kt

@@ -5,6 +5,9 @@ class Day05(
     private val updates: List<List<Int>>
 
     companion object {
+        val UNVISITED = 0
+        val OPEN = 1
+        val DONE = 2
     }
 
     init {
@@ -17,26 +20,61 @@ class Day05(
                     .map { line ->
                         line.split("|").map(String::toInt).let { it[0] to it[1] }
                     }.forEach { (before, after) ->
-                        getOrPut(after) { mutableSetOf() }.add(before)
+                        getOrPut(before) { mutableSetOf() }.add(after)
                     }
             }
         this.updates = updates.lineSequence().map { line -> line.split(",").map(String::toInt).toList() }.toList()
     }
 
+    private fun printGraph() {
+        println("digraph G {")
+
+        rules.forEach { u, vs ->
+            vs.forEach { v ->
+                println("\t$u -> $v")
+            }
+        }
+
+        println("}")
+    }
+
     override fun precompute() {
         // no-op
     }
 
     private fun isCorrect(update: List<Int>): Boolean =
-        update.withIndex().reversed().all { p ->
-            rules.getOrDefault(p.value, mutableSetOf()).all { preceding -> update.lastIndexOf(preceding) < p.index }
+        update.withIndex().all { p ->
+            rules
+                .getOrDefault(p.value, mutableSetOf())
+                .all { succeeding -> update.indexOf(succeeding).let { it == -1 || it > p.index } }
         }
 
     override fun part1(): Int = updates.filter(::isCorrect).sumOf { it[it.size / 2] }
 
-    private fun fix(update: List<Int>): List<Int> = update
+    // Toposort
+    private fun fix(xs: Set<Int>): List<Int> {
+        val ordering = mutableListOf<Int>()
+        val state = mutableMapOf<Int, Int>()
 
-    override fun part2(): Int = updates.filter { !isCorrect(it) }.map { fix(it) }.sumOf { it[it.size / 2] }
+        fun visit(u: Int) {
+            if (state.getOrDefault(u, UNVISITED) == DONE) {
+                return
+            }
+            check(state.getOrDefault(u, UNVISITED) != OPEN) { "We have found a loop ending in $u" }
+
+            state[u] = OPEN
+            // ‹.intersect()›: Recursively search only the interesting successors
+            rules.getOrDefault(u, emptySet()).intersect(xs).forEach { visit(it) }
+
+            state[u] = DONE
+            ordering.add(u)
+        }
+        xs.forEach { visit(it) }
+
+        return ordering
+    }
+
+    override fun part2(): Int = updates.filter { !isCorrect(it) }.map { fix(it.toSet()) }.sumOf { it[it.size / 2] }
 }
 
 fun main() {