3 files changed, 963 insertions(+), 0 deletions(-)

A src/main/advent2021/Day13.kt
A src/main/resources/advent2021/day13_input.txt
A src/test/advent2021/Day13KtTest.kt

@@ 0,0 1,60 @@
+package advent2021
+
+import common.Point
+import kotlin.math.abs
+
+class TransparentPaper(val points: List<Point>, val folds: List<(Point) -> Point>) {
+    companion object {
+        fun from(lines: List<String>): TransparentPaper {
+            fun foldPointLeft(p: Point, x: Int): Point {
+                val distance = abs(x - p.x)
+                val newX = if (p.x < x) p.x else x - distance
+                return Point(newX, p.y)
+            }
+
+            fun foldPointUp(p: Point, y: Int): Point {
+                val distance = abs(y - p.y)
+                val newY = if (p.y < y) p.y else y - distance
+                return Point(p.x, newY)
+            }
+
+            val points = lines.takeWhile { it.isNotEmpty() }.map { line ->
+                val (x, y) = line.split(",").map { it.toInt() }
+                Point(x, y)
+            }
+            val folds = lines.drop(points.count() + 1).map {
+                val (foldDirection, n) = it.substringAfter("fold along ").split("=")
+                when (foldDirection) {
+                    "x" -> { p: Point -> foldPointLeft(p, n.toInt()) }
+                    "y" -> { p: Point -> foldPointUp(p, n.toInt()) }
+                    else -> throw Exception("can't parse $it")
+                }
+            }
+            return TransparentPaper(points, folds)
+        }
+    }
+}
+
+fun solve13a(lines: List<String>): Int {
+    val paper = TransparentPaper.from(lines)
+    return paper.points.map { paper.folds.first()(it) }.distinct().count()
+}
+
+fun solve13b(lines: List<String>) {
+    val paper = TransparentPaper.from(lines)
+    val pointsAfterFolds = paper.folds.fold(paper.points) { points, fold -> points.map { fold(it) } }
+    val maxX = pointsAfterFolds.maxOf { it.x }
+    val maxY = pointsAfterFolds.maxOf { it.y }
+    (0..maxY).forEach { y ->
+        (0..maxX).forEach { x ->
+            if (Point(x, y) in pointsAfterFolds) print("#") else print(".")
+        }
+        println()
+    }
+}
+
+fun main() {
+    val input = input("day13")
+    println(solve13a(input)) // 712
+    solve13b(input) // BLHFJPJF
+}<
\ No newline at end of file

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+
+fold along x=655
+fold along y=447
+fold along x=327
+fold along y=223
+fold along x=163
+fold along y=111
+fold along x=81
+fold along y=55
+fold along x=40
+fold along y=27
+fold along y=13
+fold along y=6

@@ 0,0 1,37 @@
+package advent2021
+
+import org.junit.jupiter.api.Test
+
+import org.junit.jupiter.api.Assertions.*
+
+internal class Day13KtTest {
+
+    val lines = listOf(
+        "6,10",
+        "0,14",
+        "9,10",
+        "0,3",
+        "10,4",
+        "4,11",
+        "6,0",
+        "6,12",
+        "4,1",
+        "0,13",
+        "10,12",
+        "3,4",
+        "3,0",
+        "8,4",
+        "1,10",
+        "2,14",
+        "8,10",
+        "9,0",
+        "",
+        "fold along y=7",
+        "fold along x=5"
+    )
+
+    @Test
+    fun example13a() {
+        assertEquals(17, solve13a(lines))
+    }
+}<
\ No newline at end of file