/**
 * Empty iterator
 */
export const IATEND: Iterator<any> = {
  next: function () {
    return { done: true, value: undefined };
  },
};

/**
 * Empty iterable
 */
export const IEMPTY: Iterable<any> = {
  [Symbol.iterator]: () => IATEND,
};

/**
 * Iterable constructor, from an initial value, and a step value
 */
export function irecur<T, S>(start: T, step: (a: T) => T | null): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      let val: T | null = start;
      do {
        yield val;
        val = step(val);
      } while (val !== null);
    },
  };
}

/**
 * Iterable constructor, from an array
 *
 * The iterator will yield the next value each time it is called, then undefined when the array's end is reached.
 */
export function iarray<T>(array: T[], offset = 0): Iterable<T> {
  return {
    [Symbol.iterator]: function () {
      return array.slice(offset)[Symbol.iterator]();
    },
  };
}

/**
 * Iterable constructor, from a single value
 *
 * The value will be yielded only once, not repeated over.
 */
export function isingle<T>(value: T): Iterable<T> {
  return iarray([value]);
}

/**
 * Iterable that repeats the same value.
 */
export function irepeat<T>(value: T, count = -1): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      let n = count;
      while (n != 0) {
        yield value;
        n--;
      }
    },
  };
}

/**
 * Equivalent of Array.forEach for all iterables.
 *
 * If the callback returns *stopper*, the iteration is stopped.
 */
export function iforeach<T>(
  iterable: Iterable<T>,
  callback: (_: T) => any,
  stopper: any = null,
): void {
  for (let value of iterable) {
    if (callback(value) === stopper) {
      break;
    }
  }
}

/**
 * Returns the first item passing a predicate
 */
export function ifirst<T>(
  iterable: Iterable<T>,
  predicate: (item: T) => boolean,
): T | null {
  for (let value of iterable) {
    if (predicate(value)) {
      return value;
    }
  }
  return null;
}

/**
 * Returns the first non-null result of a value-yielding predicate, applied to each iterator element
 */
export function ifirstmap<T1, T2>(
  iterable: Iterable<T1>,
  predicate: (item: T1) => T2 | null,
): T2 | null {
  for (let value of iterable) {
    let res = predicate(value);
    if (res !== null) {
      return res;
    }
  }
  return null;
}

/**
 * Materialize an array from consuming an iterable
 *
 * To avoid materializing infinite iterators (and bursting memory), the item count is limited to 1 million, and an
 * exception is thrown when this limit is reached.
 */
export function imaterialize<T>(iterable: Iterable<T>, limit = 1000000): T[] {
  let result: T[] = [];

  for (let value of iterable) {
    result.push(value);
    if (result.length >= limit) {
      throw new Error("Length limit on iterator materialize");
    }
  }

  return result;
}

/**
 * Count items in an iterator
 *
 * To avoid counting infinite iterators (and bursting memory), the item count is limited by
 * the *limit* parameter, and Infinity will be returned after that many.
 */
export function ilength<T>(iterable: Iterable<T>, limit = 1000000): number {
  let result = 0;

  for (let _ of iterable) {
    if (result >= limit) {
      return Infinity;
    }
    result += 1;
  }

  return result;
}

/**
 * Iterate over natural integers
 *
 * If *count* is not specified, the iterator is infinite
 */
export function irange(
  count: number = -1,
  start = 0,
  step = 1,
): Iterable<number> {
  return {
    [Symbol.iterator]: function* () {
      let i = start;
      let n = count;
      while (n != 0) {
        yield i;
        i += step;
        n--;
      }
    },
  };
}

/**
 * Iterate over numbers, by applying a step taken from an other iterator
 *
 * This iterator stops when the "step iterator" stops
 *
 * With no argument, istep() == irange()
 */
export function istep(start = 0, step_iterable = irepeat(1)): Iterable<number> {
  return {
    [Symbol.iterator]: function* () {
      let i = start;
      yield i;
      for (let step of step_iterable) {
        i += step;
        yield i;
      }
    },
  };
}

/**
 * Skip a given number of values from an iterator, discarding them.
 */
export function iskip<T>(iterable: Iterable<T>, count = 1): Iterable<T> {
  return {
    [Symbol.iterator]: function () {
      let iterator = iterable[Symbol.iterator]();
      let n = count;
      while (n-- > 0) {
        iterator.next();
      }
      return iterator;
    },
  };
}

/**
 * Return the value at a given position in the iterator
 */
export function iat<T>(iterable: Iterable<T>, position: number): T | null {
  if (position < 0) {
    return null;
  } else {
    if (position > 0) {
      iterable = iskip(iterable, position);
    }
    let iterator = iterable[Symbol.iterator]();
    let state = iterator.next();
    return state.done ? null : state.value;
  }
}

/**
 * Chain an iterable of iterables.
 *
 * This will yield values from the first yielded iterator, then the second one, and so on...
 */
export function ichainit<T>(iterables: Iterable<Iterable<T>>): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      for (let iterable of iterables) {
        for (let value of iterable) {
          yield value;
        }
      }
    },
  };
}

/**
 * Chain iterables.
 *
 * This will yield values from the first iterator, then the second one, and so on...
 */
export function ichain<T>(...iterables: Iterable<T>[]): Iterable<T> {
  if (iterables.length == 0) {
    return IEMPTY;
  } else {
    return ichainit(iterables);
  }
}

/**
 * Loop an iterator for a number of times.
 *
 * If count is negative, if will loop forever (infinite iterator).
 *
 * onloop may be used to know when the iterator resets.
 */
export function iloop<T>(
  base: Iterable<T>,
  count = -1,
  onloop?: Function,
): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      let n = count;
      let start = false;
      while (n-- != 0) {
        for (let value of base) {
          if (start) {
            if (onloop) {
              onloop();
            }
            start = false;
          }
          yield value;
        }
        start = true;
      }
    },
  };
}

/**
 * Iterator version of "map".
 */
export function imap<T1, T2>(
  iterable: Iterable<T1>,
  mapfunc: (_: T1) => T2,
): Iterable<T2> {
  return {
    [Symbol.iterator]: function* () {
      for (let value of iterable) {
        yield mapfunc(value);
      }
    },
  };
}

/**
 * Iterator version of "reduce".
 */
export function ireduce<T>(
  iterable: Iterable<T>,
  reduce: (item1: T, item2: T) => T,
  init: T,
): T {
  let result = init;
  for (let value of iterable) {
    result = reduce(result, value);
  }
  return result;
}

/**
 * Iterator version of "filter".
 */
export function ifilter<T>(
  iterable: Iterable<T>,
  filterfunc: (_: T) => boolean,
): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      for (let value of iterable) {
        if (filterfunc(value)) {
          yield value;
        }
      }
    },
  };
}

/**
 * Apply map, and filter invalid results in the same pass.
 */
export function imapfilter<T1, T2>(
  iterable: Iterable<T1>,
  mapfunc: (_: T1) => T2 | null,
): Iterable<Exclude<T2, null>> {
  return {
    [Symbol.iterator]: function* () {
      for (let value of iterable) {
        const mapped = mapfunc(value);
        if (mapped !== null) {
          yield mapped as Exclude<T2, null>;
        }
      }
    },
  };
}

/**
 * Type filter, to return a list of instances of a given type
 */
export function ifiltertype<T>(
  iterable: Iterable<any>,
  filter: (item: any) => item is T,
): Iterable<T> {
  return ifilter(iterable, filter);
}

/**
 * Class filter, to return a list of instances of a given type
 */
export function ifilterclass<T>(
  iterable: Iterable<any>,
  classref: { new (...args: any[]): T },
): Iterable<T> {
  return ifilter(iterable, (item): item is T => item instanceof classref);
}

/**
 * Combine two iterables.
 *
 * This iterates through the second one several times, so if one iterator may be infinite,
 * it should be the first one.
 */
export function icombine<T1, T2>(
  it1: Iterable<T1>,
  it2: Iterable<T2>,
): Iterable<[T1, T2]> {
  return ichainit(imap(it1, (v1) => imap(it2, (v2): [T1, T2] => [v1, v2])));
}

/**
 * Enumerate another iterable, adding the index before the value.
 */
export function ienumerate<T>(it: Iterable<T>): Iterable<[number, T]> {
  return {
    [Symbol.iterator]: function* () {
      const iterator = it[Symbol.iterator]();
      let state = iterator.next();
      let idx = 0;
      while (!state.done) {
        yield [idx, state.value];
        state = iterator.next();
        idx += 1;
      }
    },
  };
}

/**
 * Advance through two iterables at the same time, yielding item pairs
 *
 * Iteration will stop at the first of the two iterators that stops.
 */
export function izip<T1, T2>(
  it1: Iterable<T1>,
  it2: Iterable<T2>,
): Iterable<[T1, T2]> {
  return {
    [Symbol.iterator]: function* () {
      let iterator1 = it1[Symbol.iterator]();
      let iterator2 = it2[Symbol.iterator]();
      let state1 = iterator1.next();
      let state2 = iterator2.next();
      while (!state1.done && !state2.done) {
        yield [state1.value, state2.value];
        state1 = iterator1.next();
        state2 = iterator2.next();
      }
    },
  };
}

/**
 * Advance two iterables at the same time, yielding item pairs (greedy version)
 *
 * Iteration will stop when both iterators are consumed, returning partial couples (undefined in the peer) if needed.
 */
export function izipg<T1, T2>(
  it1: Iterable<T1>,
  it2: Iterable<T2>,
): Iterable<[T1 | undefined, T2 | undefined]> {
  return {
    [Symbol.iterator]: function* () {
      let iterator1 = it1[Symbol.iterator]();
      let iterator2 = it2[Symbol.iterator]();
      let state1 = iterator1.next();
      let state2 = iterator2.next();
      while (!state1.done || !state2.done) {
        yield [state1.value, state2.value];
        state1 = iterator1.next();
        state2 = iterator2.next();
      }
    },
  };
}

/**
 * Partition in two iterables, one with values that pass the predicate, the other with values that don't
 */
export function ipartition<T>(
  iterable: Iterable<T>,
  predicate: (item: T) => boolean,
): [Iterable<T>, Iterable<T>] {
  return [
    ifilter(iterable, predicate),
    ifilter(iterable, (x) => !predicate(x)),
  ];
}

/**
 * Alternate between several iterables (pick one from the first one, then one from the second...)
 */
export function ialternate<T>(iterables: Iterable<T>[]): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      let iterators = iterables.map((iterable) => iterable[Symbol.iterator]());
      let done: boolean;
      do {
        done = false;
        // TODO Remove "dried-out" iterators
        for (let iterator of iterators) {
          let state = iterator.next();
          if (!state.done) {
            done = true;
            yield state.value;
          }
        }
      } while (done);
    },
  };
}

/**
 * Yield items from an iterator only once.
 *
 * Beware that even if this function is not materializing, it keeps track of yielded item, and may choke on
 * infinite or very long streams. Thus, no more than *limit* items will be yielded (an error is thrown
 * when this limit is reached).
 *
 * This function is O(n²)
 */
export function iunique<T>(
  iterable: Iterable<T>,
  limit = 1000000,
): Iterable<T> {
  return {
    [Symbol.iterator]: function* () {
      let done: Set<T> = new Set();
      let n = limit;
      for (let value of iterable) {
        if (!done.has(value)) {
          if (n-- > 0) {
            done.add(value);
            yield value;
          } else {
            throw new Error("Unique count limit on iterator");
          }
        }
      }
    },
  };
}

/**
 * Common reduce shortcuts
 */
export const isum = (iterable: Iterable<number>) =>
  ireduce(iterable, (a, b) => a + b, 0);
export const icat = (iterable: Iterable<string>) =>
  ireduce(iterable, (a, b) => a + b, "");
export const imin = (iterable: Iterable<number>) =>
  ireduce(iterable, Math.min, Infinity);
export const imax = (iterable: Iterable<number>) =>
  ireduce(iterable, Math.max, -Infinity);

/**
 * Min/max by key function
 */
export function iminBy<T>(
  iterable: Iterable<T>,
  key: (value: T) => number,
): T | undefined {
  return ireduce(
    iterable,
    (item1, item2) =>
      (typeof item1 == "undefined" || typeof item2 == "undefined") ||
        (key(item1) > key(item2))
        ? item2
        : item1,
    undefined,
  );
}
export function imaxBy<T>(
  iterable: Iterable<T>,
  key: (value: T) => number,
): T | undefined {
  return iminBy(iterable, (value) => -key(value));
}