Day 06

input/day06

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+Time:        61     67     75     71
+Distance:   430   1036   1307   1150

src/day06.scala

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+package aoc
+package day06
+
+val dayNumber = "06"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String) =
+  val Array(times, distances) = input
+    .split("\n")
+    .map(_.dropWhile(_ != ' ').trim.split("""\W+""").map(_.toInt))
+
+  times
+    .zip(distances)
+    .map((time, distanceToBeat) =>
+      for
+        pushDuration <- (0 to time)
+        distance = (time - pushDuration) * pushDuration
+        if distance > distanceToBeat
+      yield pushDuration
+    )
+    .map(_.length)
+    .product
+    .toString
+
+def part2(input: String) =
+
+  val Array(time, distance) = input
+    .split("\n")
+    .map(_.replaceAll(" ", "").dropWhile(!_.isDigit).toDouble)
+
+  // We are looking for x such that
+  // d < (t - x) * t
+  // Which is
+  // x² - t * x + d > 0
+  // This is a parabola and we need to find its roots
+  val disc = Math.sqrt(time * time - 4 * distance)
+  val root1 = time / 2 - disc / 2
+  val root2 = time / 2 + disc / 2
+
+  // Count integers between roots
+  val int1 = root1.ceil.toLong
+  val res1 = if int1 == root1 then int1 + 1 else int1
+  val int2 = root2.floor.toLong
+  val res2 = if int2 == root2 then int2 - 1 else int2
+  (res2 - res1 + 1L).toString