'use strict';

const {none, final, isFinalValue, getFinalValue, many, isMany, getManyValues} = require('../defs');

const next = async function*(value, fns, index) {
  for (let i = index; i <= fns.length; ++i) {
    if (value && typeof value.then == 'function') {
      // thenable
      value = await value;
    }
    if (value === none) break;
    if (isFinalValue(value)) {
      const val = getFinalValue(value);
      if (val !== none) yield val;
      break;
    }
    if (isMany(value)) {
      const values = getManyValues(value);
      if (i == fns.length) {
        yield* values;
      } else {
        for (let j = 0; j < values.length; ++j) {
          yield* next(values[j], fns, i);
        }
      }
      break;
    }
    if (value && typeof value.next == 'function') {
      // generator
      for (;;) {
        let data = value.next();
        if (data && typeof data.then == 'function') {
          data = await data;
        }
        if (data.done) break;
        if (i == fns.length) {
          yield data.value;
        } else {
          yield* next(data.value, fns, i);
        }
      }
      break;
    }
    if (i == fns.length) {
      yield value;
      break;
    }
    value = fns[i](value);
  }
};

const nop = async function*() {};

const asGen = (...fns) => {
  fns = fns.filter(fn => fn);
  if (!fns.length) return nop;
  if (Symbol.asyncIterator && fns[0][Symbol.asyncIterator]) {
    fns[0] = fns[0][Symbol.asyncIterator];
  } else if (Symbol.iterator && fns[0][Symbol.iterator]) {
    fns[0] = fns[0][Symbol.iterator];
  }
  return async function*(value) {
    yield* next(value, fns, 0);
  };
};

asGen.next = next;

asGen.none = none;
asGen.final = final;
asGen.isFinalValue = isFinalValue;
asGen.getFinalValue = getFinalValue;
asGen.many = many;
asGen.isMany = isMany;
asGen.getManyValues = getManyValues;

module.exports = asGen;