From 01187e58f2012632d783af29c7b862509320a5d6 Mon Sep 17 00:00:00 2001
From: Jamie Mansfield <jmansfield@cadixdev.org>
Date: Fri, 2 Apr 2021 21:31:23 +0100
Subject: [PATCH] LibJS: ArrayBuffer.prototype.slice

Implements the aforementioned native Javascript function, following the
specification's [1] implementation.

[1] https://tc39.es/ecma262/#sec-arraybuffer.prototype.slice
---
 .../Libraries/LibJS/Runtime/ArrayBuffer.cpp   | 11 +++++
 .../Libraries/LibJS/Runtime/ArrayBuffer.h     |  2 +
 .../LibJS/Runtime/ArrayBufferPrototype.cpp    | 41 ++++++++++++++++++-
 .../ArrayBuffer.prototype.slice.js            | 29 +++++++++++++
 4 files changed, 82 insertions(+), 1 deletion(-)
 create mode 100644 Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.slice.js

diff --git a/Userland/Libraries/LibJS/Runtime/ArrayBuffer.cpp b/Userland/Libraries/LibJS/Runtime/ArrayBuffer.cpp
index 007e61c0fa..1c506f0b79 100644
--- a/Userland/Libraries/LibJS/Runtime/ArrayBuffer.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ArrayBuffer.cpp
@@ -34,12 +34,23 @@ ArrayBuffer* ArrayBuffer::create(GlobalObject& global_object, size_t byte_size)
     return global_object.heap().allocate<ArrayBuffer>(global_object, byte_size, *global_object.array_buffer_prototype());
 }
 
+ArrayBuffer* ArrayBuffer::create(GlobalObject& global_object, ByteBuffer& buffer)
+{
+    return global_object.heap().allocate<ArrayBuffer>(global_object, buffer, *global_object.array_buffer_prototype());
+}
+
 ArrayBuffer::ArrayBuffer(size_t byte_size, Object& prototype)
     : Object(prototype)
     , m_buffer(ByteBuffer::create_zeroed(byte_size))
 {
 }
 
+ArrayBuffer::ArrayBuffer(ByteBuffer& buffer, Object& prototype)
+    : Object(prototype)
+    , m_buffer(buffer)
+{
+}
+
 ArrayBuffer::~ArrayBuffer()
 {
 }
diff --git a/Userland/Libraries/LibJS/Runtime/ArrayBuffer.h b/Userland/Libraries/LibJS/Runtime/ArrayBuffer.h
index d2724cd96e..adf52a5249 100644
--- a/Userland/Libraries/LibJS/Runtime/ArrayBuffer.h
+++ b/Userland/Libraries/LibJS/Runtime/ArrayBuffer.h
@@ -36,8 +36,10 @@ class ArrayBuffer : public Object {
 
 public:
     static ArrayBuffer* create(GlobalObject&, size_t);
+    static ArrayBuffer* create(GlobalObject&, ByteBuffer&);
 
     ArrayBuffer(size_t, Object& prototype);
+    ArrayBuffer(ByteBuffer& buffer, Object& prototype);
     virtual ~ArrayBuffer() override;
 
     size_t byte_length() const { return m_buffer.size(); }
diff --git a/Userland/Libraries/LibJS/Runtime/ArrayBufferPrototype.cpp b/Userland/Libraries/LibJS/Runtime/ArrayBufferPrototype.cpp
index 846acbd562..5bcd583379 100644
--- a/Userland/Libraries/LibJS/Runtime/ArrayBufferPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ArrayBufferPrototype.cpp
@@ -66,12 +66,51 @@ static ArrayBuffer* array_buffer_object_from(VM& vm, GlobalObject& global_object
     return static_cast<ArrayBuffer*>(&this_object);
 }
 
+// 25.1.5.3 ArrayBuffer.prototype.slice, https://tc39.es/ecma262/#sec-arraybuffer.prototype.slice
 JS_DEFINE_NATIVE_FUNCTION(ArrayBufferPrototype::slice)
 {
+    const auto start = vm.argument(0);
+    const auto end = vm.argument(1);
+
     auto array_buffer_object = array_buffer_object_from(vm, global_object);
     if (!array_buffer_object)
         return {};
-    TODO();
+
+    // FIXME: Check for shared buffer
+    // FIXME: Check for detached buffer
+
+    const auto len = array_buffer_object->byte_length();
+
+    const auto relative_start = start.is_negative_infinity()
+        ? 0
+        : start.to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    const auto first = relative_start < 0
+        ? max(len + relative_start, 0.0)
+        : min(relative_start, static_cast<double>(len));
+
+    const auto relative_end = end.is_undefined()
+        ? len
+        : end.to_integer_or_infinity(global_object);
+    if (vm.exception())
+        return {};
+
+    double final_;
+    if (end.is_negative_infinity())
+        final_ = 0;
+    else if (relative_end < 0)
+        final_ = max(len + relative_end, 0.0);
+    else
+        final_ = min(relative_end, static_cast<double>(len));
+
+    const auto new_len = max(final_ - first, 0.0);
+
+    // FIXME: this is a bit more involved in the specification
+    auto sliced = array_buffer_object->buffer().slice(first, new_len);
+    auto buffer = ArrayBuffer::create(global_object, sliced);
+    return buffer;
 }
 
 JS_DEFINE_NATIVE_GETTER(ArrayBufferPrototype::byte_length_getter)
diff --git a/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.slice.js b/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.slice.js
new file mode 100644
index 0000000000..a8dd527d9b
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/ArrayBuffer/ArrayBuffer.prototype.slice.js
@@ -0,0 +1,29 @@
+test("single parameter", () => {
+    const buffer = new ArrayBuffer(16);
+    const fullView = new Int32Array(buffer);
+
+    // modify some value that we can check in the sliced buffer
+    fullView[3] = 7;
+
+    // slice the buffer and use a new int32 view to perform basic checks
+    const slicedBuffer = buffer.slice(12);
+    const slicedView = new Int32Array(slicedBuffer);
+
+    expect(slicedView).toHaveLength(1);
+    expect(slicedView[0]).toBe(7);
+});
+
+test("both parameters", () => {
+    const buffer = new ArrayBuffer(16);
+    const fullView = new Int32Array(buffer);
+
+    // modify some value that we can check in the sliced buffer
+    fullView[1] = 12;
+
+    // slice the buffer and use a new int32 view to perform basic checks
+    const slicedBuffer = buffer.slice(4, 8);
+    const slicedView = new Int32Array(slicedBuffer);
+
+    expect(slicedView).toHaveLength(1);
+    expect(slicedView[0]).toBe(12);
+});