1 files changed, 155 insertions, 0 deletions
diff --git a/src/go/printer/comment.go b/src/go/printer/comment.go
new file mode 100644
index 0000000..9012714
--- /dev/null
+++ b/src/go/printer/comment.go
@@ -0,0 +1,155 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import (
+	"go/ast"
+	"go/doc/comment"
+	"strings"
+)
+
+// formatDocComment reformats the doc comment list,
+// returning the canonical formatting.
+func formatDocComment(list []*ast.Comment) []*ast.Comment {
+	// Extract comment text (removing comment markers).
+	var kind, text string
+	var directives []*ast.Comment
+	if len(list) == 1 && strings.HasPrefix(list[0].Text, "/*") {
+		kind = "/*"
+		text = list[0].Text
+		if !strings.Contains(text, "\n") || allStars(text) {
+			// Single-line /* .. */ comment in doc comment position,
+			// or multiline old-style comment like
+			//	/*
+			//	 * Comment
+			//	 * text here.
+			//	 */
+			// Should not happen, since it will not work well as a
+			// doc comment, but if it does, just ignore:
+			// reformatting it will only make the situation worse.
+			return list
+		}
+		text = text[2 : len(text)-2] // cut /* and */
+	} else if strings.HasPrefix(list[0].Text, "//") {
+		kind = "//"
+		var b strings.Builder
+		for _, c := range list {
+			after, found := strings.CutPrefix(c.Text, "//")
+			if !found {
+				return list
+			}
+			// Accumulate //go:build etc lines separately.
+			if isDirective(after) {
+				directives = append(directives, c)
+				continue
+			}
+			b.WriteString(strings.TrimPrefix(after, " "))
+			b.WriteString("\n")
+		}
+		text = b.String()
+	} else {
+		// Not sure what this is, so leave alone.
+		return list
+	}
+
+	if text == "" {
+		return list
+	}
+
+	// Parse comment and reformat as text.
+	var p comment.Parser
+	d := p.Parse(text)
+
+	var pr comment.Printer
+	text = string(pr.Comment(d))
+
+	// For /* */ comment, return one big comment with text inside.
+	slash := list[0].Slash
+	if kind == "/*" {
+		c := &ast.Comment{
+			Slash: slash,
+			Text:  "/*\n" + text + "*/",
+		}
+		return []*ast.Comment{c}
+	}
+
+	// For // comment, return sequence of // lines.
+	var out []*ast.Comment
+	for text != "" {
+		var line string
+		line, text, _ = strings.Cut(text, "\n")
+		if line == "" {
+			line = "//"
+		} else if strings.HasPrefix(line, "\t") {
+			line = "//" + line
+		} else {
+			line = "// " + line
+		}
+		out = append(out, &ast.Comment{
+			Slash: slash,
+			Text:  line,
+		})
+	}
+	if len(directives) > 0 {
+		out = append(out, &ast.Comment{
+			Slash: slash,
+			Text:  "//",
+		})
+		for _, c := range directives {
+			out = append(out, &ast.Comment{
+				Slash: slash,
+				Text:  c.Text,
+			})
+		}
+	}
+	return out
+}
+
+// isDirective reports whether c is a comment directive.
+// See go.dev/issue/37974.
+// This code is also in go/ast.
+func isDirective(c string) bool {
+	// "//line " is a line directive.
+	// "//extern " is for gccgo.
+	// "//export " is for cgo.
+	// (The // has been removed.)
+	if strings.HasPrefix(c, "line ") || strings.HasPrefix(c, "extern ") || strings.HasPrefix(c, "export ") {
+		return true
+	}
+
+	// "//[a-z0-9]+:[a-z0-9]"
+	// (The // has been removed.)
+	colon := strings.Index(c, ":")
+	if colon <= 0 || colon+1 >= len(c) {
+		return false
+	}
+	for i := 0; i <= colon+1; i++ {
+		if i == colon {
+			continue
+		}
+		b := c[i]
+		if !('a' <= b && b <= 'z' || '0' <= b && b <= '9') {
+			return false
+		}
+	}
+	return true
+}
+
+// allStars reports whether text is the interior of an
+// old-style /* */ comment with a star at the start of each line.
+func allStars(text string) bool {
+	for i := 0; i < len(text); i++ {
+		if text[i] == '\n' {
+			j := i + 1
+			for j < len(text) && (text[j] == ' ' || text[j] == '\t') {
+				j++
+			}
+			if j < len(text) && text[j] != '*' {
+				return false
+			}
+		}
+	}
+	return true
+}