Merge branch 'feature/day20' into develop

0legg · 0legg · commit d026f628f6df · 2024-12-19T21:18:55.000-08:00
diff --git a/year2024/src/main/kotlin/net/olegg/aoc/year2024/day20/Day20.kt b/year2024/src/main/kotlin/net/olegg/aoc/year2024/day20/Day20.kt
@@ -0,0 +1,64 @@
+package net.olegg.aoc.year2024.day20
+
+import net.olegg.aoc.someday.SomeDay
+import net.olegg.aoc.utils.Directions.Companion.NEXT_4
+import net.olegg.aoc.utils.Vector2D
+import net.olegg.aoc.utils.find
+import net.olegg.aoc.utils.get
+import net.olegg.aoc.utils.pairs
+import net.olegg.aoc.year2024.DayOf2024
+
+/**
+ * See [Year 2024, Day 20](https://adventofcode.com/2024/day/20)
+ */
+object Day20 : DayOf2024(20) {
+  override fun first(): Any? {
+    return solve(2)
+  }
+
+  override fun second(): Any? {
+    return solve(20)
+  }
+
+  private fun solve(maxCheat: Int, minSave: Int = 100): Int {
+    val start = matrix.find('S')!!
+    val end = matrix.find('E')!!
+
+    val queue = ArrayDeque(listOf(start to 0))
+    val seen = mutableMapOf<Vector2D, Int>()
+
+    while (queue.isNotEmpty()) {
+      val (curr, step) = queue.removeFirst()
+
+      if (curr in seen) {
+        continue
+      }
+      seen[curr] = step
+      if (curr == end) {
+        break
+      }
+
+      queue += NEXT_4.map { curr + it.step }
+        .filter { it == end || matrix[it] == '.'}
+        .filter { it !in seen }
+        .map { it to step + 1 }
+    }
+
+    val path = generateSequence(end to seen[end]!!) { (curr, step) ->
+      NEXT_4.map { curr + it.step }
+        .firstOrNull { seen[it] == step - 1 }
+        ?.let { it to step - 1 }
+    }.toList().reversed()
+
+    return path.pairs().count { (first, second) ->
+      val (firstPos, firstStep) = first
+      val (secondPos, secondStep) = second
+
+      val delta = (secondPos - firstPos).manhattan()
+
+      delta <= maxCheat && secondStep - firstStep - delta >= minSave
+    }
+  }
+}
+
+fun main() = SomeDay.mainify(Day20)
