mjackson/mach

var parseMediaValue = require('../utils/parseMediaValue');
var parseMediaValues = require('../utils/parseMediaValues');
var qualityFactorForMediaValue = require('../utils/qualityFactorForMediaValue');
var stringifyMediaValues = require('../utils/stringifyMediaValues');
var Header = require('../Header');

function byHighestPrecedence(a, b) {
  // "*" gets least precedence, all others are equal
  return a === '*' ? -1 : (b === '*' ? 1 : 0);
}

/**
 * Represents an HTTP Accept-Encoding header and provides several methods
 * for determining acceptable content encodings.
 *
 * http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
 */
class AcceptEncoding extends Header {

  constructor(value) {
    super('Accept-Encoding', value);
  }

  /**
   * Returns the value of this header as a string.
   */
  get value() {
    return stringifyMediaValues(this._mediaValues) || '';
  }

  set value(value) {
    this._mediaValues = value ? parseMediaValues(value) : [];
  }

  /**
   * Returns true if the given encoding is acceptable.
   */
  accepts(encoding) {
    return this.qualityFactorForEncoding(encoding) !== 0;
  }

  /**
   * Returns the quality factor for the given encoding.
   */
  qualityFactorForEncoding(encoding) {
    var values = this._mediaValues;

    var givenValue = parseMediaValue(encoding);
    var matchingValues = values.filter(function (value) {
      if (value.type === '*')
        return true;

      return value.type === givenValue.type;
    }).sort(byHighestPrecedence);

    // From RFC 2616:
    // The "identity" content-coding is always acceptable, unless
    // specifically refused because the Accept-Encoding field includes
    // "identity;q=0", or because the field includes "*;q=0" and does
    // not explicitly include the "identity" content-coding. If the
    // Accept-Encoding field-value is empty, then only the "identity"
    // encoding is acceptable.
    if (givenValue.type === 'identity') {
      if (matchingValues.length && matchingValues[0].type === 'identity')
        return qualityFactorForMediaValue(matchingValues[0]);

      return 1;
    }

    if (!matchingValues.length)
      return 0;

    return qualityFactorForMediaValue(matchingValues[0]);
  }

}

module.exports = AcceptEncoding;