// Copyright 2019 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

namespace array_shift {
  extern builtin ArrayShift(Context, JSFunction, Object, int32);

  macro TryFastArrayShift(implicit context: Context)(
      receiver: Object, arguments: Arguments): Object
      labels Slow {
    const array: FastJSArray = Cast<FastJSArray>(receiver) otherwise Slow;
    let witness = NewFastJSArrayWitness(array);

    witness.EnsureArrayPushable() otherwise Slow;

    if (array.length == 0) {
      return Undefined;
    }

    try {
      const newLength = array.length - 1;

      // Check that we're not supposed to right-trim the backing store, as
      // implemented in elements.cc:ElementsAccessorBase::SetLengthImpl.
      if ((newLength + newLength + kMinAddedElementsCapacity) <
          array.elements.length) {
        goto Runtime;
      }

      // Check that we're not supposed to left-trim the backing store, as
      // implemented in elements.cc:FastElementsAccessor::MoveElements.
      if (newLength > kMaxCopyElements) goto Runtime;

      const result = witness.LoadElementOrUndefined(0);
      witness.ChangeLength(newLength);
      witness.MoveElements(0, 1, Convert<intptr>(newLength));
      witness.StoreHole(newLength);
      return result;
    }
    label Runtime {
      tail ArrayShift(
          context, LoadTargetFromFrame(), Undefined,
          Convert<int32>(arguments.length));
    }
  }

  transitioning macro GenericArrayShift(implicit context:
                                            Context)(receiver: Object): Object {
    // 1. Let O be ? ToObject(this value).
    const object: JSReceiver = ToObject_Inline(context, receiver);

    // 2. Let len be ? ToLength(? Get(O, "length")).
    const length: Number = GetLengthProperty(object);

    // 3. If len is zero, then
    if (length == 0) {
      // a. Perform ? Set(O, "length", 0, true).
      SetProperty(object, kLengthString, Convert<Smi>(0));
      // b. Return undefined.
      return Undefined;
    }

    // 4. Let first be ? Get(O, "0").
    const first = GetProperty(object, Convert<Smi>(0));
    // 5. Let k be 1.
    let k: Number = 1;
    // 6. Repeat, while k < len
    while (k < length) {
      // a. Let from be ! ToString(k).
      const from: Number = k;

      // b. Let to be ! ToString(k - 1).
      const to: Number = k - 1;

      // c. Let fromPresent be ? HasProperty(O, from).
      const fromPresent: Boolean = HasProperty(object, from);

      // d. If fromPresent is true, then
      if (fromPresent == True) {
        // i. Let fromVal be ? Get(O, from).
        const fromValue: Object = GetProperty(object, from);

        // ii. Perform ? Set(O, to, fromValue, true).
        SetProperty(object, to, fromValue);
      } else {
        // i. Perform ? DeletePropertyOrThrow(O, to).
        DeleteProperty(object, to, kStrict);
      }

      // f. Increase k by 1.
      k++;
    }

    // 7. Perform ? DeletePropertyOrThrow(O, ! ToString(len - 1)).
    DeleteProperty(object, length - 1, kStrict);

    // 8. Perform ? Set(O, "length", len - 1, true).
    SetProperty(object, kLengthString, length - 1);

    // 9. Return first.
    return first;
  }

  // https://tc39.github.io/ecma262/#sec-array.prototype.shift
  transitioning javascript builtin ArrayPrototypeShift(
      js-implicit context: Context, receiver: Object)(...arguments): Object {
    try {
      return TryFastArrayShift(receiver, arguments) otherwise Slow;
    }
    label Slow {
      return GenericArrayShift(receiver);
    }
  }
}