[Pisces] / iterable.js

// ♓🌟 Piscēs ∷ iterable.js
// ====================================================================
//
// Copyright © 2023 Lady [@ Lady’s Computer].
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at <https://mozilla.org/MPL/2.0/>.

import { bind, call, identity } from "./function.js";
import {
  defineOwnProperty,
  getPrototype,
  objectCreate,
} from "./object.js";
import { ITERATOR, TO_STRING_TAG } from "./value.js";

export const {
  /**
   * Returns an iterator function which iterates over the values in its
   * provided arraylike object and yields them.
   *
   * If a first argument is provided, it must be a generator function.
   * The yielded values are instead those yielded by calling that
   * function with each value in turn.
   *
   * If a second argument is provided, it is used as the string tag for
   * the resulting iterator.
   *
   * The resulting function also takes a second argument, which will be
   * used as the `this` value when calling the provided generator
   * function, if provided.
   *
   * ※ The returned function is an ordinary nonconstructible (arrow)
   * function which, when called with an array, returns an iterator.
   *
   * ※ The prototypes of iterators returned by a single iterator
   * function will all be the same.
   */
  arrayIteratorFunction,

  /**
   * Returns an iterator function which iterates over the values
   * yielded by its provided generator function and yields them.
   *
   * If a first argument is provided, it must be a generator function.
   * The yielded values are instead those yielded by calling that
   * function with each value in turn.
   *
   * If a second argument is provided, it is used as the string tag for
   * the resulting iterator.
   *
   * The resulting function also takes a second argument, which will be
   * used as the `this` value when calling the provided generator
   * function, if provided.
   *
   * ※ The returned function is an ordinary nonconstructible (arrow)
   * function which, when called with a generator function, returns an
   * iterator.
   *
   * ☡ There is no way to detect whether the function provided to the
   * returned function is, in fact, a generator function. Consequently,
   * if a non‐generator function is provided, it will not throw until
   * the first attempt at reading a value.
   *
   * ※ The prototypes of iterators returned by a single iterator
   * function will all be the same.
   */
  generatorIteratorFunction,

  /**
   * Returns an iterator function which iterates over the values in its
   * provided map and yields them.
   *
   * If a first argument is provided, it must be a generator function.
   * The yielded values are instead those yielded by calling that
   * function with each value in turn.
   *
   * If a second argument is provided, it is used as the string tag for
   * the resulting iterator.
   *
   * The resulting function also takes a second argument, which will be
   * used as the `this` value when calling the provided generator
   * function, if provided.
   *
   * ※ The returned function is an ordinary nonconstructible (arrow)
   * function which, when called with a map, returns an iterator.
   *
   * ※ The prototypes of iterators returned by a single iterator
   * function will all be the same.
   */
  mapIteratorFunction,

  /**
   * Returns an iterator function which iterates over the values in its
   * provided set and yields them.
   *
   * If a first argument is provided, it must be a generator function.
   * The yielded values are instead those yielded by calling that
   * function with each value in turn.
   *
   * If a second argument is provided, it is used as the string tag for
   * the resulting iterator.
   *
   * The resulting function also takes a second argument, which will be
   * used as the `this` value when calling the provided generator
   * function, if provided.
   *
   * ※ The returned function is an ordinary nonconstructible (arrow)
   * function which, when called with a set, returns an iterator.
   *
   * ※ The prototypes of iterators returned by a single iterator
   * function will all be the same.
   */
  setIteratorFunction,

  /**
   * Returns an iterator function which iterates over the characters in
   * its provided string and yields them.
   *
   * If a first argument is provided, it must be a generator function.
   * The yielded values are instead those yielded by calling that
   * function with each value in turn.
   *
   * If a second argument is provided, it is used as the string tag for
   * the resulting iterator.
   *
   * The resulting function also takes a second argument, which will be
   * used as the `this` value when calling the provided generator
   * function, if provided.
   *
   * ※ This iterator function iterates over characters; use
   * `arrayIteratorFunction` to iterate over code units.
   *
   * ※ The returned function is an ordinary nonconstructible (arrow)
   * function which, when called with a string, returns an iterator.
   *
   * ※ The prototypes of iterators returned by a single iterator
   * function will all be the same.
   */
  stringIteratorFunction,
} = (() => {
  const { [ITERATOR]: arrayIterator } = Array.prototype;
  const arrayIteratorPrototype = getPrototype(
    [][ITERATOR](),
  );
  const { next: arrayIteratorNext } = arrayIteratorPrototype;
  const { [ITERATOR]: stringIterator } = String.prototype;
  const stringIteratorPrototype = getPrototype(
    ""[ITERATOR](),
  );
  const { next: stringIteratorNext } = stringIteratorPrototype;
  const { [ITERATOR]: mapIterator } = Map.prototype;
  const mapIteratorPrototype = getPrototype(
    new Map()[ITERATOR](),
  );
  const { next: mapIteratorNext } = mapIteratorPrototype;
  const { [ITERATOR]: setIterator } = Set.prototype;
  const setIteratorPrototype = getPrototype(
    new Set()[ITERATOR](),
  );
  const { next: setIteratorNext } = setIteratorPrototype;
  const generatorIteratorPrototype = getPrototype(
    function* () {}.prototype,
  );
  const { next: generatorIteratorNext } = generatorIteratorPrototype;
  const iteratorPrototype = getPrototype(
    generatorIteratorPrototype,
  );

  /**
   * An iterator generated by an iterator function.
   *
   * This class provides the internal data structure of all the
   * iterator functions as well as the `::next` behaviour they all use.
   *
   * ※ This class extends the identity function to allow for arbitrary
   * construction of its superclass instance based on the provided
   * prototype.
   *
   * ※ This class is not exposed.
   */
  const Iterator = class extends identity {
    #baseIterator;
    #baseIteratorNext;
    #generateNext;
    #nextIterator = null;

    /**
     * Constructs a new iterator with the provided prototype which
     * wraps the provided base iterator (advance·able using the
     * provided next method) and yields the result of calling the
     * provided generator function with each value (or just yields each
     * value if no generator function is provided).
     *
     * ☡ It is not possible to type·check the provided next method or
     * generator function to ensure that they actually are correct and
     * appropriately callable; if they aren’t, an error will be thrown
     * when attempting to yield the first value.
     */
    constructor(
      prototype,
      baseIterator,
      baseIteratorNext,
      generateNext = null,
    ) {
      super(objectCreate(prototype));
      this.#baseIterator = baseIterator;
      this.#baseIteratorNext = baseIteratorNext;
      this.#generateNext = generateNext;
    }

    /**
     * Returns an object conforming to the requirements of the iterator
     * protocol, yielding successive iterator values.
     */
    next() {
      const baseIteratorNext = this.#baseIteratorNext;
      const generateNext = this.#generateNext;
      while (true) {
        // This function sometimes needs to repeat its processing steps
        // in the case that a generator function was provided.
        //
        // To avoid potentially large amounts of recursive calls, it is
        // defined in a loop which will exit the first time a suitable
        // response is acquired.
        const baseIterator = this.#baseIterator;
        const nextIterator = this.#nextIterator;
        if (baseIterator === null) {
          // The base iterator has already been exhausted.
          return { value: undefined, done: true };
        } else if (nextIterator === null) {
          // This iterator is not currently yielding values from the
          // provided generator function, either because it doesn’t
          // exist or because its values have been exhausted.
          //
          // Get the next value in the base iterator and either provide
          // it to the generator function or yield it, as appropriate.
          const {
            value,
            done,
          } = call(baseIteratorNext, baseIterator, []);
          if (done) {
            // The base iterator is exhausted.
            //
            // Free it up from memory and rerun these steps to signal
            // that the iteration has completed.
            this.#baseIterator = null;
            this.#generateNext = null;
            continue;
          } else if (generateNext !== null) {
            // The base iterator has yielded another value and there is
            // a generator function.
            //
            // Initialize a new iterator of result values by calling
            // the function with the current value, and then rerun
            // these steps to actually yield a value from it.
            this.#nextIterator = generateNext(value);
            continue;
          } else {
            // The base iterator has yielded another value and there is
            // no generator function.
            //
            // Just yield the value.
            return { value, done: false };
          }
        } else {
          // This iterator is currently yielding values from the
          // provided generator function.
          const {
            value,
            done,
          } = call(generatorIteratorNext, nextIterator, []);
          if (done) {
            // The current iterator of values from the provided
            // generator function is exhausted.
            //
            // Remove it, and rerun these steps to advance to the next
            // value in the base iterator.
            this.#nextIterator = null;
            continue;
          } else {
            // The current iterator of values has yielded another
            // value; reyield it.
            return { value, done: false };
          }
        }
      }
    }
  };

  const {
    next: iteratorNext,
  } = Iterator.prototype;
  const makePrototype = (stringTag) =>
    objectCreate(
      iteratorPrototype,
      {
        next: {
          configurable: true,
          enumerable: false,
          value: function next() {
            return call(iteratorNext, this, []);
          },
          writable: true,
        },
        [TO_STRING_TAG]: {
          configurable: true,
          enumerable: false,
          value: stringTag,
          writable: false,
        },
      },
    );

  /**
   * Returns a new function capable of generating iterators using the
   * provided iterator generation method, iterator advancement method,
   * optional generator function, and optional string tag.
   *
   * ※ The first two arguments to this function are bound to generate
   * the actual exported iterator function makers.
   *
   * ※ This function is not exposed.
   */
  const iteratorFunction = (
    makeBaseIterator,
    baseIteratorNext,
    generateNext = null,
    stringTag = "Iterator",
  ) => {
    const prototype = makePrototype(stringTag); // intentionally cached
    return ($, thisArg = undefined) =>
      new Iterator(
        prototype,
        call(makeBaseIterator, $, []),
        baseIteratorNext,
        generateNext === null ? null : bind(generateNext, thisArg, []),
      );
  };

  return {
    arrayIteratorFunction: defineOwnProperty(
      bind(
        iteratorFunction,
        undefined,
        [arrayIterator, arrayIteratorNext],
      ),
      "name",
      { value: "arrayIteratorFunction" },
    ),
    generatorIteratorFunction: defineOwnProperty(
      bind(
        iteratorFunction,
        undefined,
        [
          function () {
            return this();
          },
          generatorIteratorNext,
        ],
      ),
      "name",
      { value: "generatorIteratorFunction" },
    ),
    mapIteratorFunction: defineOwnProperty(
      bind(
        iteratorFunction,
        undefined,
        [mapIterator, mapIteratorNext],
      ),
      "name",
      { value: "mapIteratorFunction" },
    ),
    setIteratorFunction: defineOwnProperty(
      bind(
        iteratorFunction,
        undefined,
        [setIterator, setIteratorNext],
      ),
      "name",
      { value: "setIteratorFunction" },
    ),
    stringIteratorFunction: defineOwnProperty(
      bind(
        iteratorFunction,
        undefined,
        [stringIterator, stringIteratorNext],
      ),
      "name",
      { value: "stringIteratorFunction" },
    ),
  };
})();