Day 10.

src/Utils.kt

@@ -196,10 +196,17 @@ class CharGrid {
     fun matchRelative(c: Int, r: Int, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
         relposes.filter { predicate(get(c + it.dc, r + it.dr)) }
 
+    fun marchMatching(pos: RelPos, relposes: Iterable<RelPos>, predicate: (char: Char) -> Boolean): List<RelPos> =
+        relposes.map { pos.translate(it) }.filter { predicate(get(it)) }
+
     fun findMatches(predicate: (char: Char) -> Boolean): List<Pair<Int, Int>> {
         return generateGridPos().filter { predicate(get(it)) }.map { it.dc to it.dr }.toList()
     }
 
+    fun findMatchesRelPos(predicate: (char: Char) -> Boolean): List<RelPos> {
+        return generateGridPos().filter { predicate(get(it)) }.toList()
+    }
+
     fun collectMatches(predicate: (char: Char) -> Boolean): List<Pair<Char, RelPos>> {
         return generateGridPos().filter { predicate(get(it)) }.map { get(it) to it }.toList()
     }

src/aoc2024/Day10.kt

@@ -0,0 +1,64 @@
+package aoc2024
+
+import CharGrid
+import RelPos
+import println
+import readInput
+
+/*
+--- Day 10: Hoof It ---
+https://adventofcode.com/2024/day/10
+*/
+fun main() {
+
+    val inlineTestInput = """
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
+"""
+
+    fun part1(input: List<String>): Int {
+        val grid = CharGrid(input)
+        val starts = grid.findMatchesRelPos { it == '0' }
+        var sum = 0
+        for (startPos in starts) {
+            var nodes = listOf(startPos)
+            for (c in '1'..'9') {
+                nodes = nodes.flatMap { grid.marchMatching(it, CharGrid.PLUS_POS) { char -> char == c } }.distinct()
+                if (nodes.isEmpty()) break
+            }
+            sum += nodes.size
+        }
+
+        return sum
+    }
+
+    fun walkTrail(grid: CharGrid, c: Char, p: RelPos): Int {
+        val newNodes = grid.marchMatching(p, CharGrid.PLUS_POS) { it == c }
+        return if (c == '9') newNodes.size else newNodes.sumOf { walkTrail(grid, c + 1, it) }
+    }
+
+    fun part2(input: List<String>): Int {
+        val grid = CharGrid(input)
+        return grid.findMatchesRelPos { it == '0' }.sumOf { walkTrail(grid, '1', it) }
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day10_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 36)
+    check(testInputPart2Result == 81)
+
+    val input = readInput("aoc2024/Day10")
+    part1(input).println()
+    part2(input).println()
+}