go/doc: group play example imports

When synthesizing a program from a playable example, preserve
the grouping of imports. That is, maintain blank lines between
imports while removing unused ones.

People are used to having those groups because that is what goimports
does.  It's disconcerting to see the all imports placed together, as
the existing code does, especially when the user has already grouped
them.

For an example, see golang/go#43658.

This is an improvement to a fix in pkgsite's fork of go/doc
(https://go.googlesource.com/pkgsite/+/7b10ef3861af4a863bf215f63b6de94c681d5af0/internal/godoc/internal/doc/example_pkgsite.go#405).
Here I've managed to avoid using a token.FileSet.

Change-Id: I65605e6dd53d742a3fe1210c3f982b54e3706198
Reviewed-on: https://go-review.googlesource.com/c/go/+/384837
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

src/go/doc/example.go

@@ -270,8 +270,22 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 	// Use unresolved identifiers to determine the imports used by this
 	// example. The heuristic assumes package names match base import
 	// paths for imports w/o renames (should be good enough most of the time).
-	namedImports := make(map[string]string) // [name]path
+	var namedImports []ast.Spec
 	var blankImports []ast.Spec // _ imports
+
+	// To preserve the blank lines between groups of imports, find the
+	// start position of each group, and assign that position to all
+	// imports from that group.
+	groupStarts := findImportGroupStarts(file.Imports)
+	groupStart := func(s *ast.ImportSpec) token.Pos {
+		for i, start := range groupStarts {
+			if s.Path.ValuePos < start {
+				return groupStarts[i-1]
+			}
+		}
+		return groupStarts[len(groupStarts)-1]
+	}
+
 	for _, s := range file.Imports {
 		p, err := strconv.Unquote(s.Path.Value)
 		if err != nil {
@@ -295,7 +309,12 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 			}
 		}
 		if unresolved[n] {
-			namedImports[n] = p
+			// Copy the spec and its path to avoid modifying the original.
+			spec := *s
+			path := *s.Path
+			spec.Path = &path
+			spec.Path.ValuePos = groupStart(&spec)
+			namedImports = append(namedImports, &spec)
 			delete(unresolved, n)
 		}
 	}
@@ -345,14 +364,7 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 		Lparen: 1, // Need non-zero Lparen and Rparen so that printer
 		Rparen: 1, // treats this as a factored import.
 	}
-	for n, p := range namedImports {
-		s := &ast.ImportSpec{Path: &ast.BasicLit{Value: strconv.Quote(p)}}
-		if path.Base(p) != n {
-			s.Name = ast.NewIdent(n)
-		}
-		importDecl.Specs = append(importDecl.Specs, s)
-	}
-	importDecl.Specs = append(importDecl.Specs, blankImports...)
+	importDecl.Specs = append(namedImports, blankImports...)
 
 	// Synthesize main function.
 	funcDecl := &ast.FuncDecl{
@@ -369,7 +381,6 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 	sort.Slice(decls, func(i, j int) bool {
 		return decls[i].Pos() < decls[j].Pos()
 	})
-
 	sort.Slice(comments, func(i, j int) bool {
 		return comments[i].Pos() < comments[j].Pos()
 	})
@@ -382,6 +393,41 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 	}
 }
 
+// findImportGroupStarts finds the start positions of each sequence of import
+// specs that are not separated by a blank line.
+func findImportGroupStarts(imps []*ast.ImportSpec) []token.Pos {
+	startImps := findImportGroupStarts1(imps)
+	groupStarts := make([]token.Pos, len(startImps))
+	for i, imp := range startImps {
+		groupStarts[i] = imp.Pos()
+	}
+	return groupStarts
+}
+
+// Helper for findImportGroupStarts to ease testing.
+func findImportGroupStarts1(origImps []*ast.ImportSpec) []*ast.ImportSpec {
+	// Copy to avoid mutation.
+	imps := make([]*ast.ImportSpec, len(origImps))
+	copy(imps, origImps)
+	// Assume the imports are sorted by position.
+	sort.Slice(imps, func(i, j int) bool { return imps[i].Pos() < imps[j].Pos() })
+	// Assume gofmt has been applied, so there is a blank line between adjacent imps
+	// if and only if they are more than 2 positions apart (newline, tab).
+	var groupStarts []*ast.ImportSpec
+	prevEnd := token.Pos(-2)
+	for _, imp := range imps {
+		if imp.Pos()-prevEnd > 2 {
+			groupStarts = append(groupStarts, imp)
+		}
+		prevEnd = imp.End()
+		// Account for end-of-line comments.
+		if imp.Comment != nil {
+			prevEnd = imp.Comment.End()
+		}
+	}
+	return groupStarts
+}
+
 // playExampleFile takes a whole file example and synthesizes a new *ast.File
 // such that the example is function main in package main.
 func playExampleFile(file *ast.File) *ast.File {

src/go/doc/example_internal_test.go

@@ -0,0 +1,121 @@
+// 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 doc
+
+import (
+	"go/parser"
+	"go/token"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestImportGroupStarts(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		in   string
+		want []string // paths of group-starting imports
+	}{
+		{
+			name: "one group",
+			in: `package p
+import (
+	"a"
+	"b"
+	"c"
+	"d"
+)
+`,
+			want: []string{"a"},
+		},
+		{
+			name: "several groups",
+			in: `package p
+import (
+	"a"
+
+	"b"
+	"c"
+
+	"d"
+)
+`,
+			want: []string{"a", "b", "d"},
+		},
+		{
+			name: "extra space",
+			in: `package p
+import (
+	"a"
+
+
+	"b"
+	"c"
+
+
+	"d"
+)
+`,
+			want: []string{"a", "b", "d"},
+		},
+		{
+			name: "line comment",
+			in: `package p
+import (
+	"a" // comment
+	"b" // comment
+
+	"c"
+)`,
+			want: []string{"a", "c"},
+		},
+		{
+			name: "named import",
+			in: `package p
+import (
+	"a"
+	n "b"
+
+	m "c"
+	"d"
+)`,
+			want: []string{"a", "c"},
+		},
+		{
+			name: "blank import",
+			in: `package p
+import (
+	"a"
+
+	_ "b"
+
+	_ "c"
+	"d"
+)`,
+			want: []string{"a", "b", "c"},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			fset := token.NewFileSet()
+			file, err := parser.ParseFile(fset, "test.go", strings.NewReader(test.in), parser.ParseComments)
+			if err != nil {
+				t.Fatal(err)
+			}
+			imps := findImportGroupStarts1(file.Imports)
+			got := make([]string, len(imps))
+			for i, imp := range imps {
+				got[i], err = strconv.Unquote(imp.Path.Value)
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+			if !reflect.DeepEqual(got, test.want) {
+				t.Errorf("got %v, want %v", got, test.want)
+			}
+		})
+	}
+
+}

src/go/doc/example_test.go

@@ -522,6 +522,7 @@ func TestExampleEmpty(t *testing.T) {
 }
 
 func formatFile(t *testing.T, fset *token.FileSet, n *ast.File) string {
+	t.Helper()
 	if n == nil {
 		return "<nil>"
 	}