LibJS: Add spec comments to ArrayBuffer.prototype.slice

Userland/Libraries/LibJS/Runtime/ArrayBufferPrototype.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2020, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2021-2022, Jamie Mansfield <jmansfield@cadixdev.org>
  * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -37,51 +38,89 @@ ArrayBufferPrototype::~ArrayBufferPrototype()
 // 25.1.5.3 ArrayBuffer.prototype.slice ( start, end ), https://tc39.es/ecma262/#sec-arraybuffer.prototype.slice
 JS_DEFINE_NATIVE_FUNCTION(ArrayBufferPrototype::slice)
 {
+    // 1. Let O be the this value.
+    // 2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
     auto* array_buffer_object = TRY(typed_this_value(global_object));
+
+    // 3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
     // FIXME: Check for shared buffer
+
+    // 4. If IsDetachedBuffer(O) is true, throw a TypeError exception.
     if (array_buffer_object->is_detached())
         return vm.throw_completion<TypeError>(global_object, ErrorType::DetachedArrayBuffer);
 
+    // 5. Let len be O.[[ArrayBufferByteLength]].
     auto length = array_buffer_object->byte_length();
 
+    // 6. Let relativeStart be ? ToIntegerOrInfinity(start).
     auto relative_start = TRY(vm.argument(0).to_integer_or_infinity(global_object));
 
     double first;
-    if (relative_start < 0)
+    // 7. If relativeStart is -∞, let first be 0.
+    if (Value(relative_start).is_negative_infinity())
+        first = 0;
+    // 8. Else if relativeStart < 0, let first be max(len + relativeStart, 0).
+    else if (relative_start < 0)
         first = max(length + relative_start, 0.0);
+    // 9. Else, let first be min(relativeStart, len).
     else
         first = min(relative_start, (double)length);
 
+    // 10. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
     auto relative_end = vm.argument(1).is_undefined() ? length : TRY(vm.argument(1).to_integer_or_infinity(global_object));
 
     double final;
-    if (relative_end < 0)
+    // 11. If relativeEnd is -∞, let final be 0.
+    if (Value(relative_end).is_negative_infinity())
+        final = 0;
+    // 12. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
+    else if (relative_end < 0)
         final = max(length + relative_end, 0.0);
+    // 13. Else, let final be min(relativeEnd, len).
     else
         final = min(relative_end, (double)length);
 
+    // 14. Let newLen be max(final - first, 0).
     auto new_length = max(final - first, 0.0);
 
+    // 15. Let ctor be ? SpeciesConstructor(O, %ArrayBuffer%).
     auto* constructor = TRY(species_constructor(global_object, *array_buffer_object, *global_object.array_buffer_constructor()));
+
+    // 16. Let new be ? Construct(ctor, « 𝔽(newLen) »).
     auto* new_array_buffer = TRY(construct(global_object, *constructor, Value(new_length)));
 
+    // 17. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
     if (!is<ArrayBuffer>(new_array_buffer))
         return vm.throw_completion<TypeError>(global_object, ErrorType::SpeciesConstructorDidNotCreate, "an ArrayBuffer");
     auto* new_array_buffer_object = static_cast<ArrayBuffer*>(new_array_buffer);
 
+    // 18. If IsSharedArrayBuffer(new) is true, throw a TypeError exception.
     // FIXME: Check for shared buffer
+
+    // 19. If IsDetachedBuffer(new) is true, throw a TypeError exception.
     if (new_array_buffer_object->is_detached())
         return vm.throw_completion<TypeError>(global_object, ErrorType::SpeciesConstructorReturned, "a detached ArrayBuffer");
+
+    // 20. If SameValue(new, O) is true, throw a TypeError exception.
     if (same_value(new_array_buffer_object, array_buffer_object))
         return vm.throw_completion<TypeError>(global_object, ErrorType::SpeciesConstructorReturned, "same ArrayBuffer instance");
+
+    // 21. If new.[[ArrayBufferByteLength]] < newLen, throw a TypeError exception.
     if (new_array_buffer_object->byte_length() < new_length)
         return vm.throw_completion<TypeError>(global_object, ErrorType::SpeciesConstructorReturned, "an ArrayBuffer smaller than requested");
 
+    // 22. NOTE: Side-effects of the above steps may have detached O.
+    // 23. If IsDetachedBuffer(O) is true, throw a TypeError exception.
     if (array_buffer_object->is_detached())
         return vm.throw_completion<TypeError>(global_object, ErrorType::DetachedArrayBuffer);
 
-    // This is ugly, is there a better way to do this?
+    // 24. Let fromBuf be O.[[ArrayBufferData]].
+    // 25. Let toBuf be new.[[ArrayBufferData]].
+    // 26. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
+    // FIXME: Implement this to specification
     array_buffer_object->buffer().span().slice(first, new_length).copy_to(new_array_buffer_object->buffer().span());
+
+    // 27. Return new.
     return new_array_buffer_object;
 }