lint.go

// Copyright (c) 2013 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 or at
// https://developers.google.com/open-source/licenses/bsd.

// Package lint contains a linter for Go source code.
package lint // import "golang.org/x/lint"

import (
	"bufio"
	"bytes"
	"fmt"
	"go/ast"
	"go/parser"
	"go/printer"
	"go/token"
	"go/types"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"unicode"
	"unicode/utf8"

	"golang.org/x/tools/go/ast/astutil"
	"golang.org/x/tools/go/gcexportdata"
)

const styleGuideBase = "https://golang.org/wiki/CodeReviewComments"

// A Linter lints Go source code.
type Linter struct {
	LintExported         bool // run linter on exported types.
	LintPackageComments  bool
	LintImports          bool
	LintBlankImports     bool
	LintNames            bool
	LintVarDecls         bool
	LintElses            bool
	LintRanges           bool
	LintErrorf           bool
	LintErrors           bool
	LintErrorStrings     bool
	LintReceiverNames    bool
	LintIncDec           bool
	LintErrorReturn      bool
	LintUnexportedReturn bool
	LintTimeNames        bool
	LintContextKeyTypes  bool
	LintContextArgs      bool
}

// NewLinter will create an instance of linter with all checks enabled
func NewLinter() *Linter {
	return &Linter{
		LintExported:         true, // run linter on exported types.
		LintPackageComments:  true,
		LintImports:          true,
		LintBlankImports:     true,
		LintNames:            true,
		LintVarDecls:         true,
		LintElses:            true,
		LintRanges:           true,
		LintErrorf:           true,
		LintErrors:           true,
		LintErrorStrings:     true,
		LintReceiverNames:    true,
		LintIncDec:           true,
		LintErrorReturn:      true,
		LintUnexportedReturn: true,
		LintTimeNames:        true,
		LintContextKeyTypes:  true,
		LintContextArgs:      true,
	}
}

// Problem represents a problem in some source code.
type Problem struct {
	Position   token.Position // position in source file
	Text       string         // the prose that describes the problem
	Link       string         // (optional) the link to the style guide for the problem
	Confidence float64        // a value in (0,1] estimating the confidence in this problem's correctness
	LineText   string         // the source line
	Category   string         // a short name for the general category of the problem

	// If the problem has a suggested fix (the minority case),
	// ReplacementLine is a full replacement for the relevant line of the source file.
	ReplacementLine string
}

func (p *Problem) String() string {
	if p.Link != "" {
		return p.Text + "\n\n" + p.Link
	}
	return p.Text
}

type byPosition []Problem

func (p byPosition) Len() int      { return len(p) }
func (p byPosition) Swap(i, j int) { p[i], p[j] = p[j], p[i] }

func (p byPosition) Less(i, j int) bool {
	pi, pj := p[i].Position, p[j].Position

	if pi.Filename != pj.Filename {
		return pi.Filename < pj.Filename
	}
	if pi.Line != pj.Line {
		return pi.Line < pj.Line
	}
	if pi.Column != pj.Column {
		return pi.Column < pj.Column
	}

	return p[i].Text < p[j].Text
}

// Lint lints src.
func (l *Linter) Lint(filename string, src []byte) ([]Problem, error) {
	return l.LintFiles(map[string][]byte{filename: src})
}

// Create a boolean array out of the lint options
func (l *Linter) lintSet() [18]bool {
	return [18]bool{
		l.LintExported,
		l.LintPackageComments,
		l.LintImports,
		l.LintBlankImports,
		l.LintNames,
		l.LintVarDecls,
		l.LintElses,
		l.LintRanges,
		l.LintErrorf,
		l.LintErrors,
		l.LintErrorStrings,
		l.LintReceiverNames,
		l.LintIncDec,
		l.LintErrorReturn,
		l.LintUnexportedReturn,
		l.LintTimeNames,
		l.LintContextKeyTypes,
		l.LintContextArgs,
	}
}

// LintFiles lints a set of files of a single package.
// The argument is a map of filename to source.
func (l *Linter) LintFiles(files map[string][]byte) ([]Problem, error) {
	pkg := &pkg{
		fset:  token.NewFileSet(),
		files: make(map[string]*file),
	}
	var pkgName string
	for filename, src := range files {
		if isGenerated(src) {
			continue // See issue #239
		}
		f, err := parser.ParseFile(pkg.fset, filename, src, parser.ParseComments)
		if err != nil {
			return nil, err
		}
		if pkgName == "" {
			pkgName = f.Name.Name
		} else if f.Name.Name != pkgName {
			return nil, fmt.Errorf("%s is in package %s, not %s", filename, f.Name.Name, pkgName)
		}
		pkg.files[filename] = &file{
			pkg:      pkg,
			f:        f,
			fset:     pkg.fset,
			src:      src,
			filename: filename,
		}
	}
	if len(pkg.files) == 0 {
		return nil, nil
	}
	return pkg.lint(l.lintSet()), nil
}

var (
	genHdr = []byte("// Code generated ")
	genFtr = []byte(" DO NOT EDIT.")
)

// isGenerated reports whether the source file is generated code
// according the rules from https://golang.org/s/generatedcode.
func isGenerated(src []byte) bool {
	sc := bufio.NewScanner(bytes.NewReader(src))
	for sc.Scan() {
		b := sc.Bytes()
		if bytes.HasPrefix(b, genHdr) && bytes.HasSuffix(b, genFtr) && len(b) >= len(genHdr)+len(genFtr) {
			return true
		}
	}
	return false
}

// pkg represents a package being linted.
type pkg struct {
	fset  *token.FileSet
	files map[string]*file

	typesPkg  *types.Package
	typesInfo *types.Info

	// sortable is the set of types in the package that implement sort.Interface.
	sortable map[string]bool
	// main is whether this is a "main" package.
	main bool

	problems []Problem
}

func (p *pkg) lint(flags [18]bool) []Problem {
	if err := p.typeCheck(); err != nil {
		/* TODO(dsymonds): Consider reporting these errors when golint operates on entire packages.
		if e, ok := err.(types.Error); ok {
			pos := p.fset.Position(e.Pos)
			conf := 1.0
			if strings.Contains(e.Msg, "can't find import: ") {
				// Golint is probably being run in a context that doesn't support
				// typechecking (e.g. package files aren't found), so don't warn about it.
				conf = 0
			}
			if conf > 0 {
				p.errorfAt(pos, conf, category("typechecking"), e.Msg)
			}

			// TODO(dsymonds): Abort if !e.Soft?
		}
		*/
	}

	p.scanSortable()
	p.main = p.isMain()

	for _, f := range p.files {
		f.lint(flags)
	}

	sort.Sort(byPosition(p.problems))

	return p.problems
}

// file represents a file being linted.
type file struct {
	pkg      *pkg
	f        *ast.File
	fset     *token.FileSet
	src      []byte
	filename string
}

func (f *file) isTest() bool { return strings.HasSuffix(f.filename, "_test.go") }

func (f *file) lint(flags [18]bool) {
	funcs := []func(){
		f.lintExported,
		f.lintPackageComment,
		f.lintImports,
		f.lintBlankImports,
		f.lintNames,
		f.lintVarDecls,
		f.lintElses,
		f.lintRanges,
		f.lintErrorf,
		f.lintErrors,
		f.lintErrorStrings,
		f.lintReceiverNames,
		f.lintIncDec,
		f.lintErrorReturn,
		f.lintUnexportedReturn,
		f.lintTimeNames,
		f.lintContextKeyTypes,
		f.lintContextArgs,
	}
	if len(funcs) != len(flags) {
		panic("Not enough flags registered") // this should never happen
	}
	for i, b := range flags {
		if b {
			funcs[i]()
		}
	}
}

type link string
type category string

// The variadic arguments may start with link and category types,
// and must end with a format string and any arguments.
// It returns the new Problem.
func (f *file) errorf(n ast.Node, confidence float64, args ...interface{}) *Problem {
	pos := f.fset.Position(n.Pos())
	if pos.Filename == "" {
		pos.Filename = f.filename
	}
	return f.pkg.errorfAt(pos, confidence, args...)
}

func (p *pkg) errorfAt(pos token.Position, confidence float64, args ...interface{}) *Problem {
	problem := Problem{
		Position:   pos,
		Confidence: confidence,
	}
	if pos.Filename != "" {
		// The file might not exist in our mapping if a //line directive was encountered.
		if f, ok := p.files[pos.Filename]; ok {
			problem.LineText = srcLine(f.src, pos)
		}
	}

argLoop:
	for len(args) > 1 { // always leave at least the format string in args
		switch v := args[0].(type) {
		case link:
			problem.Link = string(v)
		case category:
			problem.Category = string(v)
		default:
			break argLoop
		}
		args = args[1:]
	}

	problem.Text = fmt.Sprintf(args[0].(string), args[1:]...)

	p.problems = append(p.problems, problem)
	return &p.problems[len(p.problems)-1]
}

var newImporter = func(fset *token.FileSet) types.ImporterFrom {
	return gcexportdata.NewImporter(fset, make(map[string]*types.Package))
}

func (p *pkg) typeCheck() error {
	config := &types.Config{
		// By setting a no-op error reporter, the type checker does as much work as possible.
		Error:    func(error) {},
		Importer: newImporter(p.fset),
	}
	info := &types.Info{
		Types:  make(map[ast.Expr]types.TypeAndValue),
		Defs:   make(map[*ast.Ident]types.Object),
		Uses:   make(map[*ast.Ident]types.Object),
		Scopes: make(map[ast.Node]*types.Scope),
	}
	var anyFile *file
	var astFiles []*ast.File
	for _, f := range p.files {
		anyFile = f
		astFiles = append(astFiles, f.f)
	}
	pkg, err := config.Check(anyFile.f.Name.Name, p.fset, astFiles, info)
	// Remember the typechecking info, even if config.Check failed,
	// since we will get partial information.
	p.typesPkg = pkg
	p.typesInfo = info
	return err
}

func (p *pkg) typeOf(expr ast.Expr) types.Type {
	if p.typesInfo == nil {
		return nil
	}
	return p.typesInfo.TypeOf(expr)
}

func (p *pkg) isNamedType(typ types.Type, importPath, name string) bool {
	n, ok := typ.(*types.Named)
	if !ok {
		return false
	}
	tn := n.Obj()
	return tn != nil && tn.Pkg() != nil && tn.Pkg().Path() == importPath && tn.Name() == name
}

// scopeOf returns the tightest scope encompassing id.
func (p *pkg) scopeOf(id *ast.Ident) *types.Scope {
	var scope *types.Scope
	if obj := p.typesInfo.ObjectOf(id); obj != nil {
		scope = obj.Parent()
	}
	if scope == p.typesPkg.Scope() {
		// We were given a top-level identifier.
		// Use the file-level scope instead of the package-level scope.
		pos := id.Pos()
		for _, f := range p.files {
			if f.f.Pos() <= pos && pos < f.f.End() {
				scope = p.typesInfo.Scopes[f.f]
				break
			}
		}
	}
	return scope
}

func (p *pkg) scanSortable() {
	p.sortable = make(map[string]bool)

	// bitfield for which methods exist on each type.
	const (
		Len = 1 << iota
		Less
		Swap
	)
	nmap := map[string]int{"Len": Len, "Less": Less, "Swap": Swap}
	has := make(map[string]int)
	for _, f := range p.files {
		f.walk(func(n ast.Node) bool {
			fn, ok := n.(*ast.FuncDecl)
			if !ok || fn.Recv == nil || len(fn.Recv.List) == 0 {
				return true
			}
			// TODO(dsymonds): We could check the signature to be more precise.
			recv := receiverType(fn)
			if i, ok := nmap[fn.Name.Name]; ok {
				has[recv] |= i
			}
			return false
		})
	}
	for typ, ms := range has {
		if ms == Len|Less|Swap {
			p.sortable[typ] = true
		}
	}
}

func (p *pkg) isMain() bool {
	for _, f := range p.files {
		if f.isMain() {
			return true
		}
	}
	return false
}

func (f *file) isMain() bool {
	if f.f.Name.Name == "main" {
		return true
	}
	return false
}

// lintPackageComment checks package comments. It complains if
// there is no package comment, or if it is not of the right form.
// This has a notable false positive in that a package comment
// could rightfully appear in a different file of the same package,
// but that's not easy to fix since this linter is file-oriented.
func (f *file) lintPackageComment() {
	if f.isTest() {
		return
	}

	const ref = styleGuideBase + "#package-comments"
	prefix := "Package " + f.f.Name.Name + " "

	// Look for a detached package comment.
	// First, scan for the last comment that occurs before the "package" keyword.
	var lastCG *ast.CommentGroup
	for _, cg := range f.f.Comments {
		if cg.Pos() > f.f.Package {
			// Gone past "package" keyword.
			break
		}
		lastCG = cg
	}
	if lastCG != nil && strings.HasPrefix(lastCG.Text(), prefix) {
		endPos := f.fset.Position(lastCG.End())
		pkgPos := f.fset.Position(f.f.Package)
		if endPos.Line+1 < pkgPos.Line {
			// There isn't a great place to anchor this error;
			// the start of the blank lines between the doc and the package statement
			// is at least pointing at the location of the problem.
			pos := token.Position{
				Filename: endPos.Filename,
				// Offset not set; it is non-trivial, and doesn't appear to be needed.
				Line:   endPos.Line + 1,
				Column: 1,
			}
			f.pkg.errorfAt(pos, 0.9, link(ref), category("comments"), "package comment is detached; there should be no blank lines between it and the package statement")
			return
		}
	}

	if f.f.Doc == nil {
		f.errorf(f.f, 0.2, link(ref), category("comments"), "should have a package comment, unless it's in another file for this package")
		return
	}
	s := f.f.Doc.Text()
	if ts := strings.TrimLeft(s, " \t"); ts != s {
		f.errorf(f.f.Doc, 1, link(ref), category("comments"), "package comment should not have leading space")
		s = ts
	}
	// Only non-main packages need to keep to this form.
	if !f.pkg.main && !strings.HasPrefix(s, prefix) {
		f.errorf(f.f.Doc, 1, link(ref), category("comments"), `package comment should be of the form "%s..."`, prefix)
	}
}

// lintBlankImports complains if a non-main package has blank imports that are
// not documented.
func (f *file) lintBlankImports() {
	// In package main and in tests, we don't complain about blank imports.
	if f.pkg.main || f.isTest() {
		return
	}

	// The first element of each contiguous group of blank imports should have
	// an explanatory comment of some kind.
	for i, imp := range f.f.Imports {
		pos := f.fset.Position(imp.Pos())

		if !isBlank(imp.Name) {
			continue // Ignore non-blank imports.
		}
		if i > 0 {
			prev := f.f.Imports[i-1]
			prevPos := f.fset.Position(prev.Pos())
			if isBlank(prev.Name) && prevPos.Line+1 == pos.Line {
				continue // A subsequent blank in a group.
			}
		}

		// This is the first blank import of a group.
		if imp.Doc == nil && imp.Comment == nil {
			ref := ""
			f.errorf(imp, 1, link(ref), category("imports"), "a blank import should be only in a main or test package, or have a comment justifying it")
		}
	}
}

// lintImports examines import blocks.
func (f *file) lintImports() {
	for i, is := range f.f.Imports {
		_ = i
		if is.Name != nil && is.Name.Name == "." && !f.isTest() {
			f.errorf(is, 1, link(styleGuideBase+"#import-dot"), category("imports"), "should not use dot imports")
		}

	}
}

const docCommentsLink = styleGuideBase + "#doc-comments"

// lintExported examines the exported names.
// It complains if any required doc comments are missing,
// or if they are not of the right form. The exact rules are in
// lintFuncDoc, lintTypeDoc and lintValueSpecDoc; this function
// also tracks the GenDecl structure being traversed to permit
// doc comments for constants to be on top of the const block.
// It also complains if the names stutter when combined with
// the package name.
func (f *file) lintExported() {
	if f.isTest() {
		return
	}

	var lastGen *ast.GenDecl // last GenDecl entered.

	// Set of GenDecls that have already had missing comments flagged.
	genDeclMissingComments := make(map[*ast.GenDecl]bool)

	f.walk(func(node ast.Node) bool {
		switch v := node.(type) {
		case *ast.GenDecl:
			if v.Tok == token.IMPORT {
				return false
			}
			// token.CONST, token.TYPE or token.VAR
			lastGen = v
			return true
		case *ast.FuncDecl:
			f.lintFuncDoc(v)
			if v.Recv == nil {
				// Only check for stutter on functions, not methods.
				// Method names are not used package-qualified.
				f.checkStutter(v.Name, "func")
			}
			// Don't proceed inside funcs.
			return false
		case *ast.TypeSpec:
			// inside a GenDecl, which usually has the doc
			doc := v.Doc
			if doc == nil {
				doc = lastGen.Doc
			}
			f.lintTypeDoc(v, doc)
			f.checkStutter(v.Name, "type")
			// Don't proceed inside types.
			return false
		case *ast.ValueSpec:
			f.lintValueSpecDoc(v, lastGen, genDeclMissingComments)
			return false
		}
		return true
	})
}

var (
	allCapsRE = regexp.MustCompile(`^[A-Z0-9_]+$`)
	anyCapsRE = regexp.MustCompile(`[A-Z]`)
)

// knownNameExceptions is a set of names that are known to be exempt from naming checks.
// This is usually because they are constrained by having to match names in the
// standard library.
var knownNameExceptions = map[string]bool{
	"LastInsertId": true, // must match database/sql
	"kWh":          true,
}

func isInTopLevel(f *ast.File, ident *ast.Ident) bool {
	path, _ := astutil.PathEnclosingInterval(f, ident.Pos(), ident.End())
	for _, f := range path {
		switch f.(type) {
		case *ast.File, *ast.GenDecl, *ast.ValueSpec, *ast.Ident:
			continue
		}
		return false
	}
	return true
}

// lintNames examines all names in the file.
// It complains if any use underscores or incorrect known initialisms.
func (f *file) lintNames() {
	// Package names need slightly different handling than other names.
	if strings.Contains(f.f.Name.Name, "_") && !strings.HasSuffix(f.f.Name.Name, "_test") {
		f.errorf(f.f, 1, link("http://golang.org/doc/effective_go.html#package-names"), category("naming"), "don't use an underscore in package name")
	}
	if anyCapsRE.MatchString(f.f.Name.Name) {
		f.errorf(f.f, 1, link("http://golang.org/doc/effective_go.html#package-names"), category("mixed-caps"), "don't use MixedCaps in package name; %s should be %s", f.f.Name.Name, strings.ToLower(f.f.Name.Name))
	}

	check := func(id *ast.Ident, thing string) {
		if id.Name == "_" {
			return
		}
		if knownNameExceptions[id.Name] {
			return
		}

		// Handle two common styles from other languages that don't belong in Go.
		if len(id.Name) >= 5 && allCapsRE.MatchString(id.Name) && strings.Contains(id.Name, "_") {
			capCount := 0
			for _, c := range id.Name {
				if 'A' <= c && c <= 'Z' {
					capCount++
				}
			}
			if capCount >= 2 {
				f.errorf(id, 0.8, link(styleGuideBase+"#mixed-caps"), category("naming"), "don't use ALL_CAPS in Go names; use CamelCase")
				return
			}
		}
		if thing == "const" || (thing == "var" && isInTopLevel(f.f, id)) {
			if len(id.Name) > 2 && id.Name[0] == 'k' && id.Name[1] >= 'A' && id.Name[1] <= 'Z' {
				should := string(id.Name[1]+'a'-'A') + id.Name[2:]
				f.errorf(id, 0.8, link(styleGuideBase+"#mixed-caps"), category("naming"), "don't use leading k in Go names; %s %s should be %s", thing, id.Name, should)
			}
		}

		should := lintName(id.Name)
		if id.Name == should {
			return
		}

		if len(id.Name) > 2 && strings.Contains(id.Name[1:], "_") {
			f.errorf(id, 0.9, link("http://golang.org/doc/effective_go.html#mixed-caps"), category("naming"), "don't use underscores in Go names; %s %s should be %s", thing, id.Name, should)
			return
		}
		f.errorf(id, 0.8, link(styleGuideBase+"#initialisms"), category("naming"), "%s %s should be %s", thing, id.Name, should)
	}
	checkList := func(fl *ast.FieldList, thing string) {
		if fl == nil {
			return
		}
		for _, f := range fl.List {
			for _, id := range f.Names {
				check(id, thing)
			}
		}
	}
	f.walk(func(node ast.Node) bool {
		switch v := node.(type) {
		case *ast.AssignStmt:
			if v.Tok == token.ASSIGN {
				return true
			}
			for _, exp := range v.Lhs {
				if id, ok := exp.(*ast.Ident); ok {
					check(id, "var")
				}
			}
		case *ast.FuncDecl:
			if f.isTest() && (strings.HasPrefix(v.Name.Name, "Example") || strings.HasPrefix(v.Name.Name, "Test") || strings.HasPrefix(v.Name.Name, "Benchmark")) {
				return true
			}

			thing := "func"
			if v.Recv != nil {
				thing = "method"
			}

			// Exclude naming warnings for functions that are exported to C but
			// not exported in the Go API.
			// See https://github.com/golang/lint/issues/144.
			if ast.IsExported(v.Name.Name) || !isCgoExported(v) {
				check(v.Name, thing)
			}

			checkList(v.Type.Params, thing+" parameter")
			checkList(v.Type.Results, thing+" result")
		case *ast.GenDecl:
			if v.Tok == token.IMPORT {
				return true
			}
			var thing string
			switch v.Tok {
			case token.CONST:
				thing = "const"
			case token.TYPE:
				thing = "type"
			case token.VAR:
				thing = "var"
			}
			for _, spec := range v.Specs {
				switch s := spec.(type) {
				case *ast.TypeSpec:
					check(s.Name, thing)
				case *ast.ValueSpec:
					for _, id := range s.Names {
						check(id, thing)
					}
				}
			}
		case *ast.InterfaceType:
			// Do not check interface method names.
			// They are often constrainted by the method names of concrete types.
			for _, x := range v.Methods.List {
				ft, ok := x.Type.(*ast.FuncType)
				if !ok { // might be an embedded interface name
					continue
				}
				checkList(ft.Params, "interface method parameter")
				checkList(ft.Results, "interface method result")
			}
		case *ast.RangeStmt:
			if v.Tok == token.ASSIGN {
				return true
			}
			if id, ok := v.Key.(*ast.Ident); ok {
				check(id, "range var")
			}
			if id, ok := v.Value.(*ast.Ident); ok {
				check(id, "range var")
			}
		case *ast.StructType:
			for _, f := range v.Fields.List {
				for _, id := range f.Names {
					check(id, "struct field")
				}
			}
		}
		return true
	})
}

// lintName returns a different name if it should be different.
func lintName(name string) (should string) {
	// Fast path for simple cases: "_" and all lowercase.
	if name == "_" {
		return name
	}
	allLower := true
	for _, r := range name {
		if !unicode.IsLower(r) {
			allLower = false
			break
		}
	}
	if allLower {
		return name
	}

	// Split camelCase at any lower->upper transition, and split on underscores.
	// Check each word for common initialisms.
	runes := []rune(name)
	w, i := 0, 0 // index of start of word, scan
	for i+1 <= len(runes) {
		eow := false // whether we hit the end of a word
		if i+1 == len(runes) {
			eow = true
		} else if runes[i+1] == '_' {
			// underscore; shift the remainder forward over any run of underscores
			eow = true
			n := 1
			for i+n+1 < len(runes) && runes[i+n+1] == '_' {
				n++
			}

			// Leave at most one underscore if the underscore is between two digits
			if i+n+1 < len(runes) && unicode.IsDigit(runes[i]) && unicode.IsDigit(runes[i+n+1]) {
				n--
			}

			copy(runes[i+1:], runes[i+n+1:])
			runes = runes[:len(runes)-n]
		} else if unicode.IsLower(runes[i]) && !unicode.IsLower(runes[i+1]) {
			// lower->non-lower
			eow = true
		}
		i++
		if !eow {
			continue
		}

		// [w,i) is a word.
		word := string(runes[w:i])
		if u := strings.ToUpper(word); commonInitialisms[u] {
			// Keep consistent case, which is lowercase only at the start.
			if w == 0 && unicode.IsLower(runes[w]) {
				u = strings.ToLower(u)
			}
			// All the common initialisms are ASCII,
			// so we can replace the bytes exactly.
			copy(runes[w:], []rune(u))
		} else if w > 0 && strings.ToLower(word) == word {
			// already all lowercase, and not the first word, so uppercase the first character.
			runes[w] = unicode.ToUpper(runes[w])
		}
		w = i
	}
	return string(runes)
}

// commonInitialisms is a set of common initialisms.
// Only add entries that are highly unlikely to be non-initialisms.
// For instance, "ID" is fine (Freudian code is rare), but "AND" is not.
var commonInitialisms = map[string]bool{
	"ACL":   true,
	"API":   true,
	"ASCII": true,
	"CPU":   true,
	"CSS":   true,
	"DNS":   true,
	"EOF":   true,
	"GUID":  true,
	"HTML":  true,
	"HTTP":  true,
	"HTTPS": true,
	"ID":    true,
	"IP":    true,
	"JSON":  true,
	"LHS":   true,
	"QPS":   true,
	"RAM":   true,
	"RHS":   true,
	"RPC":   true,
	"SLA":   true,
	"SMTP":  true,
	"SQL":   true,
	"SSH":   true,
	"TCP":   true,
	"TLS":   true,
	"TTL":   true,
	"UDP":   true,
	"UI":    true,
	"UID":   true,
	"UUID":  true,
	"URI":   true,
	"URL":   true,
	"UTF8":  true,
	"VM":    true,
	"XML":   true,
	"XMPP":  true,
	"XSRF":  true,
	"XSS":   true,
}

// lintTypeDoc examines the doc comment on a type.
// It complains if they are missing from an exported type,
// or if they are not of the standard form.
func (f *file) lintTypeDoc(t *ast.TypeSpec, doc *ast.CommentGroup) {
	if !ast.IsExported(t.Name.Name) {
		return
	}
	if doc == nil {
		f.errorf(t, 1, link(docCommentsLink), category("comments"), "exported type %v should have comment or be unexported", t.Name)
		return
	}

	s := doc.Text()
	articles := [...]string{"A", "An", "The"}
	for _, a := range articles {
		if strings.HasPrefix(s, a+" ") {
			s = s[len(a)+1:]
			break
		}
	}
	if !strings.HasPrefix(s, t.Name.Name+" ") {
		f.errorf(doc, 1, link(docCommentsLink), category("comments"), `comment on exported type %v should be of the form "%v ..." (with optional leading article)`, t.Name, t.Name)
	}
}

var commonMethods = map[string]bool{
	"Error":     true,
	"Read":      true,
	"ServeHTTP": true,
	"String":    true,
	"Write":     true,
}

// lintFuncDoc examines doc comments on functions and methods.
// It complains if they are missing, or not of the right form.
// It has specific exclusions for well-known methods (see commonMethods above).
func (f *file) lintFuncDoc(fn *ast.FuncDecl) {
	if !ast.IsExported(fn.Name.Name) {
		// func is unexported
		return
	}
	kind := "function"
	name := fn.Name.Name
	if fn.Recv != nil && len(fn.Recv.List) > 0 {
		// method
		kind = "method"
		recv := receiverType(fn)
		if !ast.IsExported(recv) {
			// receiver is unexported
			return
		}
		if commonMethods[name] {
			return
		}
		switch name {
		case "Len", "Less", "Swap":
			if f.pkg.sortable[recv] {
				return
			}
		}
		name = recv + "." + name
	}
	if fn.Doc == nil {
		f.errorf(fn, 1, link(docCommentsLink), category("comments"), "exported %s %s should have comment or be unexported", kind, name)
		return
	}
	s := fn.Doc.Text()
	prefix := fn.Name.Name + " "
	if !strings.HasPrefix(s, prefix) {
		f.errorf(fn.Doc, 1, link(docCommentsLink), category("comments"), `comment on exported %s %s should be of the form "%s..."`, kind, name, prefix)
	}
}

// lintValueSpecDoc examines package-global variables and constants.
// It complains if they are not individually declared,
// or if they are not suitably documented in the right form (unless they are in a block that is commented).
func (f *file) lintValueSpecDoc(vs *ast.ValueSpec, gd *ast.GenDecl, genDeclMissingComments map[*ast.GenDecl]bool) {
	kind := "var"
	if gd.Tok == token.CONST {
		kind = "const"
	}

	if len(vs.Names) > 1 {
		// Check that none are exported except for the first.
		for _, n := range vs.Names[1:] {
			if ast.IsExported(n.Name) {
				f.errorf(vs, 1, category("comments"), "exported %s %s should have its own declaration", kind, n.Name)
				return
			}
		}
	}

	// Only one name.
	name := vs.Names[0].Name
	if !ast.IsExported(name) {
		return
	}

	if vs.Doc == nil && gd.Doc == nil {
		if genDeclMissingComments[gd] {
			return
		}
		block := ""
		if kind == "const" && gd.Lparen.IsValid() {
			block = " (or a comment on this block)"
		}
		f.errorf(vs, 1, link(docCommentsLink), category("comments"), "exported %s %s should have comment%s or be unexported", kind, name, block)
		genDeclMissingComments[gd] = true
		return
	}
	// If this GenDecl has parens and a comment, we don't check its comment form.
	if gd.Lparen.IsValid() && gd.Doc != nil {
		return
	}
	// The relevant text to check will be on either vs.Doc or gd.Doc.
	// Use vs.Doc preferentially.
	doc := vs.Doc
	if doc == nil {
		doc = gd.Doc
	}
	prefix := name + " "
	if !strings.HasPrefix(doc.Text(), prefix) {
		f.errorf(doc, 1, link(docCommentsLink), category("comments"), `comment on exported %s %s should be of the form "%s..."`, kind, name, prefix)
	}
}

func (f *file) checkStutter(id *ast.Ident, thing string) {
	pkg, name := f.f.Name.Name, id.Name
	if !ast.IsExported(name) {
		// unexported name
		return
	}
	// A name stutters if the package name is a strict prefix
	// and the next character of the name starts a new word.
	if len(name) <= len(pkg) {
		// name is too short to stutter.
		// This permits the name to be the same as the package name.
		return
	}
	if !strings.EqualFold(pkg, name[:len(pkg)]) {
		return
	}
	// We can assume the name is well-formed UTF-8.
	// If the next rune after the package name is uppercase or an underscore
	// the it's starting a new word and thus this name stutters.
	rem := name[len(pkg):]
	if next, _ := utf8.DecodeRuneInString(rem); next == '_' || unicode.IsUpper(next) {
		f.errorf(id, 0.8, link(styleGuideBase+"#package-names"), category("naming"), "%s name will be used as %s.%s by other packages, and that stutters; consider calling this %s", thing, pkg, name, rem)
	}
}

// zeroLiteral is a set of ast.BasicLit values that are zero values.
// It is not exhaustive.
var zeroLiteral = map[string]bool{
	"false": true, // bool
	// runes
	`'\x00'`: true,
	`'\000'`: true,
	// strings
	`""`: true,
	"``": true,
	// numerics
	"0":   true,
	"0.":  true,
	"0.0": true,
	"0i":  true,
}

// lintVarDecls examines variable declarations. It complains about declarations with
// redundant LHS types that can be inferred from the RHS.
func (f *file) lintVarDecls() {
	var lastGen *ast.GenDecl // last GenDecl entered.

	f.walk(func(node ast.Node) bool {
		switch v := node.(type) {
		case *ast.GenDecl:
			if v.Tok != token.CONST && v.Tok != token.VAR {
				return false
			}
			lastGen = v
			return true
		case *ast.ValueSpec:
			if lastGen.Tok == token.CONST {
				return false
			}
			if len(v.Names) > 1 || v.Type == nil || len(v.Values) == 0 {
				return false
			}
			rhs := v.Values[0]
			// An underscore var appears in a common idiom for compile-time interface satisfaction,
			// as in "var _ Interface = (*Concrete)(nil)".
			if isIdent(v.Names[0], "_") {
				return false
			}
			// If the RHS is a zero value, suggest dropping it.
			zero := false
			if lit, ok := rhs.(*ast.BasicLit); ok {
				zero = zeroLiteral[lit.Value]
			} else if isIdent(rhs, "nil") {
				zero = true
			}
			if zero {
				f.errorf(rhs, 0.9, category("zero-value"), "should drop = %s from declaration of var %s; it is the zero value", f.render(rhs), v.Names[0])
				return false
			}
			lhsTyp := f.pkg.typeOf(v.Type)
			rhsTyp := f.pkg.typeOf(rhs)

			if !validType(lhsTyp) || !validType(rhsTyp) {
				// Type checking failed (often due to missing imports).
				return false
			}

			if !types.Identical(lhsTyp, rhsTyp) {
				// Assignment to a different type is not redundant.
				return false
			}

			// The next three conditions are for suppressing the warning in situations
			// where we were unable to typecheck.

			// If the LHS type is an interface, don't warn, since it is probably a
			// concrete type on the RHS. Note that our feeble lexical check here
			// will only pick up interface{} and other literal interface types;
			// that covers most of the cases we care to exclude right now.
			if _, ok := v.Type.(*ast.InterfaceType); ok {
				return false
			}
			// If the RHS is an untyped const, only warn if the LHS type is its default type.
			if defType, ok := f.isUntypedConst(rhs); ok && !isIdent(v.Type, defType) {
				return false
			}

			f.errorf(v.Type, 0.8, category("type-inference"), "should omit type %s from declaration of var %s; it will be inferred from the right-hand side", f.render(v.Type), v.Names[0])
			return false
		}
		return true
	})
}

func validType(T types.Type) bool {
	return T != nil &&
		T != types.Typ[types.Invalid] &&
		!strings.Contains(T.String(), "invalid type") // good but not foolproof
}

// lintElses examines else blocks. It complains about any else block whose if block ends in a return.
func (f *file) lintElses() {
	// We don't want to flag if { } else if { } else { } constructions.
	// They will appear as an IfStmt whose Else field is also an IfStmt.
	// Record such a node so we ignore it when we visit it.
	ignore := make(map[*ast.IfStmt]bool)

	f.walk(func(node ast.Node) bool {
		ifStmt, ok := node.(*ast.IfStmt)
		if !ok || ifStmt.Else == nil {
			return true
		}
		if elseif, ok := ifStmt.Else.(*ast.IfStmt); ok {
			ignore[elseif] = true
			return true
		}
		if ignore[ifStmt] {
			return true
		}
		if _, ok := ifStmt.Else.(*ast.BlockStmt); !ok {
			// only care about elses without conditions
			return true
		}
		if len(ifStmt.Body.List) == 0 {
			return true
		}
		shortDecl := false // does the if statement have a ":=" initialization statement?
		if ifStmt.Init != nil {
			if as, ok := ifStmt.Init.(*ast.AssignStmt); ok && as.Tok == token.DEFINE {
				shortDecl = true
			}
		}
		lastStmt := ifStmt.Body.List[len(ifStmt.Body.List)-1]
		if _, ok := lastStmt.(*ast.ReturnStmt); ok {
			extra := ""
			if shortDecl {
				extra = " (move short variable declaration to its own line if necessary)"
			}
			f.errorf(ifStmt.Else, 1, link(styleGuideBase+"#indent-error-flow"), category("indent"), "if block ends with a return statement, so drop this else and outdent its block"+extra)
		}
		return true
	})
}

// lintRanges examines range clauses. It complains about redundant constructions.
func (f *file) lintRanges() {
	f.walk(func(node ast.Node) bool {
		rs, ok := node.(*ast.RangeStmt)
		if !ok {
			return true
		}

		if isIdent(rs.Key, "_") && (rs.Value == nil || isIdent(rs.Value, "_")) {
			p := f.errorf(rs.Key, 1, category("range-loop"), "should omit values from range; this loop is equivalent to `for range ...`")

			newRS := *rs // shallow copy
			newRS.Value = nil
			newRS.Key = nil
			p.ReplacementLine = f.firstLineOf(&newRS, rs)

			return true
		}

		if isIdent(rs.Value, "_") {
			p := f.errorf(rs.Value, 1, category("range-loop"), "should omit 2nd value from range; this loop is equivalent to `for %s %s range ...`", f.render(rs.Key), rs.Tok)

			newRS := *rs // shallow copy
			newRS.Value = nil
			p.ReplacementLine = f.firstLineOf(&newRS, rs)
		}

		return true
	})
}

// lintErrorf examines errors.New and testing.Error calls. It complains if its only argument is an fmt.Sprintf invocation.
func (f *file) lintErrorf() {
	f.walk(func(node ast.Node) bool {
		ce, ok := node.(*ast.CallExpr)
		if !ok || len(ce.Args) != 1 {
			return true
		}
		isErrorsNew := isPkgDot(ce.Fun, "errors", "New")
		var isTestingError bool
		se, ok := ce.Fun.(*ast.SelectorExpr)
		if ok && se.Sel.Name == "Error" {
			if typ := f.pkg.typeOf(se.X); typ != nil {
				isTestingError = typ.String() == "*testing.T"
			}
		}
		if !isErrorsNew && !isTestingError {
			return true
		}
		if !f.imports("errors") {
			return true
		}
		arg := ce.Args[0]
		ce, ok = arg.(*ast.CallExpr)
		if !ok || !isPkgDot(ce.Fun, "fmt", "Sprintf") {
			return true
		}
		errorfPrefix := "fmt"
		if isTestingError {
			errorfPrefix = f.render(se.X)
		}
		p := f.errorf(node, 1, category("errors"), "should replace %s(fmt.Sprintf(...)) with %s.Errorf(...)", f.render(se), errorfPrefix)

		m := f.srcLineWithMatch(ce, `^(.*)`+f.render(se)+`\(fmt\.Sprintf\((.*)\)\)(.*)$`)
		if m != nil {
			p.ReplacementLine = m[1] + errorfPrefix + ".Errorf(" + m[2] + ")" + m[3]
		}

		return true
	})
}

// lintErrors examines global error vars. It complains if they aren't named in the standard way.
func (f *file) lintErrors() {
	for _, decl := range f.f.Decls {
		gd, ok := decl.(*ast.GenDecl)
		if !ok || gd.Tok != token.VAR {
			continue
		}
		for _, spec := range gd.Specs {
			spec := spec.(*ast.ValueSpec)
			if len(spec.Names) != 1 || len(spec.Values) != 1 {
				continue
			}
			ce, ok := spec.Values[0].(*ast.CallExpr)
			if !ok {
				continue
			}
			if !isPkgDot(ce.Fun, "errors", "New") && !isPkgDot(ce.Fun, "fmt", "Errorf") {
				continue
			}

			id := spec.Names[0]
			prefix := "err"
			if id.IsExported() {
				prefix = "Err"
			}
			if !strings.HasPrefix(id.Name, prefix) {
				f.errorf(id, 0.9, category("naming"), "error var %s should have name of the form %sFoo", id.Name, prefix)
			}
		}
	}
}

func lintErrorString(s string) (isClean bool, conf float64) {
	const basicConfidence = 0.8
	const capConfidence = basicConfidence - 0.2
	first, firstN := utf8.DecodeRuneInString(s)
	last, _ := utf8.DecodeLastRuneInString(s)
	if last == '.' || last == ':' || last == '!' || last == '\n' {
		return false, basicConfidence
	}
	if unicode.IsUpper(first) {
		// People use proper nouns and exported Go identifiers in error strings,
		// so decrease the confidence of warnings for capitalization.
		if len(s) <= firstN {
			return false, capConfidence
		}
		// Flag strings starting with something that doesn't look like an initialism.
		if second, _ := utf8.DecodeRuneInString(s[firstN:]); !unicode.IsUpper(second) {
			return false, capConfidence
		}
	}
	return true, 0
}

// lintErrorStrings examines error strings.
// It complains if they are capitalized or end in punctuation or a newline.
func (f *file) lintErrorStrings() {
	f.walk(func(node ast.Node) bool {
		ce, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if !isPkgDot(ce.Fun, "errors", "New") && !isPkgDot(ce.Fun, "fmt", "Errorf") {
			return true
		}
		if len(ce.Args) < 1 {
			return true
		}
		str, ok := ce.Args[0].(*ast.BasicLit)
		if !ok || str.Kind != token.STRING {
			return true
		}
		s, _ := strconv.Unquote(str.Value) // can assume well-formed Go
		if s == "" {
			return true
		}
		clean, conf := lintErrorString(s)
		if clean {
			return true
		}

		f.errorf(str, conf, link(styleGuideBase+"#error-strings"), category("errors"),
			"error strings should not be capitalized or end with punctuation or a newline")
		return true
	})
}

// lintReceiverNames examines receiver names. It complains about inconsistent
// names used for the same type and names such as "this".
func (f *file) lintReceiverNames() {
	typeReceiver := map[string]string{}
	f.walk(func(n ast.Node) bool {
		fn, ok := n.(*ast.FuncDecl)
		if !ok || fn.Recv == nil || len(fn.Recv.List) == 0 {
			return true
		}
		names := fn.Recv.List[0].Names
		if len(names) < 1 {
			return true
		}
		name := names[0].Name
		const ref = styleGuideBase + "#receiver-names"
		if name == "_" {
			f.errorf(n, 1, link(ref), category("naming"), `receiver name should not be an underscore, omit the name if it is unused`)
			return true
		}
		if name == "this" || name == "self" {
			f.errorf(n, 1, link(ref), category("naming"), `receiver name should be a reflection of its identity; don't use generic names such as "this" or "self"`)
			return true
		}
		recv := receiverType(fn)
		if prev, ok := typeReceiver[recv]; ok && prev != name {
			f.errorf(n, 1, link(ref), category("naming"), "receiver name %s should be consistent with previous receiver name %s for %s", name, prev, recv)
			return true
		}
		typeReceiver[recv] = name
		return true
	})
}

// lintIncDec examines statements that increment or decrement a variable.
// It complains if they don't use x++ or x--.
func (f *file) lintIncDec() {
	f.walk(func(n ast.Node) bool {
		as, ok := n.(*ast.AssignStmt)
		if !ok {
			return true
		}
		if len(as.Lhs) != 1 {
			return true
		}
		if !isOne(as.Rhs[0]) {
			return true
		}
		var suffix string
		switch as.Tok {
		case token.ADD_ASSIGN:
			suffix = "++"
		case token.SUB_ASSIGN:
			suffix = "--"
		default:
			return true
		}
		f.errorf(as, 0.8, category("unary-op"), "should replace %s with %s%s", f.render(as), f.render(as.Lhs[0]), suffix)
		return true
	})
}

// lintErrorReturn examines function declarations that return an error.
// It complains if the error isn't the last parameter.
func (f *file) lintErrorReturn() {
	f.walk(func(n ast.Node) bool {
		fn, ok := n.(*ast.FuncDecl)
		if !ok || fn.Type.Results == nil {
			return true
		}
		ret := fn.Type.Results.List
		if len(ret) <= 1 {
			return true
		}
		if isIdent(ret[len(ret)-1].Type, "error") {
			return true
		}
		// An error return parameter should be the last parameter.
		// Flag any error parameters found before the last.
		for _, r := range ret[:len(ret)-1] {
			if isIdent(r.Type, "error") {
				f.errorf(fn, 0.9, category("arg-order"), "error should be the last type when returning multiple items")
				break // only flag one
			}
		}
		return true
	})
}

// lintUnexportedReturn examines exported function declarations.
// It complains if any return an unexported type.
func (f *file) lintUnexportedReturn() {
	f.walk(func(n ast.Node) bool {
		fn, ok := n.(*ast.FuncDecl)
		if !ok {
			return true
		}
		if fn.Type.Results == nil {
			return false
		}
		if !fn.Name.IsExported() {
			return false
		}
		thing := "func"
		if fn.Recv != nil && len(fn.Recv.List) > 0 {
			thing = "method"
			if !ast.IsExported(receiverType(fn)) {
				// Don't report exported methods of unexported types,
				// such as private implementations of sort.Interface.
				return false
			}
		}
		for _, ret := range fn.Type.Results.List {
			typ := f.pkg.typeOf(ret.Type)
			if exportedType(typ) {
				continue
			}
			f.errorf(ret.Type, 0.8, category("unexported-type-in-api"),
				"exported %s %s returns unexported type %s, which can be annoying to use",
				thing, fn.Name.Name, typ)
			break // only flag one
		}
		return false
	})
}

// exportedType reports whether typ is an exported type.
// It is imprecise, and will err on the side of returning true,
// such as for composite types.
func exportedType(typ types.Type) bool {
	switch T := typ.(type) {
	case *types.Named:
		// Builtin types have no package.
		return T.Obj().Pkg() == nil || T.Obj().Exported()
	case *types.Map:
		return exportedType(T.Key()) && exportedType(T.Elem())
	case interface {
		Elem() types.Type
	}: // array, slice, pointer, chan
		return exportedType(T.Elem())
	}
	// Be conservative about other types, such as struct, interface, etc.
	return true
}

// timeSuffixes is a list of name suffixes that imply a time unit.
// This is not an exhaustive list.
var timeSuffixes = []string{
	"Sec", "Secs", "Seconds",
	"Msec", "Msecs",
	"Milli", "Millis", "Milliseconds",
	"Usec", "Usecs", "Microseconds",
	"MS", "Ms",
}

func (f *file) lintTimeNames() {
	f.walk(func(node ast.Node) bool {
		v, ok := node.(*ast.ValueSpec)
		if !ok {
			return true
		}
		for _, name := range v.Names {
			origTyp := f.pkg.typeOf(name)
			// Look for time.Duration or *time.Duration;
			// the latter is common when using flag.Duration.
			typ := origTyp
			if pt, ok := typ.(*types.Pointer); ok {
				typ = pt.Elem()
			}
			if !f.pkg.isNamedType(typ, "time", "Duration") {
				continue
			}
			suffix := ""
			for _, suf := range timeSuffixes {
				if strings.HasSuffix(name.Name, suf) {
					suffix = suf
					break
				}
			}
			if suffix == "" {
				continue
			}
			f.errorf(v, 0.9, category("time"), "var %s is of type %v; don't use unit-specific suffix %q", name.Name, origTyp, suffix)
		}
		return true
	})
}

// lintContextKeyTypes checks for call expressions to context.WithValue with
// basic types used for the key argument.
// See: https://golang.org/issue/17293
func (f *file) lintContextKeyTypes() {
	f.walk(func(node ast.Node) bool {
		switch node := node.(type) {
		case *ast.CallExpr:
			f.checkContextKeyType(node)
		}

		return true
	})
}

// checkContextKeyType reports an error if the call expression calls
// context.WithValue with a key argument of basic type.
func (f *file) checkContextKeyType(x *ast.CallExpr) {
	sel, ok := x.Fun.(*ast.SelectorExpr)
	if !ok {
		return
	}
	pkg, ok := sel.X.(*ast.Ident)
	if !ok || pkg.Name != "context" {
		return
	}
	if sel.Sel.Name != "WithValue" {
		return
	}

	// key is second argument to context.WithValue
	if len(x.Args) != 3 {
		return
	}
	key := f.pkg.typesInfo.Types[x.Args[1]]

	if ktyp, ok := key.Type.(*types.Basic); ok && ktyp.Kind() != types.Invalid {
		f.errorf(x, 1.0, category("context"), fmt.Sprintf("should not use basic type %s as key in context.WithValue", key.Type))
	}
}

// lintContextArgs examines function declarations that contain an
// argument with a type of context.Context
// It complains if that argument isn't the first parameter.
func (f *file) lintContextArgs() {
	f.walk(func(n ast.Node) bool {
		fn, ok := n.(*ast.FuncDecl)
		if !ok || len(fn.Type.Params.List) <= 1 {
			return true
		}
		// A context.Context should be the first parameter of a function.
		// Flag any that show up after the first.
		for _, arg := range fn.Type.Params.List[1:] {
			if isPkgDot(arg.Type, "context", "Context") {
				f.errorf(fn, 0.9, link("https://golang.org/pkg/context/"), category("arg-order"), "context.Context should be the first parameter of a function")
				break // only flag one
			}
		}
		return true
	})
}

// containsComments returns whether the interval [start, end) contains any
// comments without "// MATCH " prefix.
func (f *file) containsComments(start, end token.Pos) bool {
	for _, cgroup := range f.f.Comments {
		comments := cgroup.List
		if comments[0].Slash >= end {
			// All comments starting with this group are after end pos.
			return false
		}
		if comments[len(comments)-1].Slash < start {
			// Comments group ends before start pos.
			continue
		}
		for _, c := range comments {
			if start <= c.Slash && c.Slash < end && !strings.HasPrefix(c.Text, "// MATCH ") {
				return true
			}
		}
	}
	return false
}

// receiverType returns the named type of the method receiver, sans "*",
// or "invalid-type" if fn.Recv is ill formed.
func receiverType(fn *ast.FuncDecl) string {
	switch e := fn.Recv.List[0].Type.(type) {
	case *ast.Ident:
		return e.Name
	case *ast.StarExpr:
		if id, ok := e.X.(*ast.Ident); ok {
			return id.Name
		}
	}
	// The parser accepts much more than just the legal forms.
	return "invalid-type"
}

func (f *file) walk(fn func(ast.Node) bool) {
	ast.Walk(walker(fn), f.f)
}

func (f *file) render(x interface{}) string {
	var buf bytes.Buffer
	if err := printer.Fprint(&buf, f.fset, x); err != nil {
		panic(err)
	}
	return buf.String()
}

func (f *file) debugRender(x interface{}) string {
	var buf bytes.Buffer
	if err := ast.Fprint(&buf, f.fset, x, nil); err != nil {
		panic(err)
	}
	return buf.String()
}

// walker adapts a function to satisfy the ast.Visitor interface.
// The function return whether the walk should proceed into the node's children.
type walker func(ast.Node) bool

func (w walker) Visit(node ast.Node) ast.Visitor {
	if w(node) {
		return w
	}
	return nil
}

func isIdent(expr ast.Expr, ident string) bool {
	id, ok := expr.(*ast.Ident)
	return ok && id.Name == ident
}

// isBlank returns whether id is the blank identifier "_".
// If id == nil, the answer is false.
func isBlank(id *ast.Ident) bool { return id != nil && id.Name == "_" }

func isPkgDot(expr ast.Expr, pkg, name string) bool {
	sel, ok := expr.(*ast.SelectorExpr)
	return ok && isIdent(sel.X, pkg) && isIdent(sel.Sel, name)
}

func isOne(expr ast.Expr) bool {
	lit, ok := expr.(*ast.BasicLit)
	return ok && lit.Kind == token.INT && lit.Value == "1"
}

func isCgoExported(f *ast.FuncDecl) bool {
	if f.Recv != nil || f.Doc == nil {
		return false
	}

	cgoExport := regexp.MustCompile(fmt.Sprintf("(?m)^//export %s$", regexp.QuoteMeta(f.Name.Name)))
	for _, c := range f.Doc.List {
		if cgoExport.MatchString(c.Text) {
			return true
		}
	}
	return false
}

var basicTypeKinds = map[types.BasicKind]string{
	types.UntypedBool:    "bool",
	types.UntypedInt:     "int",
	types.UntypedRune:    "rune",
	types.UntypedFloat:   "float64",
	types.UntypedComplex: "complex128",
	types.UntypedString:  "string",
}

// isUntypedConst reports whether expr is an untyped constant,
// and indicates what its default type is.
// scope may be nil.
func (f *file) isUntypedConst(expr ast.Expr) (defType string, ok bool) {
	// Re-evaluate expr outside of its context to see if it's untyped.
	// (An expr evaluated within, for example, an assignment context will get the type of the LHS.)
	exprStr := f.render(expr)
	tv, err := types.Eval(f.fset, f.pkg.typesPkg, expr.Pos(), exprStr)
	if err != nil {
		return "", false
	}
	if b, ok := tv.Type.(*types.Basic); ok {
		if dt, ok := basicTypeKinds[b.Kind()]; ok {
			return dt, true
		}
	}

	return "", false
}

// firstLineOf renders the given node and returns its first line.
// It will also match the indentation of another node.
func (f *file) firstLineOf(node, match ast.Node) string {
	line := f.render(node)
	if i := strings.Index(line, "\n"); i >= 0 {
		line = line[:i]
	}
	return f.indentOf(match) + line
}

func (f *file) indentOf(node ast.Node) string {
	line := srcLine(f.src, f.fset.Position(node.Pos()))
	for i, r := range line {
		switch r {
		case ' ', '\t':
		default:
			return line[:i]
		}
	}
	return line // unusual or empty line
}

func (f *file) srcLineWithMatch(node ast.Node, pattern string) (m []string) {
	line := srcLine(f.src, f.fset.Position(node.Pos()))
	line = strings.TrimSuffix(line, "\n")
	rx := regexp.MustCompile(pattern)
	return rx.FindStringSubmatch(line)
}

// imports returns true if the current file imports the specified package path.
func (f *file) imports(importPath string) bool {
	all := astutil.Imports(f.fset, f.f)
	for _, p := range all {
		for _, i := range p {
			uq, err := strconv.Unquote(i.Path.Value)
			if err == nil && importPath == uq {
				return true
			}
		}
	}
	return false
}

// srcLine returns the complete line at p, including the terminating newline.
func srcLine(src []byte, p token.Position) string {
	// Run to end of line in both directions if not at line start/end.
	lo, hi := p.Offset, p.Offset+1
	for lo > 0 && src[lo-1] != '\n' {
		lo--
	}
	for hi < len(src) && src[hi-1] != '\n' {
		hi++
	}
	return string(src[lo:hi])
}