var TokenIterator = module.exports = function(tokens, startIndex) {
    this._tokens = tokens;
    this._startIndex = startIndex || 0;
};

TokenIterator.prototype.head = function() {
    return this._tokens[this._startIndex];
};

TokenIterator.prototype.tail = function(startIndex) {
    return new TokenIterator(this._tokens, this._startIndex + 1);
};

TokenIterator.prototype.toArray = function() {
    return this._tokens.slice(this._startIndex);
};

TokenIterator.prototype.end = function() {
    return this._tokens[this._tokens.length - 1];
};

// TODO: doesn't need to be a method, can be a separate function,
// which simplifies implementation of the TokenIterator interface
TokenIterator.prototype.to = function(end) {
    var start = this.head().source;
    var endToken = end.head() || end.end();
    return start.to(endToken.source);
};