Add WIP solution to day 12

Co-authored-by: Patrick Maroney <maroney.tm@gmail.com>

src/Day12.kt

@@ -0,0 +1,62 @@
+val potentialSprings = listOf('#', '?')
+
+fun main() {
+
+    fun part1(input: List<String>): Int {
+        return input.sumOf { line ->
+            val (springs, constraints) = line.split(' ')
+            var permutations = 1
+            val firstSolution = constraints.solveForConstraints()
+
+            println()
+            permutations
+        }
+    }
+
+    fun part2(input: List<String>): Int {
+        return input.size
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = readInput("Day12_test")
+    check(part1(testInput) == 21)
+
+    val input = readInput("Day12")
+    part1(input).println()
+    check(part2(testInput) == 0)
+    part2(input).println()
+}
+
+fun String.solveForConstraints(): String {
+    var modifiedLine = this
+    var subStart = 0
+    split(',')
+        .map { it.toInt() }
+        .forEach { constraint ->
+            var subSize = 0
+            for (i in subStart..modifiedLine.lastIndex) {
+                if (potentialSprings.contains(modifiedLine[i])) {
+                    if (subSize == 0) {
+                        subStart = i
+                    }
+                    subSize++
+                    if (subSize >= constraint) {
+                        if (modifiedLine.getOrNull(subStart + subSize) == '#') {
+                            // If character after replacement group is invalid, we can't place it here
+                            subStart++
+                            subSize--
+                        } else {
+                            modifiedLine = modifiedLine.replaceRange(subStart, subStart + subSize, "#".repeat(subSize))
+                            modifiedLine.println()
+                            subStart += subSize + 1
+                            break
+                        }
+                    }
+                } else {
+                    subStart = i
+                    subSize = 0
+                }
+            }
+        }
+    return modifiedLine
+}