Port 05AB1E's transpose

shared/src/main/scala/ListHelpers.scala

@@ -40,7 +40,6 @@ object ListHelpers:
   end filter
 
   /** A wrapper call to the generator method in interpreter */
-
   def generate(function: VFun, initial: VList)(using ctx: Context): VList =
     val firstN = initial.length match
       case 0 => ctx.settings.defaultValue
@@ -245,40 +244,43 @@ object ListHelpers:
     parts.toSeq
   end split
 
+  /** Transpose a matrix.
+    *
+    * Hangs on infinite lists of finite lists. See [[transposeSafe]] for a version that handles those.
+    * Based on [[https://github.com/Adriandmen/05AB1E/blob/master/lib/commands/matrix_commands.ex 05AB1E's implementation]].
+    */
   def transpose(iterable: VList, filler: Option[VAny] = None)(using
       ctx: Context
   ): VList =
-    /*Transposes a matrix
-    In order to handle infinite lists, it generates the transpose
-    antidiagonal by antidiagonal.
-     */
-
     val matrix = iterable.map(makeIterable(_))
 
-    def genRow(r: BigInt) =
-      val temp: LazyList[Option[VAny]] = LazyList
-        .unfold(0) { c =>
-          if !matrix.isDefinedAt(c) then None
-          else if matrix(c).isDefinedAt(r) then
-            Some((Some(matrix(c).index(r)), c + 1))
-          else if filler.isDefined then
-            Some((Some(filler.getOrElse(VNum(0))), c + 1))
-          else Some((None, c + 1))
+    val out = filler match
+      case None => LazyList.unfold(matrix) { matrix =>
+          val remaining = matrix.filter(_.nonEmpty)
+          Option.when(remaining.nonEmpty) {
+            (remaining.map(_.head), remaining.map(_.tail))
+          }
+        }
+      case Some(filler) => LazyList.unfold(matrix) { matrix =>
+          Option.when(matrix.exists(_.nonEmpty)) {
+            (matrix.flatMap(_.headOption.getOrElse(filler)), matrix.map(_.tail))
+          }
         }
-      VList.from(temp.flatten)
-
-    val out: LazyList[VAny] = LazyList
-      .unfold(0) { r =>
-        if !matrix.exists(_.isDefinedAt(r)) then None
-        else
-          val row = genRow(r)
-          if row.forall(_.isInstanceOf[String]) then Some((row.mkString, r + 1))
-          else Some((row, r + 1))
-      }
     VList.from(out)
-
   end transpose
 
+  /** Transpose a matrix. Uses the length of the first row of the inputted matrix
+    * as the number of columns in the resulting matrix.
+    *
+    * @see transpose
+    */
+  def transposeSafe(iterable: VList, filler: Option[VAny] = None)(using
+      ctx: Context
+  ): VList =
+    val matrix = iterable.map(makeIterable(_))
+    ???
+  end transposeSafe
+
   def vectorisedMaximum(iterable: VList, b: VVal): VList =
     VList.from(iterable.map { a =>
       (a: @unchecked) match