const VIEW    = 'view',
      LBRACK  = '[',
      RBRACK  = ']',
      LBRACE  = '{',
      RBRACE  = '}',
      COLON   = ':',
      COMMA   = ',',
      NAME    = '@',
      GT      = '>',
      ILLEGAL = /[[\]{}]/,
      DEFAULT_MARKS = {
        '*': 1,
        arc: 1,
        area: 1,
        group: 1,
        image: 1,
        line: 1,
        path: 1,
        rect: 1,
        rule: 1,
        shape: 1,
        symbol: 1,
        text: 1,
        trail: 1
      };

let DEFAULT_SOURCE,
    MARKS;

/**
 * Parse an event selector string.
 * Returns an array of event stream definitions.
 */
export default function(selector, source, marks) {
  DEFAULT_SOURCE = source || VIEW;
  MARKS = marks || DEFAULT_MARKS;
  return parseMerge(selector.trim()).map(parseSelector);
}

function isMarkType(type) {
  return MARKS[type];
}

function find(s, i, endChar, pushChar, popChar) {
  const n = s.length;
  let count = 0, c;

  for (; i < n; ++i) {
    c = s[i];
    if (!count && c === endChar) return i;
    else if (popChar && popChar.includes(c)) --count;
    else if (pushChar && pushChar.includes(c)) ++count;
  }
  return i;
}

function parseMerge(s) {
  const output = [],
        n = s.length;
  let start = 0, i = 0;

  while (i < n) {
    i = find(s, i, COMMA, LBRACK + LBRACE, RBRACK + RBRACE);
    output.push(s.substring(start, i).trim());
    start = ++i;
  }

  if (output.length === 0) {
    throw 'Empty event selector: ' + s;
  }
  return output;
}

function parseSelector(s) {
  return s[0] === '['
    ? parseBetween(s)
    : parseStream(s);
}

function parseBetween(s) {
  const n = s.length;
  let i = 1, b;

  i = find(s, i, RBRACK, LBRACK, RBRACK);
  if (i === n) {
    throw 'Empty between selector: ' + s;
  }

  b = parseMerge(s.substring(1, i));
  if (b.length !== 2) {
    throw 'Between selector must have two elements: ' + s;
  }

  s = s.slice(i + 1).trim();
  if (s[0] !== GT) {
    throw 'Expected \'>\' after between selector: ' + s;
  }

  b = b.map(parseSelector);

  const stream = parseSelector(s.slice(1).trim());
  if (stream.between) {
    return {
      between: b,
      stream: stream
    };
  } else {
    stream.between = b;
  }

  return stream;
}

function parseStream(s) {
  const stream = {source: DEFAULT_SOURCE},
        source = [];

  let throttle = [0, 0],
      markname = 0,
      start = 0,
      n = s.length,
      i = 0, j,
      filter;

  // extract throttle from end
  if (s[n-1] === RBRACE) {
    i = s.lastIndexOf(LBRACE);
    if (i >= 0) {
      try {
        throttle = parseThrottle(s.substring(i+1, n-1));
      } catch (e) {
        throw 'Invalid throttle specification: ' + s;
      }
      s = s.slice(0, i).trim();
      n = s.length;
    } else throw 'Unmatched right brace: ' + s;
    i = 0;
  }

  if (!n) throw s;

  // set name flag based on first char
  if (s[0] === NAME) markname = ++i;

  // extract first part of multi-part stream selector
  j = find(s, i, COLON);
  if (j < n) {
    source.push(s.substring(start, j).trim());
    start = i = ++j;
  }

  // extract remaining part of stream selector
  i = find(s, i, LBRACK);
  if (i === n) {
    source.push(s.substring(start, n).trim());
  } else {
    source.push(s.substring(start, i).trim());
    filter = [];
    start = ++i;
    if (start === n) throw 'Unmatched left bracket: ' + s;
  }

  // extract filters
  while (i < n) {
    i = find(s, i, RBRACK);
    if (i === n) throw 'Unmatched left bracket: ' + s;
    filter.push(s.substring(start, i).trim());
    if (i < n-1 && s[++i] !== LBRACK) throw 'Expected left bracket: ' + s;
    start = ++i;
  }

  // marshall event stream specification
  if (!(n = source.length) || ILLEGAL.test(source[n-1])) {
    throw 'Invalid event selector: ' + s;
  }

  if (n > 1) {
    stream.type = source[1];
    if (markname) {
      stream.markname = source[0].slice(1);
    } else if (isMarkType(source[0])) {
      stream.marktype = source[0];
    } else {
      stream.source = source[0];
    }
  } else {
    stream.type = source[0];
  }
  if (stream.type.slice(-1) === '!') {
    stream.consume = true;
    stream.type = stream.type.slice(0, -1);
  }
  if (filter != null) stream.filter = filter;
  if (throttle[0]) stream.throttle = throttle[0];
  if (throttle[1]) stream.debounce = throttle[1];

  return stream;
}

function parseThrottle(s) {
  const a = s.split(COMMA);
  if (!s.length || a.length > 2) throw s;
  return a.map(_ => {
    const x = +_;
    if (x !== x) throw s;
    return x;
  });
}
