2 files changed, 102 insertions, 0 deletions
diff --git a/src/Day04.kt b/src/Day04.kt
new file mode 100644
index 0000000..6db98dd
--- /dev/null
+++ b/src/Day04.kt
@@ -0,0 +1,95 @@
+class Bingo(private val revealQueue: ArrayDeque<Int>, private val boards: List<Board>) {
+    companion object {
+        fun fromString(input: String): Bingo {
+            val sections = input.trim().split("(\\r?\\n){2}".toRegex())
+
+            val revealQueueString = sections.first()
+            val revealQueue = ArrayDeque(revealQueueString.split(",").map { it.toInt() })
+
+            val boards = sections.drop(1).map { Board.fromMatrixString(it) }
+
+            return Bingo(revealQueue, boards)
+        }
+    }
+
+    fun getResults() = sequence {
+        while (revealQueue.isNotEmpty() && boards.any { !it.didWin }) {
+            val number = revealQueue.removeFirst()
+
+            for (board in boards.filter { !it.didWin }) {
+                board.reveal(number)
+
+                if (board.didWin) {
+                    yield(board.sumOfUnmarked * number)
+                }
+            }
+        }
+    }
+
+    class Board(private var rows: List<List<Square>>) {
+        companion object {
+            fun fromMatrixString(matrixString: String): Board = Board(
+                matrixString.trim().split("\\r?\\n".toRegex()).map { rowString ->
+                    rowString.trim().split("\\s+".toRegex()).map { squareString ->
+                        Square.Unmarked(squareString.toInt())
+                    }
+                }
+            )
+
+            private const val SIZE = 5
+
+            private val ROWS = (0 until SIZE).map { row ->
+                (0 until SIZE).map { square ->
+                    Pair(row, square)
+                }
+            }
+
+            private val COLUMNS = (0 until SIZE).map { column ->
+                (0 until SIZE).map { square ->
+                    Pair(square, column)
+                }
+            }
+
+            private val WINNING_CONFIGURATIONS = ROWS + COLUMNS
+        }
+
+        fun reveal(number: Int) {
+            rows = rows.map { row ->
+                row.map { square ->
+                    if (square is Square.Unmarked && square.number == number) Square.Marked else square
+                }
+            }
+        }
+
+        val didWin: Boolean
+            get() = WINNING_CONFIGURATIONS.any { configuration ->
+                configuration.all { (row, column) -> rows[row][column] is Square.Marked }
+            }
+
+        val sumOfUnmarked: Int
+            get() = rows.fold(0) { racc, row ->
+                racc + row.fold(0) { sacc, square ->
+                    sacc + if (square is Square.Unmarked) square.number else 0
+                }
+            }
+
+        sealed class Square {
+            object Marked : Square()
+            class Unmarked(val number: Int) : Square()
+        }
+    }
+}
+
+fun main() {
+    fun part1(input: String): Int = Bingo.fromString(input).getResults().first()
+
+    fun part2(input: String): Int = Bingo.fromString(input).getResults().last()
+
+    val testInput = readInputAsString("Day04_test")
+    check(part1(testInput) == 4512)
+    check(part2(testInput) == 1924)
+
+    val input = readInputAsString("Day04")
+    println(part1(input))
+    println(part2(input))
+}
diff --git a/src/Utils.kt b/src/Utils.kt
index 27b88b5..6268ba6 100644
--- a/src/Utils.kt
+++ b/src/Utils.kt
@@ -10,6 +10,13 @@ import java.security.MessageDigest
 fun readInputAsLines(name: String): List<String> = File("src", "$name.txt").readLines()
 
 /**
+ * Returns a string of contents of the given input txt file.
+ * @param name name of the file
+ * @return contents of file as string
+ */
+fun readInputAsString(name: String): String = File("src", "$name.txt").readText()
+
+/**
  * Read lines from the given input txt file and convert them to decimal numbers.
  * @param name name of the file
  * @return list of ints containing numbers from each of file's lines