Kotlin solutions for days 6 and 7

kotlin/day4.kts

@@ -89,6 +89,8 @@ for(i in 2 until lines.size) {
     }
 }
 
+println(boards.size)
+
 // Play
 var foundWinning = false
 for(number in drawnNumbers) {

kotlin/day6.kts

@@ -0,0 +1,36 @@
+import java.io.File
+
+fun simulateDay(population: Map<Int, Long>): Map<Int, Long> {
+    var nextPopulation = mutableMapOf<Int, Long>()
+
+    for(i in 8 downTo 0) {
+        if(i > 0) {
+            nextPopulation.put(i - 1, population.getOrDefault(i, 0))
+        }
+        else {
+            nextPopulation.put(6, nextPopulation.getOrDefault(6, 0) + population.getOrDefault(0, 0))
+            nextPopulation.put(8, nextPopulation.getOrDefault(8, 0) + population.getOrDefault(0, 0))
+        }
+    }
+
+    return nextPopulation
+}
+
+fun simulateUntil(population: Map<Int, Long>, days: Int): Map<Int, Long> {
+    var nextPopulation = population.toMap()
+
+    for(i in 1..days) {
+        nextPopulation = simulateDay(nextPopulation)
+    }
+
+    return nextPopulation
+}
+
+val fileName = if(args.size > 0) args[0] else "day6.txt"
+val lines = File(fileName).readLines()
+
+// Convert the comma-separated  list of lanternfish counters to a frequency map <Int, Long>
+var population = lines[0].split(",").map { it.trim().toInt() }.groupingBy { it }.eachCount().mapValues { it.value.toLong() }
+
+println(simulateUntil(population, 80).values.sum())
+println(simulateUntil(population, 256).values.sum())

kotlin/day7.kts

@@ -0,0 +1,34 @@
+import java.io.File
+
+val fileName = if(args.size > 0) args[0] else "day7.txt"
+val lines = File(fileName).readLines()
+
+// Convert the comma-separated list of crabs' horizontal positions to a List of Int
+var population = lines[0].split(",").map { it.trim().toInt() }
+
+fun computeFuelConsumption(population: List<Int>, secondPart: Boolean): Int {
+    var bestFuelConsumption = -1
+
+    // Try every horizontal position to see which one is the best
+    for(i in 0 until population.size) {
+        // Compute the fuel cost needed to move each crab to that position
+        var currentFuelConsumption = 0
+
+        for(j in 0 until population.size) {
+            val distance = Math.abs(population[j] - i) + 1
+            // The key to do the second part quickly was just to notice that we can use the formula
+            // n*(n-1)/2 to have the sum of the first n integers
+            val cost = if(!secondPart) distance else distance*(distance-1)/2
+
+            currentFuelConsumption += cost
+        }
+
+        if(bestFuelConsumption == -1 || currentFuelConsumption < bestFuelConsumption)
+            bestFuelConsumption = currentFuelConsumption
+    }
+
+    return bestFuelConsumption
+}
+
+println(computeFuelConsumption(population, false))
+println(computeFuelConsumption(population, true))