Added Sequence.take and Sequence.skip

These lazy iterator producing methods are useful when working with
arbitrary sequences and you need to skip or take some number of elements
at the start.

doc/site/modules/core/sequence.markdown

@@ -125,6 +125,23 @@ It is a runtime error to call this on an empty sequence.
 Similar to above, but uses `seed` for the initial value of the accumulator. If
 the sequence is empty, returns `seed`.
 
+### **skip**(count)
+
+Creates a new sequence that skips the first `count` elements of the original
+sequence.
+
+The returned sequence is *lazy*. The first `count` elements are only skipped
+once you start to iterate the returned sequence. Changes to the original
+sequence will be reflected in the filtered sequence.
+
+### **take**(count)
+
+Creates a new sequence that iterates only the first `count` elements of the
+original sequence.
+
+The returned sequence is *lazy*. Changes to the original sequence will be
+reflected in the filtered sequence.
+
 ### **toList**
 
 Creates a [list][] containing all the elements in the sequence.

src/vm/wren_core.wren

@@ -56,6 +56,10 @@ class Sequence {
 
   map(transformation) { MapSequence.new(this, transformation) }
 
+  skip(count) { SkipSequence.new(this, count) }
+
+  take(count) { TakeSequence.new(this, count) }
+
   where(predicate) { WhereSequence.new(this, predicate) }
 
   reduce(acc, f) {
@@ -112,6 +116,43 @@ class MapSequence is Sequence {
   iteratorValue(iterator) { _fn.call(_sequence.iteratorValue(iterator)) }
 }
 
+class SkipSequence is Sequence {
+  construct new(sequence, count) {
+    _sequence = sequence
+    _count = count
+  }
+
+  iterate(iterator) {
+    if (iterator) {
+      return _sequence.iterate(iterator)
+    } else {
+      iterator = _sequence.iterate(iterator)
+      var count = _count
+      while (count > 0 && iterator) {
+        iterator = _sequence.iterate(iterator)
+        count = count - 1
+      }
+      return iterator
+    }
+  }
+
+  iteratorValue(iterator) { _sequence.iteratorValue(iterator) }
+}
+
+class TakeSequence is Sequence {
+  construct new(sequence, count) {
+    _sequence = sequence
+    _count = count
+  }
+
+  iterate(iterator) {
+    if (!iterator) _taken = 1 else _taken = _taken + 1
+    return _taken > _count ? null : _sequence.iterate(iterator)
+  }
+
+  iteratorValue(iterator) { _sequence.iteratorValue(iterator) }
+}
+
 class WhereSequence is Sequence {
   construct new(sequence, fn) {
     _sequence = sequence

src/vm/wren_core.wren.inc

@@ -58,6 +58,10 @@ static const char* coreModuleSource =
 "\n"
 "  map(transformation) { MapSequence.new(this, transformation) }\n"
 "\n"
+"  skip(count) { SkipSequence.new(this, count) }\n"
+"\n"
+"  take(count) { TakeSequence.new(this, count) }\n"
+"\n"
 "  where(predicate) { WhereSequence.new(this, predicate) }\n"
 "\n"
 "  reduce(acc, f) {\n"
@@ -114,6 +118,43 @@ static const char* coreModuleSource =
 "  iteratorValue(iterator) { _fn.call(_sequence.iteratorValue(iterator)) }\n"
 "}\n"
 "\n"
+"class SkipSequence is Sequence {\n"
+"  construct new(sequence, count) {\n"
+"    _sequence = sequence\n"
+"    _count = count\n"
+"  }\n"
+"\n"
+"  iterate(iterator) {\n"
+"    if (iterator) {\n"
+"      return _sequence.iterate(iterator)\n"
+"    } else {\n"
+"      iterator = _sequence.iterate(iterator)\n"
+"      var count = _count\n"
+"      while (count > 0 && iterator) {\n"
+"        iterator = _sequence.iterate(iterator)\n"
+"        count = count - 1\n"
+"      }\n"
+"      return iterator\n"
+"    }\n"
+"  }\n"
+"\n"
+"  iteratorValue(iterator) { _sequence.iteratorValue(iterator) }\n"
+"}\n"
+"\n"
+"class TakeSequence is Sequence {\n"
+"  construct new(sequence, count) {\n"
+"    _sequence = sequence\n"
+"    _count = count\n"
+"  }\n"
+"\n"
+"  iterate(iterator) {\n"
+"    if (!iterator) _taken = 1 else _taken = _taken + 1\n"
+"    return _taken > _count ? null : _sequence.iterate(iterator)\n"
+"  }\n"
+"\n"
+"  iteratorValue(iterator) { _sequence.iteratorValue(iterator) }\n"
+"}\n"
+"\n"
 "class WhereSequence is Sequence {\n"
 "  construct new(sequence, fn) {\n"
 "    _sequence = sequence\n"

test/core/sequence/skip.wren

@@ -0,0 +1,28 @@
+class TestSequence is Sequence {
+  construct new() {}
+
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 3) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+var test = TestSequence.new().skip(0)
+
+System.print(test is Sequence) // expect: true
+System.print(test) // expect: instance of SkipSequence
+
+// Skipping 0 changes nothing
+System.print(test.toList) // expect: [1, 2, 3]
+
+// Skipping 1 works
+System.print(test.skip(1).toList) // expect: [2, 3]
+
+// Skipping more than length of sequence produces empty list
+System.print(test.skip(4).isEmpty) // expect: true
+
+// Skipping less than 0 changes nothing
+System.print(test.skip(-10).toList) // expect: [1, 2, 3]

test/core/sequence/take.wren

@@ -0,0 +1,28 @@
+class TestSequence is Sequence {
+  construct new() {}
+
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 3) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+var test = TestSequence.new().take(3)
+
+System.print(test is Sequence) // expect: true
+System.print(test) // expect: instance of TakeSequence
+
+// Taking 0 produces empty list
+System.print(test.take(0).isEmpty) // expect: true
+
+// Taking 1 works
+System.print(test.take(1).toList) // expect: [1]
+
+// Taking more than length of sequence produces whole sequence
+System.print(test.take(4).toList) // expect: [1, 2, 3]
+
+// Taking less than 0 produces empty list
+System.print(test.take(-10).isEmpty) // expect: true