day(01): solve first day

Signed-off-by: Matej Focko <me@mfocko.xyz>

inputs/day01/sample.txt

@@ -0,0 +1,6 @@
+3   4
+4   3
+2   5
+1   3
+3   9
+3   3

src/Day01.kt

@@ -1,21 +1,44 @@
+fun parseInput(inputType: String): Pair<MutableList<Int>, MutableList<Int>> =
+    readInput(1, inputType).fold(mutableListOf<Int>() to mutableListOf<Int>()) { (left, right), line ->
+        val nums = line.split(Regex("\\s+"))
+
+        left.add(nums[0].toInt())
+        right.add(nums[1].toInt())
+
+        left to right
+    }
+
+fun precompute(input: Pair<MutableList<Int>, MutableList<Int>>) {
+    input.first.sort()
+    input.second.sort()
+}
+
+fun part1(input: Pair<List<Int>, List<Int>>): Int =
+    input.first.zip(input.second).sumOf { (l, r) -> maxOf(l - r, r - l) }
+
+fun part2(input: Pair<List<Int>, List<Int>>): Int {
+    val (left, right) = input
+    val freqs = buildMap<Int, Int> {
+        right.forEach { it ->
+            put(it, 1 + getOrDefault(it, 0))
+        }
+    }
+
+    return left.sumOf { it -> it * freqs.getOrDefault(it, 0) }
+}
+
 fun main() {
-    fun part1(input: List<String>): Int {
+    // Test of sample
-        return input.size
+    val testInput = parseInput("sample")
-    }
+    precompute(testInput)
 
-    fun part2(input: List<String>): Int {
+    check(part1(testInput) == 11)
-        return input.size
+    check(part2(testInput) == 31)
-    }
 
-    // Test if implementation meets criteria from the description, like:
+    // Challenge input
-    check(part1(listOf("test_input")) == 1)
+    val input = parseInput("input")
+    precompute(input)
 
-    // Or read a large test input from the `src/Day01_test.txt` file:
-    val testInput = readInput("Day01_test")
-    check(part1(testInput) == 1)
-
-    // Read the input from the `src/Day01.txt` file.
-    val input = readInput("Day01")
     part1(input).println()
     part2(input).println()
 }