add 'notesRecord' to check footnote existence fast

It is necessary to use vector for 'notes' instead of map to keep
footnotes ordered. But checking for presence in a vector is not
efficient. So add a map variable 'notesRecord' to tackle this problem.

inline.go

@@ -488,6 +488,7 @@ func link(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 			}
 
 			p.notes = append(p.notes, ref)
+			p.notesRecord[string(ref.link)] = true
 
 			link = ref.link
 			title = ref.title
@@ -501,6 +502,7 @@ func link(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 			if t == linkDeferredFootnote && !p.isFootnote(lr) {
 				lr.noteId = len(p.notes) + 1
 				p.notes = append(p.notes, lr)
+				p.notesRecord[string(lr.link)] = true
 			}
 
 			// keep link and title from reference

markdown.go

@@ -218,7 +218,8 @@ type parser struct {
 	// Footnotes need to be ordered as well as available to quickly check for
 	// presence. If a ref is also a footnote, it's stored both in refs and here
 	// in notes. Slice is nil if footnotes not enabled.
-	notes []*reference
+	notes       []*reference
+	notesRecord map[string]bool
 }
 
 func (p *parser) getRef(refid string) (ref *reference, found bool) {
@@ -242,13 +243,8 @@ func (p *parser) getRef(refid string) (ref *reference, found bool) {
 }
 
 func (p *parser) isFootnote(ref *reference) bool {
-	for _, v := range p.notes {
-		if string(ref.link) == string(v.link) {
-			return true
-		}
-	}
-
-	return false
+	_, ok := p.notesRecord[string(ref.link)]
+	return ok
 }
 
 //
@@ -386,6 +382,7 @@ func MarkdownOptions(input []byte, renderer Renderer, opts Options) []byte {
 
 	if extensions&EXTENSION_FOOTNOTES != 0 {
 		p.notes = make([]*reference, 0)
+		p.notesRecord = make(map[string]bool)
 	}
 
 	first := firstPass(p, input)