Day2

build.sbt

@@ -8,5 +8,6 @@ lazy val core = (project in file("core")).settings(
     "org.typelevel" %% "cats-effect" % "3.0.0-M4",
     "co.fs2" %% "fs2-core" % "3.0.0-M6",
     "co.fs2" %% "fs2-io" % "3.0.0-M6",
+    "org.typelevel" %% "cats-parse" % "0.1-31-923a513",
   ),
 )

core/src/main/scala/tf/bug/aoc/y2020/Day1.scala

@@ -10,10 +10,10 @@ import tf.bug.aoc.Day
 
 object Day1 extends Day:
 
-  def part1[F[_]: Sync]: Pipe[F, String, String] =
+  override def part1[F[_]: Sync]: Pipe[F, String, String] =
     parts[F](2)
 
-  def part2[F[_]: Sync]: Pipe[F, String, String] =
+  override def part2[F[_]: Sync]: Pipe[F, String, String] =
     parts[F](3)
 
   def parts[F[_]: Sync](nums: Int): Pipe[F, String, String] =

core/src/main/scala/tf/bug/aoc/y2020/Day2.scala

@@ -0,0 +1,47 @@
+package tf.bug.aoc.y2020
+
+import cats.effect._
+import cats.parse.{Parser => P, Parser1, Numbers}
+import cats.syntax.all._
+import fs2._
+import tf.bug.aoc.Day
+
+object Day2 extends Day:
+
+  case class Policy(min: Int, max: Int, letter: Char)
+  case class Entry(policy: Policy, password: String)
+
+  val parsePolicy: Parser1[Policy] =
+    (Numbers.nonNegativeIntString, P.char('-'), Numbers.nonNegativeIntString, P.char(' '), P.anyChar).mapN {
+      case (min, (), max, (), letter) =>
+        Policy(min.toInt, max.toInt, letter)
+    }
+  
+  val parseEntry: Parser1[Entry] =
+    (parsePolicy, P.string1(": "), P.until1(P.end)).mapN {
+      case (policy, (), password) =>
+        Entry(policy, password)
+    }
+
+  override def part1[F[_]: Sync]: Pipe[F, String, String] =
+    parts[F] { ent =>
+      val charCount = ent.password.count(_ == ent.policy.letter)
+      ent.policy.min <= charCount && charCount <= ent.policy.max
+    }
+
+  override def part2[F[_]: Sync]: Pipe[F, String, String] =
+    parts[F] { ent =>
+      val a = ent.policy.min <= ent.password.length && ent.password.charAt(ent.policy.min - 1) == ent.policy.letter
+      val b = ent.policy.max <= ent.password.length && ent.password.charAt(ent.policy.max - 1) == ent.policy.letter
+      (a || b) && !(a && b)
+    }
+
+  def parts[F[_]: Sync](validation: Entry => Boolean): Pipe[F, String, String] =
+    (lines: Stream[F, String]) => {
+      lines.map(parseEntry.parseAll)
+        .collect {
+          case Right(entry) => entry
+        }
+        .foldMap(ent => if(validation(ent)) 1 else 0)
+        .map(_.show)
+    }

core/src/main/scala/tf/bug/aoc/y2020/y2020.scala

@@ -5,4 +5,5 @@ import tf.bug.aoc.{Day, Year}
 object y2020 extends Year:
   override def days: Map[Int, Day] = Map(
     1 -> Day1,
+    2 -> Day2,
   )