Day 2

Author: WVerlaek

src/main/kotlin/y24/Day2.kt

@@ -0,0 +1,56 @@
+package y24
+
+import common.puzzle.solvePuzzle
+import common.puzzle.Input
+import common.puzzle.Puzzle
+import common.puzzle.splitToInts
+import kotlin.math.*
+
+fun main() = solvePuzzle(year = 2024, day = 2) { Day2(it) }
+
+class Day2(val input: Input) : Puzzle {
+
+    override fun solveLevel1(): Any {
+        return input.lines.count {
+            isSafe(it.splitToInts(" "), 0, 1)
+        }
+    }
+
+    private fun isSafe(level: List<Int>, i0: Int, i1: Int, skipIndex: Int = level.size): Boolean {
+        if (i1 >= level.size || (i1 >= skipIndex && i1 + 1 >= level.size)) {
+            return true
+        }
+
+        val first = if (skipIndex == 0) 1 else 0
+        val second = if (skipIndex <= 1) 2 else 1
+        val sign = (level[second] - level[first]).sign
+
+        val v0 = if (i0 >= skipIndex) level[i0 + 1] else level[i0]
+        val v1 = if (i1 >= skipIndex) level[i1 + 1] else level[i1]
+
+        if (areValuesSafe(sign, v0, v1)) {
+            return isSafe(level, i0 + 1, i1 + 1, skipIndex)
+        }
+
+        return false
+    }
+
+    private fun areValuesSafe(sign: Int, val0: Int, val1: Int): Boolean {
+        val diff = abs(val1 - val0)
+        if (diff == 0 || diff > 3) {
+            return false
+        }
+        return (val1 - val0).sign == sign
+    }
+
+    override fun solveLevel2(): Any {
+        return input.lines.count {
+            val report = it.splitToInts(" ")
+
+            // Try skipping each index, or not skipping at all (index = report.size).
+            (0..report.size).any { skipIndex ->
+                isSafe(report, 0, 1, skipIndex)
+            }
+        }
+    }
+}

src/test/kotlin/y24/Day2Test.kt

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+package y24
+
+import common.puzzle.Input
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+internal class Day2Test {
+    private val sample = Input("""
+    7 6 4 2 1
+    1 2 7 8 9
+    9 7 6 2 1
+    1 3 2 4 5
+    8 6 4 4 1
+    1 3 6 7 9
+    """.trimIndent())
+
+    private val day = Day2(sample)
+
+    @Test
+    fun solveLevel1() {
+        assertEquals(2, day.solveLevel1())
+    }
+
+    @Test
+    fun solveLevel2() {
+        assertEquals(4, day.solveLevel2())
+
+        assertEquals(1, Day2(Input("29 32 30 31 34 35 37")).solveLevel2())
+        assertEquals(1, Day2(Input("27 25 26 27 30 31")).solveLevel2())
+    }
+}