spec-md/src/client/linkSelections.js

Renders Markdown with some additions into an HTML format commonly used for writing technical specification documents. Markdown additions include code syntax highlighting, edit annotations, and the definition of algorithms and grammar productions.

// RFC4648 url-safe base-64 encoding
var URL64Code =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
var selectionLink;
var article;
var currentRange;
var currentEncodedRange;

document.addEventListener("selectionchange", handleSelectionChange);
window.addEventListener("resize", renderCurrentRange);
window.addEventListener("hashchange", scrollToWindowLocation);
window.addEventListener("load", scrollToWindowLocation);

function onClickHash(event) {
  scrollToSelectionHash(new URL(event.target.href));
}

function scrollToWindowLocation() {
  scrollToSelectionHash(window.location);
}

// Given a URL with a selection hash, decode it, apply it, and scroll to it.
function scrollToSelectionHash(url) {
  var match = url.hash.match(/^#sel-([A-Za-z0-9-_]+)$/);
  if (!match) {
    return;
  }
  currentEncodedRange = match[1];
  currentRange = decodeRange(currentEncodedRange);
  var rect = currentRange.getBoundingClientRect();
  var topOffset = Math.max(
    20,
    Math.floor((window.innerHeight - rect.height) * 0.4)
  );
  window.scrollTo(0, window.scrollY + rect.y - topOffset);
  var selection = document.getSelection();
  selection.empty();
  selection.addRange(currentRange);
  renderCurrentRange();
}

// If there is currently a selection on the page, render a link encoding it.
function handleSelectionChange(event) {
  var selection = document.getSelection();
  if (selection.isCollapsed) {
    if (selectionLink) {
      selectionLink.parentNode.removeChild(selectionLink);
      selectionLink = null;
    }
  } else {
    var range = selection.getRangeAt(0);
    if (
      !currentRange ||
      range.compareBoundaryPoints(Range.START_TO_START, currentRange) !== 0 ||
      range.compareBoundaryPoints(Range.END_TO_END, currentRange) !== 0
    ) {
      currentRange = range;
      currentEncodedRange = encodeRange(currentRange);
      renderCurrentRange();
    }
  }
}

function renderCurrentRange() {
  if (!currentRange) {
    return
  }
  if (!article) {
    article = document.getElementsByTagName("article")[0];
  }
  if (!selectionLink) {
    selectionLink = document.createElement("a");
    document.body.appendChild(selectionLink);
  }
  selectionLink.href = "#sel-" + currentEncodedRange;
  selectionLink.onclick = onClickHash;
  selectionLink.className = currentRange.isOutdated
    ? "outdated-selection-link"
    : "selection-link";
  selectionLink.innerText = currentRange.isOutdated ? "!" : "\u201F";
  var smallScreen = window.innerWidth < 720;
  var rect = currentRange.getBoundingClientRect();
  if (smallScreen) {
    selectionLink.style.left = Math.floor(rect.x + rect.width / 2 + window.scrollX - 13) + "px";
    selectionLink.style.top = Math.floor(rect.bottom + window.scrollY + 10) + "px";
  } else {
    var left = article.getBoundingClientRect().x;
    selectionLink.style.left = Math.floor(left + window.scrollX - 37) + "px";
    selectionLink.style.top = Math.floor(rect.y + window.scrollY - 3) + "px";
  }
}

// Encodes the range of a selection on the page as a string. The string is a
// URL-safe Base64 encoded hexad stream. While we could have used a byte stream,
// using hexads removes the need to convert Base64's hexads to bytes.
// A range is encoded as three lists of unsigned ints. The first list is the
// tree traversal path to the common ancestor node of the selection. The second
// is the tree traversal path from the common node to the start container,
// followed by the index into the text at that node, and the third is from the
// common node to the end container and the end text index.
function encodeRange(range) {
  var encoded = "";
  var startPath = encodeNodePath(range.startContainer);
  var endPath = encodeNodePath(range.endContainer);
  var commonPath = getCommonPath(startPath, endPath);
  writeList(commonPath);
  writeList(startPath.slice(commonPath.length).concat(range.startOffset));
  writeList(endPath.slice(commonPath.length).concat(range.endOffset));
  writeInt(getFNVChecksum(range.toString()));
  return encoded;

  // Unsigned ints are represented in a go-style varint encoding. Each hexad
  // holds 5 bits of value and a MSB indicating if there are subsequent hexads
  // representing this int.
  function writeInt(number) {
    do {
      encoded += URL64Code[(number & 0x1f) | (number > 0x1f ? 0x20 : 0)];
      number >>= 5;
    } while (number > 0);
  }

  // Lists are written as one int indicating the list's length, followed by each
  // element in that list.
  function writeList(list) {
    writeInt(list.length);
    for (var i = 0; i < list.length; i++) {
      writeInt(list[i]);
    }
  }
}

function decodeRange(encoded) {
  var URL64Decode = new Array(64);
  for (var i = 0; i < 64; i++) {
    URL64Decode[URL64Code.charCodeAt(i)] = i;
  }
  var offset = 0;
  var commonPath = readList();
  var startPath = readList();
  var endPath = readList();
  var expectedChecksum = readInt();
  var startOffset = startPath.pop();
  var startNode = decodeNodePath(commonPath.concat(startPath));
  var endOffset = endPath.pop();
  var endNode = decodeNodePath(commonPath.concat(endPath));
  var range = document.createRange();
  range.setStart(startNode, startOffset);
  range.setEnd(endNode, endOffset);
  range.isOutdated =
    expectedChecksum !== undefined &&
    expectedChecksum !== getFNVChecksum(range.toString());
  return range;

  function readInt() {
    var number = 0;
    var sign = 0;
    while (offset < encoded.length) {
      var byte = URL64Decode[encoded.charCodeAt(offset++)];
      number |= (byte & 0x1f) << sign;
      sign += 5;
      if (byte < 0x20) {
        return number;
      }
    }
  }

  function readList() {
    var length = readInt();
    if (length != undefined) {
      var list = new Array(length);
      for (var i = 0; i < length; i++) {
        list[i] = readInt();
      }
      return list;
    }
  }
}

// A node's identity is encoded as a list of integers representing the tree
// traversal path from the document body.
function encodeNodePath(node) {
  var path = [];
  while (node != document.body) {
    var parentNode = node.parentNode;
    path.push(Array.prototype.indexOf.call(parentNode.childNodes, node));
    node = parentNode;
  }
  return path.reverse();
}

function decodeNodePath(path) {
  var node = document.body;
  for (var i = 0; i < path.length && node; i++) {
    node = node.childNodes[path[i]];
  }
  return node;
}

// Given two arrays of integers, returns the common prefix of the two.
function getCommonPath(p1, p2) {
  var i = 0;
  while (i < p1.length && i < p2.length && p1[i] === p2[i]) {
    i++;
  }
  return p1.slice(0, i);
}

// Given a string, produce a 15-bit unsigned int checksum.
// Later used to catch if a range may have changed since the link was created.
function getFNVChecksum(str) {
  var sum = 0x811c9dc5;
  for (var i = 0; i < str.length; ++i) {
    sum ^= str.charCodeAt(i);
    sum += (sum << 1) + (sum << 4) + (sum << 7) + (sum << 8) + (sum << 24);
  }
  return ((sum >> 15) ^ sum) & 0x7fff;
}