Issue 507 - Implement lightweight array.slice() method

lib/typedObjects/arrayTypes.js

Index: lib/typedObjects/arrayTypes.js
diff --git a/lib/typedObjects/arrayTypes.js b/lib/typedObjects/arrayTypes.js
index 03e48ba0addada242f0f263ecc28b7c8f990e8fe..051d294a1c4db2d4c2193deb77fe0b3102dd15af 100644
--- a/lib/typedObjects/arrayTypes.js
+++ b/lib/typedObjects/arrayTypes.js
@@ -17,7 +17,7 @@
 
 "use strict";
 
-let {ilog2, nextPow2, alloc, dealloc, addBuffer, removeBuffer} = require("typedObjects/utils");
+let {ilog2, nextPow2, alloc, fixedPropertyDescriptor, dealloc, addBuffer, removeBuffer} = require("typedObjects/utils");
 
 function defaultArrayConstructor()
 {
@@ -26,8 +26,22 @@ function defaultArrayConstructor()
 
 function defaultArrayDestructor()
 {
-  this.length = 0;
-  this.size = 0;
+  if (this._copy)
+  {
+    // Carefully zero length + size if this is a slice of an array
+    Object.defineProperties(this, {
+      length: fixedPropertyDescriptor(0),
+      size: fixedPropertyDescriptor(0)
+    });
+  }
+  else
+  {
+    // Remove read-only length property for previously sliced arrays
+    delete this.length;
+
+    this.length = 0;
+    this.size = 0;
+  }
 };
 
 function createGetter(elementGetter, elementShift)
@@ -212,6 +226,52 @@ function pop()
   return result;
 }
 
+function createSlicer(elementShift)
+{
+  let {STATE_UNINITIALIZED} = require("typedObjects/objectTypes");
+
+  return function slice(start, end)
+  {
+    start = start | 0;
+    end = end | 0;
+
+    if (start < 0)
+      start = Math.max(0, this.length + start);
+    else
+      start = Math.min(start, this.length);
+
+    if (end > 0)
+      end = Math.min(0, end - this.length);
+    else
+      end = Math.max(end, -1 * this.length);
+
+    Object.defineProperties(this, {
+      length: {value: this.length, configurable: true}
+    });
+    this.retain();
+
+    let result = Object.create(this, {
+      arrayByteOffset: fixedPropertyDescriptor(
+        this.arrayByteOffset + start << elementShift
+      ),
+      length: {value: this.length - start + end, configurable: true},
+      _refCount: {value: 1, writable: true},
+      _copy: fixedPropertyDescriptor(true),
+      _state: {value: STATE_UNINITIALIZED, writable: true}
+    });
+
+    // TODO
+    //  - A sliced array will have a read-only length even if all slices are
+    //    released. We could fix this by keeping a count of slices, but is it
+    //    worth it?
+    //  - A slice of an array still keeps a reference to the array in it's
+    //    __proto__ property after it's released. (We can't just replace it
+    //    with null as modifying an object's prototype is apparently very slow.)
+
+    return result;
+  };
+}
+
 function splice(index, count)
 {
   index = index | 0;
@@ -325,6 +385,7 @@ function createArrayType(elementType, typeDescriptor, meta)
   typeDescriptor.shift = shift;
 
   let elementShift = ilog2(elementType.referenceLength | 0);
+  typeDescriptor.slice = createSlicer(elementShift);
   typeDescriptor.get = createGetter(elementGetter, elementShift);
   typeDescriptor.set = createSetter(elementSetter, elementShift);