// Returns a promise which resolves with a Document representing a parsed SVG
// from the given text file. The top-level group tag is given an id equal to
// the original file name (without an extension).
function svgDocument(file) {
  var reader = new FileReader;
  return new Promise((resolve, reject) => {
    reader.onload = function(e) {
      var text = e.target.result;
      var parser = new DOMParser();
      var xmlDocument = parser.parseFromString(text, "text/xml");
      var name = file.name.substring(0, file.name.indexOf(".svg"));
      
      var svg = xmlDocument.querySelector('svg');

      if (svg != null) {
        var g = svg.querySelector('g');

        if (g != null) {
          // Set top-level group id to be the filename.
          g.setAttribute('id', name);
        } else {
          // Enclose SVG elements in a parent group tag.
          let newGroup = document.createElement('g');
          newGroup.setAttribute('id', name);
          for (let i = 0; i < svg.children.length; i++) {
            let child = svg.children[i];
            newGroup.appendChild(child);
          }
          svg.appendChild(newGroup);
        }
      }
      
      resolve(xmlDocument);
    };

    reader.readAsText(file);
    reader.onerror = reject;
  }); 
}

// Returns [Document, Metadata]
function sanitizeSvg(svgDocument) {
  let tags = ["path", "g", "polyline", "polygon"],
      notes = {
        fill: false, // True if a fill rule other than `fill: #000000;` is used.
        stroke: false // True if a stroke attribute is used.
      };

  tags.forEach((tag) => {
    svgDocument.querySelectorAll(tag).forEach((element) => {
      let fill = element.getAttribute("fill");
      if (fill &&  fill !== "#000000") {
        notes.fill = true;
      }

      let stroke = element.getAttribute("stroke");
      if (stroke) {
        notes.stroke = true;
      }
      
      element.removeAttribute("fill");
      element.removeAttribute("stroke");
    });
  });

  // TODO: Return useful metadata regarding the svgDocument.
  return [svgDocument, notes];
}

// Returns a string representing a sprite sheet for the given XML documents.
function svgSpritesheet(documents) {
  let serializer = new XMLSerializer(),
      s = "",
      data = [];

  documents.forEach((d) => {
    let sanitizedSvg,
        datum,
        formattedSvg,
        symbolizedSvg;

    [sanitizedSvg, datum] = sanitizeSvg(d);
    formattedSvg = serializer.serializeToString(sanitizedSvg);
    symbolizedSvg = formattedSvg
        .replace("<svg", "<symbol")
        .replace("</svg>", "</symbol>");
    s += symbolizedSvg;
    data.push(datum);
  });

  return ['<svg xmlns="http://www.w3.org/2000/svg"><defs>'
      + s
      + '</defs></svg>',
      data];
}