/*
 * headers - Detect when fetch Headers are undefined and use a partial polyfill.
 *
 * A full polyfill is not used in order to keep package size as small as possible.
 * Since this is only used internally and is not added to the window object,
 * the full interface doesn't need to be supported.
 *
 * This implementation is modified from whatwg-fetch:
 * https://github.com/github/fetch
 */
function headers(headers) {
  if (typeof Headers === 'undefined') {
    return new FetchHeaders(headers);
  }

  return new Headers(headers);
}

function normalizeName(name) {
  if (typeof name !== 'string') {
    name = String(name);
  }
  return name.toLowerCase();
}

function normalizeValue(value) {
  if (typeof value !== 'string') {
    value = String(value);
  }
  return value;
}

function iteratorFor(items) {
  var iterator = {
    next: function () {
      var value = items.shift();
      return { done: value === undefined, value: value };
    },
  };

  return iterator;
}

function FetchHeaders(headers) {
  this.map = {};

  if (headers instanceof FetchHeaders) {
    headers.forEach(function (value, name) {
      this.append(name, value);
    }, this);
  } else if (Array.isArray(headers)) {
    headers.forEach(function (header) {
      this.append(header[0], header[1]);
    }, this);
  } else if (headers) {
    Object.getOwnPropertyNames(headers).forEach(function (name) {
      this.append(name, headers[name]);
    }, this);
  }
}

FetchHeaders.prototype.append = function (name, value) {
  name = normalizeName(name);
  value = normalizeValue(value);
  var oldValue = this.map[name];
  this.map[name] = oldValue ? oldValue + ', ' + value : value;
};

FetchHeaders.prototype.get = function (name) {
  name = normalizeName(name);
  return this.has(name) ? this.map[name] : null;
};

FetchHeaders.prototype.has = function (name) {
  return this.map.hasOwnProperty(normalizeName(name));
};

FetchHeaders.prototype.forEach = function (callback, thisArg) {
  for (var name in this.map) {
    if (this.map.hasOwnProperty(name)) {
      callback.call(thisArg, this.map[name], name, this);
    }
  }
};

FetchHeaders.prototype.entries = function () {
  var items = [];
  this.forEach(function (value, name) {
    items.push([name, value]);
  });
  return iteratorFor(items);
};

module.exports = headers;