From 1d67f6a52ccc409a743c1000a8d8679d70886223 Mon Sep 17 00:00:00 2001
From: Bas Dado <bas@orbitgames.nl>
Date: Wed, 4 Dec 2019 19:42:01 +0100
Subject: [PATCH] Day 4 in a pretty streamy way

---
 .../com/basdado/adventofcode/day4/Day4.kt     | 39 +++++++++++++++++++
 src/main/resources/day/4/input.txt            |  1 +
 2 files changed, 40 insertions(+)
 create mode 100644 src/main/kotlin/com/basdado/adventofcode/day4/Day4.kt
 create mode 100644 src/main/resources/day/4/input.txt

diff --git a/src/main/kotlin/com/basdado/adventofcode/day4/Day4.kt b/src/main/kotlin/com/basdado/adventofcode/day4/Day4.kt
new file mode 100644
index 0000000..0d8a02a
--- /dev/null
+++ b/src/main/kotlin/com/basdado/adventofcode/day4/Day4.kt
@@ -0,0 +1,39 @@
+package com.basdado.adventofcode.day4
+
+import com.basdado.adventofcode.line
+import java.util.stream.IntStream
+
+const val DAY4_PATH = "/day/4/input.txt"
+
+fun main() {
+    Day4.puzzle1()
+    Day4.puzzle2()
+}
+
+object Day4 {
+
+    fun puzzle1() {
+        val range = line(DAY4_PATH).split('-').map { it.toInt() }
+        println(IntStream.range(range[0], range[1])
+            .parallel()
+            .filter(::neverDecrease)
+            .filter(::repeatingDigits)
+            .count())
+    }
+
+    fun puzzle2() {
+        val range = line(DAY4_PATH).split('-').map { it.toInt() }
+        println(IntStream.range(range[0], range[1])
+            .parallel()
+            .filter(::neverDecrease)
+            .filter{ n -> IntStream.rangeClosed(1, 9).anyMatch{ repeatsOnlyTwice(n, it)} }
+            .count())
+    }
+
+    private fun neverDecrease(n: Int) = streamDigits(n).reduce(0) { a, b -> if (a < 0) a else if (b < a) -1 else b} >= 0
+    private fun repeatingDigits(n: Int) = streamDigits(n).reduce(-1) { a, b -> if (a == 1000) a else if (a == b) 1000 else b} == 1000
+    private fun repeatsOnlyTwice(n: Int, needle: Int) = streamDigits(n).filter { it == needle }.count() == 2L
+
+    private fun streamDigits(n: Int): IntStream = IntStream.range(0, n.toString().length).map { n.toString()[it].toString().toInt() }
+
+}
\ No newline at end of file
diff --git a/src/main/resources/day/4/input.txt b/src/main/resources/day/4/input.txt
new file mode 100644
index 0000000..0d20957
--- /dev/null
+++ b/src/main/resources/day/4/input.txt
@@ -0,0 +1 @@
+128392-643281
\ No newline at end of file