Tail recursive implementation of mapConserve, s...

Tail recursive implementation of mapConserve, submitted by Eric
Willigers. Closes #2411, review by malayeri.

1 files changed, 53 insertions, 0 deletions

diff --git a/test/files/run/mapConserve.scala b/test/files/run/mapConserve.scala
new file mode 100644
index 0000000000..a285113b11
--- /dev/null
+++ b/test/files/run/mapConserve.scala
@@ -0,0 +1,53 @@
+import scala.annotation.tailrec
+import scala.collection.mutable.ListBuffer
+
+object Test {
+  val maxListLength = 7 // up to 16, but larger is slower
+  var testCount = 0
+
+  def checkStackOverflow() = {
+    var xs: List[String] = Nil
+    for (i <- 0 until 250000)
+        xs = "X" :: xs
+
+    val lowers = xs.mapConserve(_.toLowerCase)
+    assert(xs.mapConserve(x => x) eq xs)
+  }
+
+  def checkBehaviourUnchanged(input: List[_], oldOutput: List[_], newOutput: List[_]) {
+    if (oldOutput eq input)
+      assert(newOutput eq oldOutput)
+    else {
+      assert(newOutput.head == oldOutput.head)
+      checkBehaviourUnchanged(input.tail, oldOutput.tail, newOutput.tail)
+    }
+    testCount += 1
+  }
+
+    var callCount = 0
+    val lastHexDigit: Function1[BigInt, AnyRef] = { x: BigInt => callCount+=1; if (x < 16) x else x % 16 }
+
+    def main(args: Array[String]) {
+        for (length <- 0 to maxListLength;
+             bitmap <- 0 until (1 << length);
+             data = List.range(0, length) map { x: Int =>
+               if ((bitmap & (1 << x)) != 0) BigInt(x+16)
+               else BigInt(x)
+             })
+        {
+            // Behaves like map with respect to  ==
+            callCount = 0
+            val numUnconserved = data.reverse.dropWhile(_ < 16).length
+            val result = data mapConserve lastHexDigit
+            val mapResult = data map lastHexDigit
+            assert(result == mapResult)
+            assert((result drop numUnconserved) eq (data drop numUnconserved))
+            assert(callCount == 2 * length) // map, mapConserve call transform for each element in the list
+
+            // Behaves like existing mapConserve with respect to  eq
+            checkBehaviourUnchanged(data, data mapConserve lastHexDigit, data mapConserve lastHexDigit)
+        }
+
+        checkStackOverflow();
+    }
+}
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