/* String streams are the things fed to parsers (which can feed them
 * to a tokenizer if they want). They provide peek and next methods
 * for looking at the current character (next 'consumes' this
 * character, peek does not), and a get method for retrieving all the
 * text that was consumed since the last time get was called.
 *
 * An easy mistake to make is to let a StopIteration exception finish
 * the token stream while there are still characters pending in the
 * string stream (hitting the end of the buffer while parsing a
 * token). To make it easier to detect such errors, the stringstreams
 * throw an exception when this happens.
 */

// Make a stringstream stream out of an iterator that returns strings.
// This is applied to the result of traverseDOM (see codemirror.js),
// and the resulting stream is fed to the parser.
var stringStream = function(source){
  // String that's currently being iterated over.
  var current = "";
  // Position in that string.
  var pos = 0;
  // Accumulator for strings that have been iterated over but not
  // get()-ed yet.
  var accum = "";
  // Make sure there are more characters ready, or throw
  // StopIteration.
  function ensureChars() {
    while (pos == current.length) {
      accum += current;
      current = ""; // In case source.next() throws
      pos = 0;
      try {current = source.next();}
      catch (e) {
        if (e != StopIteration) throw e;
        else return false;
      }
    }
    return true;
  }

  return {
    // Return the next character in the stream.
    peek: function() {
      if (!ensureChars()) return null;
      return current.charAt(pos);
    },
    // Get the next character, throw StopIteration if at end, check
    // for unused content.
    next: function() {
      if (!ensureChars()) {
        if (accum.length > 0)
          throw "End of stringstream reached without emptying buffer ('" + accum + "').";
        else
          throw StopIteration;
      }
      return current.charAt(pos++);
    },
    // Return the characters iterated over since the last call to
    // .get().
    get: function() {
      var temp = accum;
      accum = "";
      if (pos > 0){
        temp += current.slice(0, pos);
        current = current.slice(pos);
        pos = 0;
      }
      return temp;
    },
    // Push a string back into the stream.
    push: function(str) {
      current = current.slice(0, pos) + str + current.slice(pos);
    },
    lookAhead: function(str, consume, skipSpaces, caseInsensitive) {
      function cased(str) {return caseInsensitive ? str.toLowerCase() : str;}
      str = cased(str);
      var found = false;

      var _accum = accum, _pos = pos;
      if (skipSpaces) this.nextWhileMatches(/[\s\u00a0]/);

      while (true) {
        var end = pos + str.length, left = current.length - pos;
        if (end <= current.length) {
          found = str == cased(current.slice(pos, end));
          pos = end;
          break;
        }
        else if (str.slice(0, left) == cased(current.slice(pos))) {
          accum += current; current = "";
          try {current = source.next();}
          catch (e) {break;}
          pos = 0;
          str = str.slice(left);
        }
        else {
          break;
        }
      }

      if (!(found && consume)) {
        current = accum.slice(_accum.length) + current;
        pos = _pos;
        accum = _accum;
      }

      return found;
    },

    // Utils built on top of the above
    more: function() {
      return this.peek() !== null;
    },
    applies: function(test) {
      var next = this.peek();
      return (next !== null && test(next));
    },
    nextWhile: function(test) {
      var next;
      while ((next = this.peek()) !== null && test(next))
        this.next();
    },
    matches: function(re) {
      var next = this.peek();
      return (next !== null && re.test(next));
    },
    nextWhileMatches: function(re) {
      var next;
      while ((next = this.peek()) !== null && re.test(next))
        this.next();
    },
    equals: function(ch) {
      return ch === this.peek();
    },
    endOfLine: function() {
      var next = this.peek();
      return next == null || next == "\n";
    }
  };
};
