NewCommentMap creates a new comment map by associating comment groups of the comments list with the nodes of the AST specified by node.

A comment group g is associated with a node n if:

- g starts on the same line as n ends
- g starts on the line immediately following n, and there is
  at least one empty line after g and before the next node
- g starts before n and is not associated to the node before n
  via the previous rules

NewCommentMap tries to associate a comment group to the "largest" node possible: For instance, if the comment is a line comment trailing an assignment, the comment is associated with the entire assignment rather than just the last operand in the assignment.


func NewCommentMap(fset *token.FileSet, node Node, comments []*CommentGroup) CommentMap {
	if len(comments) == 0 {
		return nil // no comments to map

	cmap := make(CommentMap)

	// set up comment reader r
	tmp := make([]*CommentGroup, len(comments))
	copy(tmp, comments) // don't change incoming comments
	r := commentListReader{fset: fset, list: tmp} // !r.eol() because len(comments) > 0

	// create node list in lexical order
	nodes := nodeList(node)
	nodes = append(nodes, nil) // append sentinel

	// set up iteration variables
	var (
		p     Node           // previous node
		pend  token.Position // end of p
		pg    Node           // previous node group (enclosing nodes of "importance")
		pgend token.Position // end of pg
		stack nodeStack      // stack of node groups

	for _, q := range nodes {
		var qpos token.Position
		if q != nil {
			qpos = fset.Position(q.Pos()) // current node position
		} else {
			// set fake sentinel position to infinity so that
			// all comments get processed before the sentinel
			const infinity = 1 << 30
			qpos.Offset = infinity
			qpos.Line = infinity

		// process comments before current node
		for r.end.Offset <= qpos.Offset {
			// determine recent node group
			if top := stack.pop(r.comment.Pos()); top != nil {
				pg = top
				pgend = fset.Position(pg.End())
			// Try to associate a comment first with a node group
			// (i.e., a node of "importance" such as a declaration);
			// if that fails, try to associate it with the most recent
			// node.
			// TODO(gri) try to simplify the logic below
			var assoc Node
			switch {
			case pg != nil &&
				(pgend.Line == r.pos.Line ||
					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line):
				// 1) comment starts on same line as previous node group ends, or
				// 2) comment starts on the line immediately after the
				//    previous node group and there is an empty line before
				//    the current node
				// => associate comment with previous node group
				assoc = pg
			case p != nil &&
				(pend.Line == r.pos.Line ||
					pend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line ||
					q == nil):
				// same rules apply as above for p rather than pg,
				// but also associate with p if we are at the end (q == nil)
				assoc = p
				// otherwise, associate comment with current node
				if q == nil {
					// we can only reach here if there was no p
					// which would imply that there were no nodes
					panic("internal error: no comments should be associated with sentinel")
				assoc = q
			cmap.addComment(assoc, r.comment)
			if r.eol() {
				return cmap

		// update previous node
		p = q
		pend = fset.Position(p.End())

		// update previous node group if we see an "important" node
		switch q.(type) {
		case *File, *Field, Decl, Spec, Stmt:

	return cmap