Day 8 part 1

2024/src/main/kotlin/day8.kt

@@ -0,0 +1,42 @@
+fun main() {
+    val grid = Grid.read("day8.txt")
+
+    val antennasByFrequency = mutableMapOf<Char, MutableList<Grid.Coordinate>>()
+    for (y in 0..<grid.height) {
+        for (x in 0..<grid.width) {
+            val value = grid.get(x, y)
+            if (value != '.') {
+                antennasByFrequency.getOrPut(value) { mutableListOf() }.add(Grid.Coordinate(x, y))
+            }
+        }
+    }
+
+    fun isAntinode(coordinate: Grid.Coordinate, frequency: Char, antennas: List<Grid.Coordinate>) =
+        antennas.any { antenna ->
+            val dx = antenna.x - coordinate.x
+            val dy = antenna.y - coordinate.y
+            val secondAntenna = Grid.Coordinate(antenna.x + dx, antenna.y + dy)
+
+            secondAntenna != antenna && grid.inside(secondAntenna) && grid.get(secondAntenna) == frequency
+        }
+
+    fun isAntinode(coordinate: Grid.Coordinate) =
+        antennasByFrequency.any { (frequency, positions) ->
+            isAntinode(coordinate, frequency, positions)
+        }
+
+    val antinodes = mutableSetOf<Grid.Coordinate>()
+
+    for (y in 0..<grid.height) {
+        for (x in 0..<grid.width) {
+
+            val coordinate = Grid.Coordinate(x, y)
+            if (isAntinode(coordinate)) {
+                antinodes.add(coordinate)
+            }
+
+        }
+    }
+
+    println("Part 1: ${antinodes.count()}")
+}