Optimize backtracking in links

lib/links.js

@@ -17,6 +17,11 @@ function parseLinkLabel(state, start) {
 
   if (state.validateInsideLink) { return -1; }
 
+  if (state.label_nest_level) {
+    state.label_nest_level--;
+    return -1;
+  }
+
   state.pos = start + 1;
   state.validateInsideLink = true;
   level = 1;
@@ -38,7 +43,12 @@ function parseLinkLabel(state, start) {
     if (!ok) { state.pending += state.src[state.pos++]; }
   }
 
-  if (found) { labelEnd = state.pos; }
+  if (found) {
+    labelEnd = state.pos;
+    state.label_nest_level = 0;
+  } else {
+    state.label_nest_level = level - 1;
+  }
 
   // restore old state
   state.pos = oldPos;

lib/rules_inline/state_inline.js

@@ -17,6 +17,7 @@ function StateInline(src, parser, options, env) {
   this.level = 0;
   this.link_content = '';
   this.pendingLevel = 0;
+  this.label_nest_level = 0; // for stmd-like backtrack optimization
 }