Day 4 Part 1

2024/src/main/kotlin/Grid.kt

@@ -0,0 +1,66 @@
+class Grid(val rows: List<String>) {
+    val width = rows.firstOrNull()?.count() ?: 0
+    val height get() = rows.count()
+
+    init {
+        assert(rows.all { it.length == width })
+    }
+
+    companion object {
+        fun read(name: String): Grid = Grid(readInput(name).toList())
+    }
+
+    fun chars(x: Int, y: Int, dx: Int, dy: Int) = Line(x, y, dx, dy)
+
+    inner class Line(val x: Int, val y: Int, val dx: Int, val dy: Int) : Sequence<Char> {
+        override fun iterator(): Iterator<Char> = iterator {
+            if (dx == 1 && dy == 0) {
+                yieldAll(rows[y].substring(startIndex = x).asSequence())
+            } else {
+
+                var x = x
+                var y = y
+
+                while (x in 0..<width && y in 0..<height) {
+                    yield(get(x, y))
+                    x += dx
+                    y += dy
+                }
+            }
+        }
+    }
+
+    fun rows(): Sequence<Line> = sequence {
+        for (y in 0..<height) {
+            yield(chars(0, y, 1, 0))
+        }
+    }
+
+    fun columns(): Sequence<Line> = sequence {
+        for (x in 0..<width) {
+            yield(chars(x, 0, 0, 1))
+        }
+    }
+
+    fun diagonals(): Sequence<Line> = sequence {
+        for (x in 0..<width) {
+            yield(chars(x, 0, 1, 1))
+            if (x > 0) {
+                yield(chars(x, 0, -1, 1))
+            }
+        }
+        for (y in 1..<height) {
+            yield(chars(0, y, 1, 1))
+            yield(chars(width - 1, y, -1, 1))
+        }
+    }
+
+    fun all(): Sequence<Line> = sequence {
+        yieldAll(rows())
+        yieldAll(columns())
+        yieldAll(diagonals())
+    }
+
+    fun get(x: Int, y: Int): Char = rows[y][x]
+
+}

2024/src/main/kotlin/day4.kt

@@ -0,0 +1,27 @@
+fun main() {
+    val grid = Grid.read("day4.txt")
+
+    val part1 = grid.all().fold(0) { acc, line ->
+        val string = line.joinToString(separator = "")
+        println(string)
+
+        acc + string.countSubstring("XMAS") + string.countSubstring("SAMX")
+    }
+
+    println("Part 1: $part1")
+}
+
+fun String.countSubstring(substring: String): Int {
+    var start = 0
+    var count = 0
+
+    while (true) {
+        val next = indexOf(substring, start)
+        if (next == -1) break
+
+        count++
+        start = next + substring.length
+    }
+
+    return count
+}