[day4, refactor] factor out logic to split input list by separator lines

src/main/scala/Day4.scala

 package aoc2020
+import aoc2020.lib._
 import aoc2020.lib.Passports._
 
 def day4(input: List[String]): String =
@@ -6,13 +7,6 @@ def day4(input: List[String]): String =
   s"complete: ${complete.length}\nvalid:    ${complete.count(_.isValid)}"
 
 def findPassports(input: List[String]): List[Passport] =
-  def go(lines: List[String], accu: List[Passport]): List[Passport] =
-    lines.span(_ != "") match {
-      case (passport, "" :: rest) => go(rest, Passport.fromStrings(passport) :: accu)
-      case (passport, Nil) => Passport.fromStrings(passport) :: accu
-      case _ => List() // malformed input
-    }
-  go(input, List())
-
-
-
+  input.split("").foldLeft(List[Passport]())(
+    (passes, lines) => Passport.fromStrings(lines) :: passes
+  )

src/main/scala/lib/extensions.scala

 package aoc2020.lib
 
+/* for splitting input with separator lines */
+extension [A](input: List[A])(using Eql[A,A])
+  def split(separator: A): LazyList[List[A]] =
+    input.span(_ != separator) match {
+        case (h,   _ :: t) => h #:: t.split(separator)
+        case (Nil, Nil)    => LazyList()
+        case (h,   Nil)    => LazyList(h)
+      }
+
+
+
 /* Using -Yexplicit-nulls isn't really ready for use with the java standard
  * library. e.g. String doesn't have `@NotNull` annotations
  */