gofix: test and fix missorted renames

Also introduce a new insertion heuristic:
insert new import next to existing import
with the longest matching prefix.

R=golang-dev, adg, gri
CC=golang-dev
https://golang.org/cl/5412053

src/cmd/gofix/fix.go

@@ -552,6 +552,15 @@ func renameTop(f *ast.File, old, new string) bool {
 	return fixed
 }
 
+// matchLen returns the length of the longest prefix shared by x and y.
+func matchLen(x, y string) int {
+	i := 0
+	for i < len(x) && i < len(y) && x[i] == y[i] {
+		i++
+	}
+	return i
+}
+
 // addImport adds the import path to the file f, if absent.
 func addImport(f *ast.File, ipath string) (added bool) {
 	if imports(f, ipath) {
@@ -572,53 +581,65 @@ func addImport(f *ast.File, ipath string) (added bool) {
 		},
 	}
 
-	var impdecl *ast.GenDecl
-
 	// Find an import decl to add to.
-	var lastImport = -1
+	var (
+		bestMatch  = -1
+		lastImport = -1
+		impDecl    *ast.GenDecl
+		impIndex   = -1
+	)
 	for i, decl := range f.Decls {
 		gen, ok := decl.(*ast.GenDecl)
-
 		if ok && gen.Tok == token.IMPORT {
 			lastImport = i
 			// Do not add to import "C", to avoid disrupting the
 			// association with its doc comment, breaking cgo.
-			if !declImports(gen, "C") {
-				impdecl = gen
-				break
+			if declImports(gen, "C") {
+				continue
+			}
+
+			// Compute longest shared prefix with imports in this block.
+			for j, spec := range gen.Specs {
+				impspec := spec.(*ast.ImportSpec)
+				n := matchLen(importPath(impspec), ipath)
+				if n > bestMatch {
+					bestMatch = n
+					impDecl = gen
+					impIndex = j
+				}
 			}
 		}
 	}
 
-	// No import decl found.  Add one.
-	if impdecl == nil {
-		impdecl = &ast.GenDecl{
+	// If no import decl found, add one after the last import.
+	if impDecl == nil {
+		impDecl = &ast.GenDecl{
 			Tok: token.IMPORT,
 		}
 		f.Decls = append(f.Decls, nil)
 		copy(f.Decls[lastImport+2:], f.Decls[lastImport+1:])
-		f.Decls[lastImport+1] = impdecl
+		f.Decls[lastImport+1] = impDecl
 	}
 
 	// Ensure the import decl has parentheses, if needed.
-	if len(impdecl.Specs) > 0 && !impdecl.Lparen.IsValid() {
-		impdecl.Lparen = impdecl.Pos()
+	if len(impDecl.Specs) > 0 && !impDecl.Lparen.IsValid() {
+		impDecl.Lparen = impDecl.Pos()
 	}
 
-	// Assume the import paths are alphabetically ordered.
-	// If they are not, the result is ugly, but legal.
-	insertAt := len(impdecl.Specs) // default to end of specs
-	for i, spec := range impdecl.Specs {
-		impspec := spec.(*ast.ImportSpec)
-		if importPath(impspec) > ipath {
-			insertAt = i
-			break
-		}
+	insertAt := impIndex + 1
+	if insertAt == 0 {
+		insertAt = len(impDecl.Specs)
+	}
+	impDecl.Specs = append(impDecl.Specs, nil)
+	copy(impDecl.Specs[insertAt+1:], impDecl.Specs[insertAt:])
+	impDecl.Specs[insertAt] = newImport
+	if insertAt > 0 {
+		// Assign same position as the previous import,
+		// so that the sorter sees it as being in the same block.
+		prev := impDecl.Specs[insertAt-1]
+		newImport.Path.ValuePos = prev.Pos()
+		newImport.EndPos = prev.Pos()
 	}
-
-	impdecl.Specs = append(impdecl.Specs, nil)
-	copy(impdecl.Specs[insertAt+1:], impdecl.Specs[insertAt:])
-	impdecl.Specs[insertAt] = newImport
 
 	f.Imports = append(f.Imports, newImport)
 	return true
@@ -682,6 +703,9 @@ func rewriteImport(f *ast.File, oldPath, newPath string) (rewrote bool) {
 	for _, imp := range f.Imports {
 		if importPath(imp) == oldPath {
 			rewrote = true
+			// record old End, beacuse the default is to compute
+			// it using the length of imp.Path.Value.
+			imp.EndPos = imp.End()
 			imp.Path.Value = strconv.Quote(newPath)
 		}
 	}

src/cmd/gofix/go1pkgrename_test.go

@@ -93,6 +93,33 @@ import poot "html/template"
 
 var _ = cmplx.Sin
 var _ = poot.Poot
+`,
+	},
+	{
+		Name: "go1rename.2",
+		In: `package foo
+
+import (
+	"fmt"
+	"http"
+	"url"
+
+	"google/secret/project/go"
+)
+
+func main() {}
+`,
+		Out: `package foo
+
+import (
+	"fmt"
+	"net/http"
+	"net/url"
+
+	"google/secret/project/go"
+)
+
+func main() {}
 `,
 	},
 }

src/cmd/gofix/import_test.go

@@ -348,17 +348,54 @@ import (
 )
 
 var addr = flag.String("addr", ":1718", "http service address") // Q=17, R=18
+`,
+	},
+	{
+		Name: "import.3",
+		Fn:   addImportFn("x/y/z", "x/a/c"),
+		In: `package main
+
+// Comment
+import "C"
+
+import (
+	"a"
+	"b"
+
+	"x/w"
+
+	"d/f"
+)
+`,
+		Out: `package main
+
+// Comment
+import "C"
+
+import (
+	"a"
+	"b"
+
+	"x/a/c"
+	"x/w"
+	"x/y/z"
+
+	"d/f"
+)
 `,
 	},
 }
 
-func addImportFn(path string) func(*ast.File) bool {
+func addImportFn(path ...string) func(*ast.File) bool {
 	return func(f *ast.File) bool {
-		if !imports(f, path) {
-			addImport(f, path)
-			return true
+		fixed := false
+		for _, p := range path {
+			if !imports(f, p) {
+				addImport(f, p)
+				fixed = true
+			}
 		}
-		return false
+		return fixed
 	}
 }