Add collapsed and full reference links

lib/lexer_inline.js

@@ -212,7 +212,7 @@ LexerInline.prototype.parse_reference = function (str, options) {
   if (pos < max && state.src.charCodeAt(pos) !== 0x0A) { return null; }
 
   return {
-    label: str.slice(1, labelEnd).trim().replace(/\s+/g, ' '),
+    label: links.normalizeReference(str.slice(1, labelEnd)),
     title: title,
     href: href,
     remaining: str.slice(pos)

lib/lexer_inline/links.js

@@ -164,13 +164,19 @@ function parseLinkTitle(state, pos) {
   return null;
 }
 
+function normalizeReference(str) {
+  return str.trim().replace(/\s+/g, ' ').toLowerCase();
+}
+
 function links(state) {
   var labelStart,
       labelEnd,
+      label,
       href,
       title,
       pos,
       ref,
+      code,
       isImage = false,
       max = state.posMax,
       start = state.pos,
@@ -235,7 +241,25 @@ function links(state) {
     //
     // Link reference
     //
-    ref = state.env.references[state.src.slice(labelStart, labelEnd).trim().replace(/\s+/g, ' ')];
+
+    // [foo]  [bar]
+    //      ^^ optional whitespace (can include newlines)
+    for (; pos < max; pos++) {
+      code = state.src.charCodeAt(pos);
+      if (code !== 0x20 && code !== 0x0A) { break; }
+    }
+
+    if (pos < max && state.src.charCodeAt(pos) === 0x5B/* [ */) {
+      start = pos + 1;
+      pos = parseLinkLabel(state, pos);
+      label = state.src.slice(start, pos);
+    }
+
+    // covers label === '' and label === undefined
+    // (collapsed reference link and shortcut reference link respectively)
+    if (!label) { label = state.src.slice(labelStart, labelEnd); }
+
+    ref = state.env.references[normalizeReference(label)];
     if (!ref) { return false; }
     href = ref.href;
     title = ref.title;
@@ -269,3 +293,4 @@ module.exports = links;
 module.exports.parseLinkLabel = parseLinkLabel;
 module.exports.parseLinkDestination = parseLinkDestination;
 module.exports.parseLinkTitle = parseLinkTitle;
+module.exports.normalizeReference = normalizeReference;