// Copyright 2019 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 golang

// This file defines the behavior of the "Add test for FUNC" command.

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"go/ast"
	"go/format"
	"go/types"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"text/template"
	"unicode"

	goastutil "golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/gopls/internal/cache"
	"golang.org/x/tools/gopls/internal/cache/metadata"
	"golang.org/x/tools/gopls/internal/cache/parsego"
	"golang.org/x/tools/gopls/internal/protocol"
	"golang.org/x/tools/gopls/internal/util/moremaps"
	"golang.org/x/tools/internal/astutil"
	"golang.org/x/tools/internal/imports"
	"golang.org/x/tools/internal/typesinternal"
)

const testTmplString = `
func {{.TestFuncName}}(t *{{.TestingPackageName}}.T) {
	{{- /* Test cases struct declaration and empty initialization. */}}
	tests := []struct {
		name string // description of this test case

		{{- $commentPrinted := false }}
		{{- if and .Receiver .Receiver.Constructor}}
		{{- range .Receiver.Constructor.Args}}
		{{- if .Name}}
		{{- if not $commentPrinted}}
		// Named input parameters for receiver constructor.
		{{- $commentPrinted = true }}
		{{- end}}
		{{.Name}} {{.Type}}
		{{- end}}
		{{- end}}
		{{- end}}

		{{- $commentPrinted := false }}
		{{- range .Func.Args}}
		{{- if .Name}}
		{{- if not $commentPrinted}}
		// Named input parameters for target function.
		{{- $commentPrinted = true }}
		{{- end}}
		{{.Name}} {{.Type}}
		{{- end}}
		{{- end}}

		{{- range $index, $res := .Func.Results}}
		{{- if eq $res.Name "gotErr"}}
		wantErr bool
		{{- else if eq $index 0}}
		want {{$res.Type}}
		{{- else}}
		want{{add $index 1}} {{$res.Type}}
		{{- end}}
		{{- end}}
	}{
		// TODO: Add test cases.
	}

	{{- /* Loop over all the test cases. */}}
	for _, tt := range tests {
		t.Run(tt.name, func(t *{{.TestingPackageName}}.T) {
			{{- /* Constructor or empty initialization. */}}
			{{- if .Receiver}}
			{{- if .Receiver.Constructor}}
			{{- /* Receiver variable by calling constructor. */}}
			{{fieldNames .Receiver.Constructor.Results ""}} := {{if .PackageName}}{{.PackageName}}.{{end}}
			{{- .Receiver.Constructor.Name}}

			{{- /* Constructor input parameters. */ -}}
			(
				{{- range $index, $arg := .Receiver.Constructor.Args}}
				{{- if ne $index 0}}, {{end}}
				{{- if .Name}}tt.{{.Name}}{{else}}{{.Value}}{{end}}
				{{- end -}}
				{{- if and .Receiver.Constructor.IsVariadic (not .Receiver.Constructor.IsUnusedVariadic) }}...{{- end -}}
			)

			{{- /* Handles the error return from constructor. */}}
			{{- $last := last .Receiver.Constructor.Results}}
			{{- if eq $last.Type "error"}}
			if err != nil {
				t.Fatalf("could not construct receiver type: %v", err)
			}
			{{- end}}
			{{- else}}
			{{- /* Receiver variable declaration. */}}
			// TODO: construct the receiver type.
			var {{.Receiver.Var.Name}} {{.Receiver.Var.Type}}
			{{- end}}
			{{- end}}

			{{- /* Got variables. */}}
			{{if .Func.Results}}{{fieldNames .Func.Results ""}} := {{end}}

			{{- /* Call expression. */}}
			{{- if .Receiver}}{{/* Call method by VAR.METHOD. */}}
			{{- .Receiver.Var.Name}}.
			{{- else if .PackageName}}{{/* Call function by PACKAGE.FUNC. */}}
			{{- .PackageName}}.
			{{- end}}{{.Func.Name}}

			{{- /* Input parameters. */ -}}
			(
				{{- range $index, $arg := .Func.Args}}
				{{- if ne $index 0}}, {{end}}
				{{- if .Name}}tt.{{.Name}}{{else}}{{.Value}}{{end}}
				{{- end -}}
				{{- if and .Func.IsVariadic (not .Func.IsUnusedVariadic) }}...{{- end -}}
			)

			{{- /* Handles the returned error before the rest of return value. */}}
			{{- $last := last .Func.Results}}
			{{- if eq $last.Type "error"}}
			if gotErr != nil {
				if !tt.wantErr {
					t.Errorf("{{$.Func.Name}}() failed: %v", gotErr)
				}
				return
			}
			if tt.wantErr {
				t.Fatal("{{$.Func.Name}}() succeeded unexpectedly")
			}
			{{- end}}

			{{- /* Compare the returned values except for the last returned error. */}}
			{{- if or (and .Func.Results (ne $last.Type "error")) (and (gt (len .Func.Results) 1) (eq $last.Type "error"))}}
			// TODO: update the condition below to compare got with tt.want.
			{{- range $index, $res := .Func.Results}}
			{{- if ne $res.Name "gotErr"}}
			if true {
				t.Errorf("{{$.Func.Name}}() = %v, want %v", {{.Name}}, tt.{{if eq $index 0}}want{{else}}want{{add $index 1}}{{end}})
			}
			{{- end}}
			{{- end}}
			{{- end}}
		})
	}
}
`

// Name is the name of the field this input parameter should reference.
// Value is the expression this input parameter should accept.
//
// Exactly one of Name or Value must be set.
type field struct {
	Name, Type, Value string
}

type function struct {
	Name    string
	Args    []field
	Results []field
	// IsVariadic holds information if the function is variadic.
	// In case of the IsVariadic=true must expand it in the function-under-test call.
	IsVariadic bool
	// IsUnusedVariadic holds information if the function's variadic args is not used.
	// In this case ... must be skipped.
	IsUnusedVariadic bool
}

type receiver struct {
	// Var is the name and type of the receiver variable.
	Var field
	// Constructor holds information about the constructor for the receiver type.
	// If no qualified constructor is found, this field will be nil.
	Constructor *function
}

type testInfo struct {
	// TestingPackageName is the package name should be used when referencing
	// package "testing"
	TestingPackageName string
	// PackageName is the package name the target function/method is declared from.
	PackageName  string
	TestFuncName string
	// Func holds information about the function or method being tested.
	Func function
	// Receiver holds information about the receiver of the function or method
	// being tested.
	// This field is nil for functions and non-nil for methods.
	Receiver *receiver
}

var testTmpl = template.Must(template.New("test").Funcs(template.FuncMap{
	"add": func(a, b int) int { return a + b },
	"last": func(slice []field) field {
		if len(slice) == 0 {
			return field{}
		}
		return slice[len(slice)-1]
	},
	"fieldNames": func(fields []field, qualifier string) (res string) {
		var names []string
		for _, f := range fields {
			names = append(names, qualifier+f.Name)
		}
		return strings.Join(names, ", ")
	},
}).Parse(testTmplString))

// AddTestForFunc adds a test for the function enclosing the given input range.
// It creates a _test.go file if one does not already exist.
// It returns the required text edits and the predicted location of the new test
// function, which is only valid after the edits have been successfully applied.
func AddTestForFunc(ctx context.Context, snapshot *cache.Snapshot, loc protocol.Location) (changes []protocol.DocumentChange, show *protocol.Location, _ error) {
	pkg, pgf, err := NarrowestPackageForFile(ctx, snapshot, loc.URI)
	if err != nil {
		return nil, nil, err
	}

	if metadata.IsCommandLineArguments(pkg.Metadata().ID) {
		return nil, nil, fmt.Errorf("current file in command-line-arguments package")
	}

	if errors := pkg.ParseErrors(); len(errors) > 0 {
		return nil, nil, fmt.Errorf("package has parse errors: %v", errors[0])
	}
	if errors := pkg.TypeErrors(); len(errors) > 0 {
		return nil, nil, fmt.Errorf("package has type errors: %v", errors[0])
	}

	// All three maps map the path of an imported package to
	// the local name if explicit or "" otherwise.
	var (
		fileImports  map[string]string         // imports in foo.go file
		testImports  map[string]string         // imports in foo_test.go file
		extraImports = make(map[string]string) // imports to add to test file
	)

	collectImports := func(file *ast.File) (map[string]string, error) {
		imps := make(map[string]string)
		for _, spec := range file.Imports {
			// TODO(hxjiang): support dot imports.
			if spec.Name != nil && spec.Name.Name == "." {
				return nil, fmt.Errorf("\"add test for func\" does not support files containing dot imports")
			}
			path, err := strconv.Unquote(spec.Path.Value)
			if err != nil {
				return nil, err
			}
			if spec.Name != nil {
				if spec.Name.Name == "_" {
					continue
				}
				imps[path] = spec.Name.Name
			} else {
				imps[path] = ""
			}
		}
		return imps, nil
	}

	// Collect all the imports from the x.go, keep track of the local package name.
	if fileImports, err = collectImports(pgf.File); err != nil {
		return nil, nil, err
	}

	testBase := strings.TrimSuffix(loc.URI.Base(), ".go") + "_test.go"
	goTestFileURI := protocol.URIFromPath(filepath.Join(loc.URI.DirPath(), testBase))

	testFH, err := snapshot.ReadFile(ctx, goTestFileURI)
	if err != nil {
		return nil, nil, err
	}

	// TODO(hxjiang): use a fresh name if the same test function name already
	// exist.

	var (
		eofRange protocol.Range // empty selection at end of new file
		// edits contains all the text edits to be applied to the test file.
		edits []protocol.TextEdit
		// xtest indicates whether the test file use package x or x_test.
		// TODO(hxjiang): We can discuss the option to interpret the user's
		// intention by which function they are selecting. Have one file for
		// x_test package testing, one file for x package testing.
		xtest = true
	)

	start, end, err := pgf.RangePos(loc.Range)
	if err != nil {
		return nil, nil, err
	}

	path, _ := goastutil.PathEnclosingInterval(pgf.File, start, end)
	if len(path) < 2 {
		return nil, nil, fmt.Errorf("no enclosing function")
	}

	decl, ok := path[len(path)-2].(*ast.FuncDecl)
	if !ok {
		return nil, nil, fmt.Errorf("no enclosing function")
	}

	fn := pkg.TypesInfo().Defs[decl.Name].(*types.Func)
	sig := fn.Signature()

	testPGF, err := snapshot.ParseGo(ctx, testFH, parsego.Header)
	if err != nil {
		if !errors.Is(err, os.ErrNotExist) {
			return nil, nil, err
		}
		changes = append(changes, protocol.DocumentChangeCreate(goTestFileURI))

		// header is the buffer containing the text to add to the beginning of the file.
		var header bytes.Buffer

		// If this test file was created by the gopls, add a copyright header and
		// package decl based on the originating file.
		// Search for something that looks like a copyright header, to replicate
		// in the new file.
		if c := CopyrightComment(pgf.File); c != nil {
			text, err := pgf.NodeText(c)
			if err != nil {
				return nil, nil, err
			}
			header.Write(text)
			// One empty line between copyright header and following.
			header.WriteString("\n\n")
		}

		// If this test file was created by gopls, add build constraints
		// matching the non-test file.
		if c := buildConstraintComment(pgf.File); c != nil {
			text, err := pgf.NodeText(c)
			if err != nil {
				return nil, nil, err
			}
			header.Write(text)
			// One empty line between build constraint and following.
			header.WriteString("\n\n")
		}

		// Determine if a new test file should use in-package test (package x)
		// or external test (package x_test). If any of the function parameters
		// reference an unexported object, we cannot write out test cases from
		// an x_test package.
		externalTestOK := func() bool {
			if !fn.Exported() {
				return false
			}
			if fn.Signature().Recv() != nil {
				if _, ident, _ := astutil.UnpackRecv(decl.Recv.List[0].Type); ident == nil || !ident.IsExported() {
					return false
				}
			}
			refsUnexported := false
			ast.Inspect(decl, func(n ast.Node) bool {
				// The original function refs to an unexported object from the
				// same package, so further inspection is unnecessary.
				if refsUnexported {
					return false
				}
				switch t := n.(type) {
				case *ast.BlockStmt:
					// Avoid inspect the function body.
					return false
				case *ast.Ident:
					// Use test variant (package foo) if the function signature
					// references any unexported objects (like types or
					// constants) from the same package.
					// Note: types.PkgName is excluded from this check as it's
					// always defined in the same package.
					if obj, ok := pkg.TypesInfo().Uses[t]; ok && !obj.Exported() && obj.Pkg() == pkg.Types() && !is[*types.PkgName](obj) {
						refsUnexported = true
					}
					return false
				default:
					return true
				}
			})
			return !refsUnexported
		}

		xtest = externalTestOK()
		if xtest {
			fmt.Fprintf(&header, "package %s_test\n", pkg.Types().Name())
		} else {
			fmt.Fprintf(&header, "package %s\n", pkg.Types().Name())
		}

		// Write the copyright and package decl to the beginning of the file.
		edits = append(edits, protocol.TextEdit{
			Range:   protocol.Range{},
			NewText: header.String(),
		})
	} else { // existing _test.go file.
		file := testPGF.File
		if !file.Name.NamePos.IsValid() {
			return nil, nil, fmt.Errorf("missing package declaration")
		}
		switch file.Name.Name {
		case pgf.File.Name.Name:
			xtest = false
		case pgf.File.Name.Name + "_test":
			xtest = true
		default:
			return nil, nil, fmt.Errorf("invalid package declaration %q in test file %q", file.Name, testPGF)
		}

		eofRange, err = testPGF.PosRange(file.FileEnd, file.FileEnd)
		if err != nil {
			return nil, nil, err
		}

		// Collect all the imports from the foo_test.go.
		if testImports, err = collectImports(file); err != nil {
			return nil, nil, err
		}
	}

	// qual qualifier determines the correct package name to use for a type in
	// foo_test.go. It does this by:
	// - Consult imports map from test file foo_test.go.
	// - If not found, consult imports map from original file foo.go.
	// If the package is not imported in test file foo_test.go, it is added to
	// extraImports map.
	qual := func(p *types.Package) string {
		// References from an in-package test should not be qualified.
		if !xtest && p == pkg.Types() {
			return ""
		}
		// Prefer using the package name if already defined in foo_test.go
		if local, ok := testImports[p.Path()]; ok {
			if local != "" {
				return local
			} else {
				return p.Name()
			}
		}
		// TODO(hxjiang): we should consult the scope of the test package to
		// ensure these new imports do not shadow any package-level names.
		// Prefer the local import name (if any) used in the package under test.
		if local, ok := fileImports[p.Path()]; ok && local != "" {
			extraImports[p.Path()] = local
			return local
		}
		// Fall back to the package name since there is no renaming.
		extraImports[p.Path()] = ""
		return p.Name()
	}

	if xtest {
		// Reject if function/method is unexported.
		if !fn.Exported() {
			return nil, nil, fmt.Errorf("cannot add test of unexported function %s to external test package %s_test", decl.Name, pgf.File.Name)
		}

		// Reject if receiver is unexported.
		if sig.Recv() != nil {
			if _, ident, _ := astutil.UnpackRecv(decl.Recv.List[0].Type); ident == nil || !ident.IsExported() {
				return nil, nil, fmt.Errorf("cannot add external test for method %s.%s as receiver type is not exported", ident.Name, decl.Name)
			}
		}
		// TODO(hxjiang): reject if the any input parameter type is unexported.
		// TODO(hxjiang): reject if any return value type is unexported. Explore
		// the option to drop the return value if the type is unexported.
	}

	testName, err := testName(fn)
	if err != nil {
		return nil, nil, err
	}

	data := testInfo{
		TestingPackageName: qual(types.NewPackage("testing", "testing")),
		PackageName:        qual(pkg.Types()),
		TestFuncName:       testName,
		Func: function{
			Name:       fn.Name(),
			IsVariadic: sig.Variadic(),
		},
	}

	isContextType := func(t types.Type) bool {
		return typesinternal.IsTypeNamed(t, "context", "Context")
	}

	isUnusedParameter := func(name string) bool {
		return name == "" || name == "_"
	}

	for i := range sig.Params().Len() {
		param := sig.Params().At(i)
		name, typ := param.Name(), param.Type()
		f := field{Type: types.TypeString(typ, qual)}
		if i == 0 && isContextType(typ) {
			f.Value = qual(types.NewPackage("context", "context")) + ".Background()"
		} else if isUnusedParameter(name) && data.Func.IsVariadic && sig.Params().Len()-1 == i {
			// The last argument is the variadic argument, and it's not used in the function body,
			// so we don't need to render it in the test case struct.
			data.Func.IsUnusedVariadic = true
			continue
		} else if isUnusedParameter(name) {
			f.Value, _ = typesinternal.ZeroString(typ, qual)
		} else {
			f.Name = name
		}
		data.Func.Args = append(data.Func.Args, f)
	}

	for i := range sig.Results().Len() {
		typ := sig.Results().At(i).Type()
		var name string
		if i == sig.Results().Len()-1 && types.Identical(typ, errorType) {
			name = "gotErr"
		} else if i == 0 {
			name = "got"
		} else {
			name = fmt.Sprintf("got%d", i+1)
		}
		data.Func.Results = append(data.Func.Results, field{
			Name: name,
			Type: types.TypeString(typ, qual),
		})
	}

	if sig.Recv() != nil {
		// Find the preferred type for the receiver. We don't use
		// typesinternal.ReceiverNamed here as we want to preserve aliases.
		recvType := sig.Recv().Type()
		if ptr, ok := recvType.(*types.Pointer); ok {
			recvType = ptr.Elem()
		}

		t, ok := recvType.(typesinternal.NamedOrAlias)
		if !ok {
			return nil, nil, fmt.Errorf("the receiver type is neither named type nor alias type")
		}

		var varName string
		{
			var possibleNames []string // list of candidates, preferring earlier entries.
			if len(sig.Recv().Name()) > 0 {
				possibleNames = append(possibleNames,
					sig.Recv().Name(),            // receiver name.
					string(sig.Recv().Name()[0]), // first character of receiver name.
				)
			}
			possibleNames = append(possibleNames,
				string(t.Obj().Name()[0]), // first character of receiver type name.
			)
			if len(t.Obj().Name()) >= 2 {
				possibleNames = append(possibleNames,
					string(t.Obj().Name()[:2]), // first two character of receiver type name.
				)
			}
			var camelCase []rune
			for i, s := range t.Obj().Name() {
				if i == 0 || unicode.IsUpper(s) {
					camelCase = append(camelCase, s)
				}
			}
			possibleNames = append(possibleNames,
				string(camelCase), // captalized initials.
			)
			for _, name := range possibleNames {
				name = strings.ToLower(name)
				if name == "" || name == "t" || name == "tt" {
					continue
				}
				varName = name
				break
			}
			if varName == "" {
				varName = "r" // default as "r" for "receiver".
			}
		}

		data.Receiver = &receiver{
			Var: field{
				Name: varName,
				Type: types.TypeString(recvType, qual),
			},
		}

		// constructor is the selected constructor for type T.
		var constructor *types.Func

		// When finding the qualified constructor, the function should return the
		// any type whose named type is the same type as T's named type.
		_, wantType := typesinternal.ReceiverNamed(sig.Recv())
		for _, name := range pkg.Types().Scope().Names() {
			f, ok := pkg.Types().Scope().Lookup(name).(*types.Func)
			if !ok {
				continue
			}
			if f.Signature().Recv() != nil {
				continue
			}
			// Unexported constructor is not visible in x_test package.
			if xtest && !f.Exported() {
				continue
			}
			// Only allow constructors returning T, T, (T, error), or (T, error).
			if f.Signature().Results().Len() > 2 || f.Signature().Results().Len() == 0 {
				continue
			}

			_, gotType := typesinternal.ReceiverNamed(f.Signature().Results().At(0))
			if gotType == nil || !types.Identical(gotType, wantType) {
				continue
			}

			if f.Signature().Results().Len() == 2 && !types.Identical(f.Signature().Results().At(1).Type(), errorType) {
				continue
			}

			if constructor == nil {
				constructor = f
			}

			// Functions named NewType are prioritized as constructors over other
			// functions that match only the signature criteria.
			if strings.EqualFold(strings.ToLower(f.Name()), strings.ToLower("new"+t.Obj().Name())) {
				constructor = f
			}
		}

		if constructor != nil {
			data.Receiver.Constructor = &function{
				Name:       constructor.Name(),
				IsVariadic: constructor.Signature().Variadic(),
			}
			for i := range constructor.Signature().Params().Len() {
				param := constructor.Signature().Params().At(i)
				name, typ := param.Name(), param.Type()
				f := field{Type: types.TypeString(typ, qual)}
				if i == 0 && isContextType(typ) {
					f.Value = qual(types.NewPackage("context", "context")) + ".Background()"
				} else if isUnusedParameter(name) && data.Receiver.Constructor.IsVariadic && constructor.Signature().Params().Len()-1 == i {
					// The last argument is the variadic argument, and it's not used in the function body,
					// so we don't need to render it in the test case struct.
					data.Receiver.Constructor.IsUnusedVariadic = true
					continue
				} else if isUnusedParameter(name) {
					f.Value, _ = typesinternal.ZeroString(typ, qual)
				} else {
					f.Name = name
				}
				data.Receiver.Constructor.Args = append(data.Receiver.Constructor.Args, f)
			}
			for i := range constructor.Signature().Results().Len() {
				typ := constructor.Signature().Results().At(i).Type()
				var name string
				if i == 0 {
					// The first return value must be of type T, *T, or a type whose named
					// type is the same as named type of T.
					name = varName
				} else if i == constructor.Signature().Results().Len()-1 && types.Identical(typ, errorType) {
					name = "err"
				} else {
					// Drop any return values beyond the first and the last.
					// e.g., "f, _, _, err := NewFoo()".
					name = "_"
				}
				data.Receiver.Constructor.Results = append(data.Receiver.Constructor.Results, field{
					Name: name,
					Type: types.TypeString(typ, qual),
				})
			}
		}
	}

	// Resolves duplicate parameter names between the function and its
	// receiver's constructor. It adds prefix to the constructor's parameters
	// until no conflicts remain.
	if data.Receiver != nil && data.Receiver.Constructor != nil {
		seen := map[string]bool{}
		for _, f := range data.Func.Args {
			if f.Name == "" {
				continue
			}
			seen[f.Name] = true
		}

		// "" for no change, "c" for constructor, "i" for input.
		for _, prefix := range []string{"", "c", "c_", "i", "i_"} {
			conflict := false
			for _, f := range data.Receiver.Constructor.Args {
				if f.Name == "" {
					continue
				}
				if seen[prefix+f.Name] {
					conflict = true
					break
				}
			}
			if !conflict {
				for i, f := range data.Receiver.Constructor.Args {
					if f.Name == "" {
						continue
					}
					data.Receiver.Constructor.Args[i].Name = prefix + data.Receiver.Constructor.Args[i].Name
				}
				break
			}
		}
	}

	// Compute edits to update imports.
	//
	// If we're adding to an existing test file, we need to adjust existing
	// imports. Otherwise, we can simply write out the imports to the new file.
	if testPGF != nil {
		var importFixes []*imports.ImportFix
		for path, name := range extraImports {
			importFixes = append(importFixes, &imports.ImportFix{
				StmtInfo: imports.ImportInfo{
					ImportPath: path,
					Name:       name,
				},
				FixType: imports.AddImport,
			})
		}
		importEdits, err := ComputeImportFixEdits(snapshot.Options().Local, testPGF.Src, importFixes...)
		if err != nil {
			return nil, nil, fmt.Errorf("could not compute the import fix edits: %w", err)
		}
		edits = append(edits, importEdits...)
	} else {
		var importsBuffer bytes.Buffer
		if len(extraImports) == 1 {
			importsBuffer.WriteString("\nimport ")
			for path, name := range extraImports {
				if name != "" {
					importsBuffer.WriteString(name + " ")
				}
				importsBuffer.WriteString(fmt.Sprintf("\"%s\"\n", path))
			}
		} else {
			importsBuffer.WriteString("\nimport(")
			// Sort for determinism.
			for path, name := range moremaps.Sorted(extraImports) {
				importsBuffer.WriteString("\n\t")
				if name != "" {
					importsBuffer.WriteString(name + " ")
				}
				importsBuffer.WriteString(fmt.Sprintf("\"%s\"", path))
			}
			importsBuffer.WriteString("\n)\n")
		}
		edits = append(edits, protocol.TextEdit{
			Range:   protocol.Range{},
			NewText: importsBuffer.String(),
		})
	}

	var test bytes.Buffer
	if err := testTmpl.Execute(&test, data); err != nil {
		return nil, nil, err
	}

	formatted, err := format.Source(test.Bytes())
	if err != nil {
		return nil, nil, err
	}

	edits = append(edits,
		protocol.TextEdit{
			Range:   eofRange,
			NewText: string(formatted),
		},
	)

	// Show the line of generated test function.
	{
		line := eofRange.Start.Line
		for i := range len(edits) - 1 { // last edits is the func decl
			e := edits[i]
			oldLines := e.Range.End.Line - e.Range.Start.Line
			newLines := uint32(strings.Count(e.NewText, "\n"))
			line += (newLines - oldLines)
		}
		show = &protocol.Location{
			URI: testFH.URI(),
			Range: protocol.Range{
				// Test function template have a new line at beginning.
				Start: protocol.Position{Line: line + 1},
				End:   protocol.Position{Line: line + 1},
			},
		}
	}

	return append(changes, protocol.DocumentChangeEdit(testFH, edits)), show, nil
}

// testName returns the name of the function to use for the new function that
// tests fn.
// Returns empty string if the fn is ill typed or nil.
func testName(fn *types.Func) (string, error) {
	if fn == nil {
		return "", fmt.Errorf("input nil function")
	}
	testName := "Test"
	if recv := fn.Signature().Recv(); recv != nil { // method declaration.
		// Retrieve the unpointered receiver type to ensure the test name is based
		// on the topmost alias or named type, not the alias' RHS type (potentially
		// unexported) type.
		// For example:
		// type Foo = foo // Foo is an exported alias for the unexported type foo
		recvType := recv.Type()
		if ptr, ok := recv.Type().(*types.Pointer); ok {
			recvType = ptr.Elem()
		}

		t, ok := recvType.(typesinternal.NamedOrAlias)
		if !ok {
			return "", fmt.Errorf("receiver type is not named type or alias type")
		}

		if !t.Obj().Exported() {
			testName += "_"
		}

		testName += t.Obj().Name() + "_"
	} else if !fn.Exported() { // unexported function declaration.
		testName += "_"
	}
	return testName + fn.Name(), nil
}
