2023 - Day 18 - part 1 and 2

src/main/kotlin/no/rodland/advent/Direction.kt

@@ -23,4 +23,17 @@ enum class Direction(val c: Char, val num: Long) {
         SOUTH -> WEST
         WEST -> NORTH
     }
+
+    operator fun times(num: Int): List<Direction> = (0..<num).map { _ -> this }
+    operator fun times(num: Long): List<Direction> = (0..<num).map { _ -> this }
+
+    companion object{
+        fun fromUDLR(c: Char): Direction = when (c) {
+            'U' -> NORTH
+            'D' -> SOUTH
+            'L' -> WEST
+            'R' -> EAST
+            else -> throw IllegalArgumentException("Unknown direction: $c")
+        }
+    }
 }

src/main/kotlin/no/rodland/advent_2023/Day18.kt

@@ -1,27 +1,68 @@
 package no.rodland.advent_2023
 
 import no.rodland.advent.Day
+import no.rodland.advent.Direction
+import no.rodland.advent.Pos
 
 // template generated: 18/12/2023
 // Fredrik Rødland 2023
 
-class Day18(val input: List<String>) : Day<Long, Long, List<String>> {
+// heavily inspired by https://www.youtube.com/watch?v=bGWK76_e-LM
+// Shoelace formula: https://en.wikipedia.org/wiki/Shoelace_formula
+// Pick's theorem: https://en.wikipedia.org/wiki/Pick%27s_theorem
+
+class Day18(val input: List<String>) : Day<Long, Long, List<Day18.Command>> {
 
     private val parsed = input.parse()
 
     override fun partOne(): Long {
-        return 2
+        val points = parsed.map { it -> it.direction to it.num }
+        return shoelacePick(points)
+//        return buildMapAndTraverseAndCount()
     }
 
     override fun partTwo(): Long {
-        return 2
+        val points = parsed.map { it.colour }.map { it.takeLast(1).toDir() to it.dropLast(1).toInt(16) }
+        return shoelacePick(points)
+    }
+
+    private fun shoelacePick(points: List<Pair<Direction, Int>>): Long {
+        var boundaryPoints = 0
+        val posList = points
+            .fold(listOf(Pos(0, 0))) { acc, (dir, num) ->
+                boundaryPoints += num
+                val pos = acc.last().next(dir, num)
+                acc + pos
+            }
+
+        // https://en.wikipedia.org/wiki/Shoelace_formula
+        // {\displaystyle A={\frac {1}{2}}\sum _{i=1}^{n}x_{i}(y_{i+1}-y_{i-1})}
+        val shoelace = posList.mapIndexed { i, p -> p.x.toLong() * (posList[(i + 1) % posList.size].y - posList[(posList.size + i) % posList.size].y) }.sum()
+
+        // https://en.wikipedia.org/wiki/Pick%27s_theorem
+        // {\displaystyle A=i+{\frac {b}{2}}-1.}
+        // => i = A - b/2 + 1
+        val interiorPoints = shoelace - boundaryPoints / 2 + 1
+        return interiorPoints + boundaryPoints
     }
 
-    override fun List<String>.parse(): List<String> {
+    private fun String.toDir(): Direction = when (this.first()) {
+        '0' -> Direction.EAST
+        '1' -> Direction.SOUTH
+        '2' -> Direction.WEST
+        '3' -> Direction.NORTH
+        else -> error("unknown dir: $this")
+    }
+
+    override fun List<String>.parse(): List<Command> {
         return map { line ->
-            line
+            val split = line.split(" ", "(", ")", "#").filterNot { it.isBlank() }
+            Command(Direction.fromUDLR(split[0].first()), split[1].toInt(), split[2])
         }
     }
 
+    data class Command(val direction: Direction, val num: Int, val colour: String)
+
     override val day = "18".toInt()
 }
+

src/test/kotlin/no/rodland/advent_2023/Day18Test.kt

@@ -15,10 +15,10 @@ internal class Day18Test {
     private val data18 = "2023/input_18.txt".readFile()
     private val test18 = "2023/input_18_test.txt".readFile()
 
-    private val resultTestOne = 2L
-    private val resultTestTwo = 2L
-    private val resultOne = 2L
-    private val resultTwo = 2L
+    private val resultTestOne = 62L
+    private val resultTestTwo = 952408144115L
+    private val resultOne = 40714L
+    private val resultTwo = 129849166997110L
 
     val test = defaultTestSuiteParseOnInit(
         Day18(data18),
@@ -29,6 +29,8 @@ internal class Day18Test {
         resultTwo,
         { Day18(data18) },
         { Day18(test18) },
+        numTestPart1 = 1000,
+        numTestPart2 = 1000,
     )
 
     @Nested

src/test/resources/2023/input_18_test.txt

@@ -0,0 +1,14 @@
+R 6 (#70c710)
+D 5 (#0dc571)
+L 2 (#5713f0)
+D 2 (#d2c081)
+R 2 (#59c680)
+D 2 (#411b91)
+L 5 (#8ceee2)
+U 2 (#caa173)
+L 1 (#1b58a2)
+U 2 (#caa171)
+R 2 (#7807d2)
+U 3 (#a77fa3)
+L 2 (#015232)
+U 2 (#7a21e3)