cmd/stringer/stringer.go - tools - Git at Google

 // Copyright 2014 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.

 // Stringer is a tool to automate the creation of methods that satisfy the fmt.Stringer
 // interface. Given the name of a (signed or unsigned) integer type T that has constants
 // defined, stringer will create a new self-contained Go source file implementing
 //	func (t T) String() string
 // The file is created in the same package and directory as the package that defines T.
 // It has helpful defaults designed for use with go generate.
 //
 // Stringer works best with constants that are consecutive values such as created using iota,
 // but creates good code regardless. In the future it might also provide custom support for
 // constant sets that are bit patterns.
 //
 // For example, given this snippet,
 //
 //	package painkiller
 //
 //	type Pill int
 //
 //	const (
 //		Placebo Pill = iota
 //		Aspirin
 //		Ibuprofen
 //		Paracetamol
 //		Acetaminophen = Paracetamol
 //	)
 //
 // running this command
 //
 //	stringer -type=Pill
 //
 // in the same directory will create the file pill_string.go, in package painkiller,
 // containing a definition of
 //
 //	func (Pill) String() string
 //
 // That method will translate the value of a Pill constant to the string representation
 // of the respective constant name, so that the call fmt.Print(painkiller.Aspirin) will
 // print the string "Aspirin".
 //
 // Typically this process would be run using go generate, like this:
 //
 //	//go:generate stringer -type=Pill
 //
 // If multiple constants have the same value, the lexically first matching name will
 // be used (in the example, Acetaminophen will print as "Paracetamol").
 //
 // With no arguments, it processes the package in the current directory.
 // Otherwise, the arguments must name a single directory holding a Go package
 // or a set of Go source files that represent a single Go package.
 //
 // The -type flag accepts a comma-separated list of types so a single run can
 // generate methods for multiple types. The default output file is t_string.go,
 // where t is the lower-cased name of the first type listed. It can be overridden
 // with the -output flag.
 //
 // The -linecomment flag tells stringer to generate the text of any line comment, trimmed
 // of leading spaces, instead of the constant name. For instance, if the constants above had a
 // Pill prefix, one could write
 //
 //	PillAspirin // Aspirin
 //
 // to suppress it in the output.
 package main // import "golang.org/x/tools/cmd/stringer"

 import (
 	"bytes"
 	"flag"
 	"fmt"
 	"go/ast"
 	"go/constant"
 	"go/format"
 	"go/token"
 	"go/types"
 	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"

 	"golang.org/x/tools/go/packages"
 )

 var (
 	typeNames   = flag.String("type", "", "comma-separated list of type names; must be set")
 	output      = flag.String("output", "", "output file name; default srcdir/<type>_string.go")
 	trimprefix  = flag.String("trimprefix", "", "trim the `prefix` from the generated constant names")
 	linecomment = flag.Bool("linecomment", false, "use line comment text as printed text when present")
 	buildTags   = flag.String("tags", "", "comma-separated list of build tags to apply")
 )

 // Usage is a replacement usage function for the flags package.
 func Usage() {
 	fmt.Fprintf(os.Stderr, "Usage of stringer:\n")
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T files... # Must be a single package\n")
 	fmt.Fprintf(os.Stderr, "For more information, see:\n")
 	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
 	fmt.Fprintf(os.Stderr, "Flags:\n")
 	flag.PrintDefaults()
 }

 func main() {
 	log.SetFlags(0)
 	log.SetPrefix("stringer: ")
 	flag.Usage = Usage
 	flag.Parse()
 	if len(*typeNames) == 0 {
 		flag.Usage()
 		os.Exit(2)
 	}
 	types := strings.Split(*typeNames, ",")
 	var tags []string
 	if len(*buildTags) > 0 {
 		tags = strings.Split(*buildTags, ",")
 	}

 	// We accept either one directory or a list of files. Which do we have?
 	args := flag.Args()
 	if len(args) == 0 {
 		// Default: process whole package in current directory.
 		args = []string{"."}
 	}

 	// Parse the package once.
 	var dir string
 	g := Generator{
 		trimPrefix:  *trimprefix,
 		lineComment: *linecomment,
 	}
 	// TODO(suzmue): accept other patterns for packages (directories, list of files, import paths, etc).
 	if len(args) == 1 && isDirectory(args[0]) {
 		dir = args[0]
 	} else {
 		if len(tags) != 0 {
 			log.Fatal("-tags option applies only to directories, not when files are specified")
 		}
 		dir = filepath.Dir(args[0])
 	}

 	g.parsePackage(args, tags)

 	// Print the header and package clause.
 	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT.\n", strings.Join(os.Args[1:], " "))
 	g.Printf("\n")
 	g.Printf("package %s", g.pkg.name)
 	g.Printf("\n")
 	g.Printf("import \"strconv\"\n") // Used by all methods.

 	// Run generate for each type.
 	for _, typeName := range types {
 		g.generate(typeName)
 	}

 	// Format the output.
 	src := g.format()

 	// Write to file.
 	outputName := *output
 	if outputName == "" {
 		baseName := fmt.Sprintf("%s_string.go", types[0])
 		outputName = filepath.Join(dir, strings.ToLower(baseName))
 	}
 	err := ioutil.WriteFile(outputName, src, 0644)
 	if err != nil {
 		log.Fatalf("writing output: %s", err)
 	}
 }

 // isDirectory reports whether the named file is a directory.
 func isDirectory(name string) bool {
 	info, err := os.Stat(name)
 	if err != nil {
 		log.Fatal(err)
 	}
 	return info.IsDir()
 }

 // Generator holds the state of the analysis. Primarily used to buffer
 // the output for format.Source.
 type Generator struct {
 	buf bytes.Buffer // Accumulated output.
 	pkg *Package     // Package we are scanning.

 	trimPrefix  string
 	lineComment bool
 }

 func (g *Generator) Printf(format string, args ...interface{}) {
 	fmt.Fprintf(&g.buf, format, args...)
 }

 // File holds a single parsed file and associated data.
 type File struct {
 	pkg  *Package  // Package to which this file belongs.
 	file *ast.File // Parsed AST.
 	// These fields are reset for each type being generated.
 	typeName string  // Name of the constant type.
 	values   []Value // Accumulator for constant values of that type.

 	trimPrefix  string
 	lineComment bool
 }

 type Package struct {
 	name  string
 	defs  map[*ast.Ident]types.Object
 	files []*File
 }

 // parsePackage analyzes the single package constructed from the patterns and tags.
 // parsePackage exits if there is an error.
 func (g *Generator) parsePackage(patterns []string, tags []string) {
 	cfg := &packages.Config{
 		Mode: packages.LoadSyntax,
 		// TODO: Need to think about constants in test files. Maybe write type_string_test.go
 		// in a separate pass? For later.
 		Tests:      false,
 		BuildFlags: []string{fmt.Sprintf("-tags=%s", strings.Join(tags, " "))},
 	}
 	pkgs, err := packages.Load(cfg, patterns...)
 	if err != nil {
 		log.Fatal(err)
 	}
 	if len(pkgs) != 1 {
 		log.Fatalf("error: %d packages found", len(pkgs))
 	}
 	g.addPackage(pkgs[0])
 }

 // addPackage adds a type checked Package and its syntax files to the generator.
 func (g *Generator) addPackage(pkg *packages.Package) {
 	g.pkg = &Package{
 		name:  pkg.Name,
 		defs:  pkg.TypesInfo.Defs,
 		files: make([]*File, len(pkg.Syntax)),
 	}

 	for i, file := range pkg.Syntax {
 		g.pkg.files[i] = &File{
 			file:        file,
 			pkg:         g.pkg,
 			trimPrefix:  g.trimPrefix,
 			lineComment: g.lineComment,
 		}
 	}
 }

 // generate produces the String method for the named type.
 func (g *Generator) generate(typeName string) {
 	values := make([]Value, 0, 100)
 	for _, file := range g.pkg.files {
 		// Set the state for this run of the walker.
 		file.typeName = typeName
 		file.values = nil
 		if file.file != nil {
 			ast.Inspect(file.file, file.genDecl)
 			values = append(values, file.values...)
 		}
 	}

 	if len(values) == 0 {
 		log.Fatalf("no values defined for type %s", typeName)
 	}
 	// Generate code that will fail if the constants change value.
 	g.Printf("func _() {\n")
 	g.Printf("\t// An \"invalid array index\" compiler error signifies that the constant values have changed.\n")
 	g.Printf("\t// Re-run the stringer command to generate them again.\n")
 	g.Printf("\tvar x [1]struct{}\n")
 	for _, v := range values {
 		g.Printf("\t_ = x[%s - %s]\n", v.originalName, v.str)
 	}
 	g.Printf("}\n")
 	runs := splitIntoRuns(values)
 	// The decision of which pattern to use depends on the number of
 	// runs in the numbers. If there's only one, it's easy. For more than
 	// one, there's a tradeoff between complexity and size of the data
 	// and code vs. the simplicity of a map. A map takes more space,
 	// but so does the code. The decision here (crossover at 10) is
 	// arbitrary, but considers that for large numbers of runs the cost
 	// of the linear scan in the switch might become important, and
 	// rather than use yet another algorithm such as binary search,
 	// we punt and use a map. In any case, the likelihood of a map
 	// being necessary for any realistic example other than bitmasks
 	// is very low. And bitmasks probably deserve their own analysis,
 	// to be done some other day.
 	switch {
 	case len(runs) == 1:
 		g.buildOneRun(runs, typeName)
 	case len(runs) <= 10:
 		g.buildMultipleRuns(runs, typeName)
 	default:
 		g.buildMap(runs, typeName)
 	}
 }

 // splitIntoRuns breaks the values into runs of contiguous sequences.
 // For example, given 1,2,3,5,6,7 it returns {1,2,3},{5,6,7}.
 // The input slice is known to be non-empty.
 func splitIntoRuns(values []Value) [][]Value {
 	// We use stable sort so the lexically first name is chosen for equal elements.
 	sort.Stable(byValue(values))
 	// Remove duplicates. Stable sort has put the one we want to print first,
 	// so use that one. The String method won't care about which named constant
 	// was the argument, so the first name for the given value is the only one to keep.
 	// We need to do this because identical values would cause the switch or map
 	// to fail to compile.
 	j := 1
 	for i := 1; i < len(values); i++ {
 		if values[i].value != values[i-1].value {
 			values[j] = values[i]
 			j++
 		}
 	}
 	values = values[:j]
 	runs := make([][]Value, 0, 10)
 	for len(values) > 0 {
 		// One contiguous sequence per outer loop.
 		i := 1
 		for i < len(values) && values[i].value == values[i-1].value+1 {
 			i++
 		}
 		runs = append(runs, values[:i])
 		values = values[i:]
 	}
 	return runs
 }

 // format returns the gofmt-ed contents of the Generator's buffer.
 func (g *Generator) format() []byte {
 	src, err := format.Source(g.buf.Bytes())
 	if err != nil {
 		// Should never happen, but can arise when developing this code.
 		// The user can compile the output to see the error.
 		log.Printf("warning: internal error: invalid Go generated: %s", err)
 		log.Printf("warning: compile the package to analyze the error")
 		return g.buf.Bytes()
 	}
 	return src
 }

 // Value represents a declared constant.
 type Value struct {
 	originalName string // The name of the constant.
 	name         string // The name with trimmed prefix.
 	// The value is stored as a bit pattern alone. The boolean tells us
 	// whether to interpret it as an int64 or a uint64; the only place
 	// this matters is when sorting.
 	// Much of the time the str field is all we need; it is printed
 	// by Value.String.
 	value  uint64 // Will be converted to int64 when needed.
 	signed bool   // Whether the constant is a signed type.
 	str    string // The string representation given by the "go/constant" package.
 }

 func (v *Value) String() string {
 	return v.str
 }

 // byValue lets us sort the constants into increasing order.
 // We take care in the Less method to sort in signed or unsigned order,
 // as appropriate.
 type byValue []Value

 func (b byValue) Len() int      { return len(b) }
 func (b byValue) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
 func (b byValue) Less(i, j int) bool {
 	if b[i].signed {
 		return int64(b[i].value) < int64(b[j].value)
 	}
 	return b[i].value < b[j].value
 }

 // genDecl processes one declaration clause.
 func (f *File) genDecl(node ast.Node) bool {
 	decl, ok := node.(*ast.GenDecl)
 	if !ok || decl.Tok != token.CONST {
 		// We only care about const declarations.
 		return true
 	}
 	// The name of the type of the constants we are declaring.
 	// Can change if this is a multi-element declaration.
 	typ := ""
 	// Loop over the elements of the declaration. Each element is a ValueSpec:
 	// a list of names possibly followed by a type, possibly followed by values.
 	// If the type and value are both missing, we carry down the type (and value,
 	// but the "go/types" package takes care of that).
 	for _, spec := range decl.Specs {
 		vspec := spec.(*ast.ValueSpec) // Guaranteed to succeed as this is CONST.
 		if vspec.Type == nil && len(vspec.Values) > 0 {
 			// "X = 1". With no type but a value. If the constant is untyped,
 			// skip this vspec and reset the remembered type.
 			typ = ""

 			// If this is a simple type conversion, remember the type.
 			// We don't mind if this is actually a call; a qualified call won't
 			// be matched (that will be SelectorExpr, not Ident), and only unusual
 			// situations will result in a function call that appears to be
 			// a type conversion.
 			ce, ok := vspec.Values[0].(*ast.CallExpr)
 			if !ok {
 				continue
 			}
 			id, ok := ce.Fun.(*ast.Ident)
 			if !ok {
 				continue
 			}
 			typ = id.Name
 		}
 		if vspec.Type != nil {
 			// "X T". We have a type. Remember it.
 			ident, ok := vspec.Type.(*ast.Ident)
 			if !ok {
 				continue
 			}
 			typ = ident.Name
 		}
 		if typ != f.typeName {
 			// This is not the type we're looking for.
 			continue
 		}
 		// We now have a list of names (from one line of source code) all being
 		// declared with the desired type.
 		// Grab their names and actual values and store them in f.values.
 		for _, name := range vspec.Names {
 			if name.Name == "_" {
 				continue
 			}
 			// This dance lets the type checker find the values for us. It's a
 			// bit tricky: look up the object declared by the name, find its
 			// types.Const, and extract its value.
 			obj, ok := f.pkg.defs[name]
 			if !ok {
 				log.Fatalf("no value for constant %s", name)
 			}
 			info := obj.Type().Underlying().(*types.Basic).Info()
 			if info&types.IsInteger == 0 {
 				log.Fatalf("can't handle non-integer constant type %s", typ)
 			}
 			value := obj.(*types.Const).Val() // Guaranteed to succeed as this is CONST.
 			if value.Kind() != constant.Int {
 				log.Fatalf("can't happen: constant is not an integer %s", name)
 			}
 			i64, isInt := constant.Int64Val(value)
 			u64, isUint := constant.Uint64Val(value)
 			if !isInt && !isUint {
 				log.Fatalf("internal error: value of %s is not an integer: %s", name, value.String())
 			}
 			if !isInt {
 				u64 = uint64(i64)
 			}
 			v := Value{
 				originalName: name.Name,
 				value:        u64,
 				signed:       info&types.IsUnsigned == 0,
 				str:          value.String(),
 			}
 			if c := vspec.Comment; f.lineComment && c != nil && len(c.List) == 1 {
 				v.name = strings.TrimSpace(c.Text())
 			} else {
 				v.name = strings.TrimPrefix(v.originalName, f.trimPrefix)
 			}
 			f.values = append(f.values, v)
 		}
 	}
 	return false
 }

 // Helpers

 // usize returns the number of bits of the smallest unsigned integer
 // type that will hold n. Used to create the smallest possible slice of
 // integers to use as indexes into the concatenated strings.
 func usize(n int) int {
 	switch {
 	case n < 1<<8:
 		return 8
 	case n < 1<<16:
 		return 16
 	default:
 		// 2^32 is enough constants for anyone.
 		return 32
 	}
 }

 // declareIndexAndNameVars declares the index slices and concatenated names
 // strings representing the runs of values.
 func (g *Generator) declareIndexAndNameVars(runs [][]Value, typeName string) {
 	var indexes, names []string
 	for i, run := range runs {
 		index, name := g.createIndexAndNameDecl(run, typeName, fmt.Sprintf("_%d", i))
 		if len(run) != 1 {
 			indexes = append(indexes, index)
 		}
 		names = append(names, name)
 	}
 	g.Printf("const (\n")
 	for _, name := range names {
 		g.Printf("\t%s\n", name)
 	}
 	g.Printf(")\n\n")

 	if len(indexes) > 0 {
 		g.Printf("var (")
 		for _, index := range indexes {
 			g.Printf("\t%s\n", index)
 		}
 		g.Printf(")\n\n")
 	}
 }

 // declareIndexAndNameVar is the single-run version of declareIndexAndNameVars
 func (g *Generator) declareIndexAndNameVar(run []Value, typeName string) {
 	index, name := g.createIndexAndNameDecl(run, typeName, "")
 	g.Printf("const %s\n", name)
 	g.Printf("var %s\n", index)
 }

 // createIndexAndNameDecl returns the pair of declarations for the run. The caller will add "const" and "var".
 func (g *Generator) createIndexAndNameDecl(run []Value, typeName string, suffix string) (string, string) {
 	b := new(bytes.Buffer)
 	indexes := make([]int, len(run))
 	for i := range run {
 		b.WriteString(run[i].name)
 		indexes[i] = b.Len()
 	}
 	nameConst := fmt.Sprintf("_%s_name%s = %q", typeName, suffix, b.String())
 	nameLen := b.Len()
 	b.Reset()
 	fmt.Fprintf(b, "_%s_index%s = [...]uint%d{0, ", typeName, suffix, usize(nameLen))
 	for i, v := range indexes {
 		if i > 0 {
 			fmt.Fprintf(b, ", ")
 		}
 		fmt.Fprintf(b, "%d", v)
 	}
 	fmt.Fprintf(b, "}")
 	return b.String(), nameConst
 }

 // declareNameVars declares the concatenated names string representing all the values in the runs.
 func (g *Generator) declareNameVars(runs [][]Value, typeName string, suffix string) {
 	g.Printf("const _%s_name%s = \"", typeName, suffix)
 	for _, run := range runs {
 		for i := range run {
 			g.Printf("%s", run[i].name)
 		}
 	}
 	g.Printf("\"\n")
 }

 // buildOneRun generates the variables and String method for a single run of contiguous values.
 func (g *Generator) buildOneRun(runs [][]Value, typeName string) {
 	values := runs[0]
 	g.Printf("\n")
 	g.declareIndexAndNameVar(values, typeName)
 	// The generated code is simple enough to write as a Printf format.
 	lessThanZero := ""
 	if values[0].signed {
 		lessThanZero = "i < 0 || "
 	}
 	if values[0].value == 0 { // Signed or unsigned, 0 is still 0.
 		g.Printf(stringOneRun, typeName, usize(len(values)), lessThanZero)
 	} else {
 		g.Printf(stringOneRunWithOffset, typeName, values[0].String(), usize(len(values)), lessThanZero)
 	}
 }

 // Arguments to format are:
 //	[1]: type name
 //	[2]: size of index element (8 for uint8 etc.)
 //	[3]: less than zero check (for signed types)
 const stringOneRun = `func (i %[1]s) String() string {
 	if %[3]si >= %[1]s(len(_%[1]s_index)-1) {
 		return "%[1]s(" + strconv.FormatInt(int64(i), 10) + ")"
 	}
 	return _%[1]s_name[_%[1]s_index[i]:_%[1]s_index[i+1]]
 }
 `

 // Arguments to format are:
 //	[1]: type name
 //	[2]: lowest defined value for type, as a string
 //	[3]: size of index element (8 for uint8 etc.)
 //	[4]: less than zero check (for signed types)
 /*
  */
 const stringOneRunWithOffset = `func (i %[1]s) String() string {
 	i -= %[2]s
 	if %[4]si >= %[1]s(len(_%[1]s_index)-1) {
 		return "%[1]s(" + strconv.FormatInt(int64(i + %[2]s), 10) + ")"
 	}
 	return _%[1]s_name[_%[1]s_index[i] : _%[1]s_index[i+1]]
 }
 `

 // buildMultipleRuns generates the variables and String method for multiple runs of contiguous values.
 // For this pattern, a single Printf format won't do.
 func (g *Generator) buildMultipleRuns(runs [][]Value, typeName string) {
 	g.Printf("\n")
 	g.declareIndexAndNameVars(runs, typeName)
 	g.Printf("func (i %s) String() string {\n", typeName)
 	g.Printf("\tswitch {\n")
 	for i, values := range runs {
 		if len(values) == 1 {
 			g.Printf("\tcase i == %s:\n", &values[0])
 			g.Printf("\t\treturn _%s_name_%d\n", typeName, i)
 			continue
 		}
 		if values[0].value == 0 && !values[0].signed {
 			// For an unsigned lower bound of 0, "0 <= i" would be redundant.
 			g.Printf("\tcase i <= %s:\n", &values[len(values)-1])
 		} else {
 			g.Printf("\tcase %s <= i && i <= %s:\n", &values[0], &values[len(values)-1])
 		}
 		if values[0].value != 0 {
 			g.Printf("\t\ti -= %s\n", &values[0])
 		}
 		g.Printf("\t\treturn _%s_name_%d[_%s_index_%d[i]:_%s_index_%d[i+1]]\n",
 			typeName, i, typeName, i, typeName, i)
 	}
 	g.Printf("\tdefault:\n")
 	g.Printf("\t\treturn \"%s(\" + strconv.FormatInt(int64(i), 10) + \")\"\n", typeName)
 	g.Printf("\t}\n")
 	g.Printf("}\n")
 }

 // buildMap handles the case where the space is so sparse a map is a reasonable fallback.
 // It's a rare situation but has simple code.
 func (g *Generator) buildMap(runs [][]Value, typeName string) {
 	g.Printf("\n")
 	g.declareNameVars(runs, typeName, "")
 	g.Printf("\nvar _%s_map = map[%s]string{\n", typeName, typeName)
 	n := 0
 	for _, values := range runs {
 		for _, value := range values {
 			g.Printf("\t%s: _%s_name[%d:%d],\n", &value, typeName, n, n+len(value.name))
 			n += len(value.name)
 		}
 	}
 	g.Printf("}\n\n")
 	g.Printf(stringMap, typeName)
 }

 // Argument to format is the type name.
 const stringMap = `func (i %[1]s) String() string {
 	if str, ok := _%[1]s_map[i]; ok {
 		return str
 	}
 	return "%[1]s(" + strconv.FormatInt(int64(i), 10) + ")"
 }
 `
	// Copyright 2014 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.

	// Stringer is a tool to automate the creation of methods that satisfy the fmt.Stringer
	// interface. Given the name of a (signed or unsigned) integer type T that has constants
	// defined, stringer will create a new self-contained Go source file implementing
	// func (t T) String() string
	// The file is created in the same package and directory as the package that defines T.
	// It has helpful defaults designed for use with go generate.
	//
	// Stringer works best with constants that are consecutive values such as created using iota,
	// but creates good code regardless. In the future it might also provide custom support for
	// constant sets that are bit patterns.
	//
	// For example, given this snippet,
	//
	// package painkiller
	//
	// type Pill int
	//
	// const (
	// Placebo Pill = iota
	// Aspirin
	// Ibuprofen
	// Paracetamol
	// Acetaminophen = Paracetamol
	// )
	//
	// running this command
	//
	// stringer -type=Pill
	//
	// in the same directory will create the file pill_string.go, in package painkiller,
	// containing a definition of
	//
	// func (Pill) String() string
	//
	// That method will translate the value of a Pill constant to the string representation
	// of the respective constant name, so that the call fmt.Print(painkiller.Aspirin) will
	// print the string "Aspirin".
	//
	// Typically this process would be run using go generate, like this:
	//
	// //go:generate stringer -type=Pill
	//
	// If multiple constants have the same value, the lexically first matching name will
	// be used (in the example, Acetaminophen will print as "Paracetamol").
	//
	// With no arguments, it processes the package in the current directory.
	// Otherwise, the arguments must name a single directory holding a Go package
	// or a set of Go source files that represent a single Go package.
	//
	// The -type flag accepts a comma-separated list of types so a single run can
	// generate methods for multiple types. The default output file is t_string.go,
	// where t is the lower-cased name of the first type listed. It can be overridden
	// with the -output flag.
	//
	// The -linecomment flag tells stringer to generate the text of any line comment, trimmed
	// of leading spaces, instead of the constant name. For instance, if the constants above had a
	// Pill prefix, one could write
	//
	// PillAspirin // Aspirin
	//
	// to suppress it in the output.
	package main // import "golang.org/x/tools/cmd/stringer"

	import (
	"bytes"
	"flag"
	"fmt"
	"go/ast"
	"go/constant"
	"go/format"
	"go/token"
	"go/types"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"
	"sort"
	"strings"

	"golang.org/x/tools/go/packages"
	)

	var (
	typeNames = flag.String("type", "", "comma-separated list of type names; must be set")
	output = flag.String("output", "", "output file name; default srcdir/<type>_string.go")
	trimprefix = flag.String("trimprefix", "", "trim the `prefix` from the generated constant names")
	linecomment = flag.Bool("linecomment", false, "use line comment text as printed text when present")
	buildTags = flag.String("tags", "", "comma-separated list of build tags to apply")
	)

	// Usage is a replacement usage function for the flags package.
	func Usage() {
	fmt.Fprintf(os.Stderr, "Usage of stringer:\n")
	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T files... # Must be a single package\n")
	fmt.Fprintf(os.Stderr, "For more information, see:\n")
	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
	fmt.Fprintf(os.Stderr, "Flags:\n")
	flag.PrintDefaults()
	}

	func main() {
	log.SetFlags(0)
	log.SetPrefix("stringer: ")
	flag.Usage = Usage
	flag.Parse()
	if len(*typeNames) == 0 {
	flag.Usage()
	os.Exit(2)
	}
	types := strings.Split(*typeNames, ",")
	var tags []string
	if len(*buildTags) > 0 {
	tags = strings.Split(*buildTags, ",")
	}

	// We accept either one directory or a list of files. Which do we have?
	args := flag.Args()
	if len(args) == 0 {
	// Default: process whole package in current directory.
	args = []string{"."}
	}

	// Parse the package once.
	var dir string
	g := Generator{
	trimPrefix: *trimprefix,
	lineComment: *linecomment,
	}
	// TODO(suzmue): accept other patterns for packages (directories, list of files, import paths, etc).
	if len(args) == 1 && isDirectory(args[0]) {
	dir = args[0]
	} else {
	if len(tags) != 0 {
	log.Fatal("-tags option applies only to directories, not when files are specified")
	}
	dir = filepath.Dir(args[0])
	}

	g.parsePackage(args, tags)

	// Print the header and package clause.
	g.Printf("// Code generated by \"stringer %s\"; DO NOT EDIT.\n", strings.Join(os.Args[1:], " "))
	g.Printf("\n")
	g.Printf("package %s", g.pkg.name)
	g.Printf("\n")
	g.Printf("import \"strconv\"\n") // Used by all methods.

	// Run generate for each type.
	for _, typeName := range types {
	g.generate(typeName)
	}

	// Format the output.
	src := g.format()

	// Write to file.
	outputName := *output
	if outputName == "" {
	baseName := fmt.Sprintf("%s_string.go", types[0])
	outputName = filepath.Join(dir, strings.ToLower(baseName))
	}
	err := ioutil.WriteFile(outputName, src, 0644)
	if err != nil {
	log.Fatalf("writing output: %s", err)
	}
	}

	// isDirectory reports whether the named file is a directory.
	func isDirectory(name string) bool {
	info, err := os.Stat(name)
	if err != nil {
	log.Fatal(err)
	}
	return info.IsDir()
	}

	// Generator holds the state of the analysis. Primarily used to buffer
	// the output for format.Source.
	type Generator struct {
	buf bytes.Buffer // Accumulated output.
	pkg *Package // Package we are scanning.

	trimPrefix string
	lineComment bool
	}

	func (g *Generator) Printf(format string, args ...interface{}) {
	fmt.Fprintf(&g.buf, format, args...)
	}

	// File holds a single parsed file and associated data.
	type File struct {
	pkg *Package // Package to which this file belongs.
	file *ast.File // Parsed AST.
	// These fields are reset for each type being generated.
	typeName string // Name of the constant type.
	values []Value // Accumulator for constant values of that type.

	trimPrefix string
	lineComment bool
	}

	type Package struct {
	name string
	defs map[*ast.Ident]types.Object
	files []*File
	}

	// parsePackage analyzes the single package constructed from the patterns and tags.
	// parsePackage exits if there is an error.
	func (g *Generator) parsePackage(patterns []string, tags []string) {
	cfg := &packages.Config{
	Mode: packages.LoadSyntax,
	// TODO: Need to think about constants in test files. Maybe write type_string_test.go
	// in a separate pass? For later.
	Tests: false,
	BuildFlags: []string{fmt.Sprintf("-tags=%s", strings.Join(tags, " "))},
	}
	pkgs, err := packages.Load(cfg, patterns...)
	if err != nil {
	log.Fatal(err)
	}
	if len(pkgs) != 1 {
	log.Fatalf("error: %d packages found", len(pkgs))
	}
	g.addPackage(pkgs[0])
	}

	// addPackage adds a type checked Package and its syntax files to the generator.
	func (g Generator) addPackage(pkg packages.Package) {
	g.pkg = &Package{
	name: pkg.Name,
	defs: pkg.TypesInfo.Defs,
	files: make([]*File, len(pkg.Syntax)),
	}

	for i, file := range pkg.Syntax {
	g.pkg.files[i] = &File{
	file: file,
	pkg: g.pkg,
	trimPrefix: g.trimPrefix,
	lineComment: g.lineComment,
	}
	}
	}

	// generate produces the String method for the named type.
	func (g *Generator) generate(typeName string) {
	values := make([]Value, 0, 100)
	for _, file := range g.pkg.files {
	// Set the state for this run of the walker.
	file.typeName = typeName
	file.values = nil
	if file.file != nil {
	ast.Inspect(file.file, file.genDecl)
	values = append(values, file.values...)
	}
	}

	if len(values) == 0 {
	log.Fatalf("no values defined for type %s", typeName)
	}
	// Generate code that will fail if the constants change value.
	g.Printf("func _() {\n")
	g.Printf("\t// An \"invalid array index\" compiler error signifies that the constant values have changed.\n")
	g.Printf("\t// Re-run the stringer command to generate them again.\n")
	g.Printf("\tvar x [1]struct{}\n")
	for _, v := range values {
	g.Printf("\t_ = x[%s - %s]\n", v.originalName, v.str)
	}
	g.Printf("}\n")
	runs := splitIntoRuns(values)
	// The decision of which pattern to use depends on the number of
	// runs in the numbers. If there's only one, it's easy. For more than
	// one, there's a tradeoff between complexity and size of the data
	// and code vs. the simplicity of a map. A map takes more space,
	// but so does the code. The decision here (crossover at 10) is
	// arbitrary, but considers that for large numbers of runs the cost
	// of the linear scan in the switch might become important, and
	// rather than use yet another algorithm such as binary search,
	// we punt and use a map. In any case, the likelihood of a map
	// being necessary for any realistic example other than bitmasks
	// is very low. And bitmasks probably deserve their own analysis,
	// to be done some other day.
	switch {
	case len(runs) == 1:
	g.buildOneRun(runs, typeName)
	case len(runs) <= 10:
	g.buildMultipleRuns(runs, typeName)
	default:
	g.buildMap(runs, typeName)
	}
	}

	// splitIntoRuns breaks the values into runs of contiguous sequences.
	// For example, given 1,2,3,5,6,7 it returns {1,2,3},{5,6,7}.
	// The input slice is known to be non-empty.
	func splitIntoRuns(values []Value) [][]Value {
	// We use stable sort so the lexically first name is chosen for equal elements.
	sort.Stable(byValue(values))
	// Remove duplicates. Stable sort has put the one we want to print first,
	// so use that one. The String method won't care about which named constant
	// was the argument, so the first name for the given value is the only one to keep.
	// We need to do this because identical values would cause the switch or map
	// to fail to compile.
	j := 1
	for i := 1; i < len(values); i++ {
	if values[i].value != values[i-1].value {
	values[j] = values[i]
	j++
	}
	}
	values = values[:j]
	runs := make([][]Value, 0, 10)
	for len(values) > 0 {
	// One contiguous sequence per outer loop.
	i := 1
	for i < len(values) && values[i].value == values[i-1].value+1 {
	i++
	}
	runs = append(runs, values[:i])
	values = values[i:]
	}
	return runs
	}

	// format returns the gofmt-ed contents of the Generator's buffer.
	func (g *Generator) format() []byte {
	src, err := format.Source(g.buf.Bytes())
	if err != nil {
	// Should never happen, but can arise when developing this code.
	// The user can compile the output to see the error.
	log.Printf("warning: internal error: invalid Go generated: %s", err)
	log.Printf("warning: compile the package to analyze the error")
	return g.buf.Bytes()
	}
	return src
	}

	// Value represents a declared constant.
	type Value struct {
	originalName string // The name of the constant.
	name string // The name with trimmed prefix.
	// The value is stored as a bit pattern alone. The boolean tells us
	// whether to interpret it as an int64 or a uint64; the only place
	// this matters is when sorting.
	// Much of the time the str field is all we need; it is printed
	// by Value.String.
	value uint64 // Will be converted to int64 when needed.
	signed bool // Whether the constant is a signed type.
	str string // The string representation given by the "go/constant" package.
	}

	func (v *Value) String() string {
	return v.str
	}

	// byValue lets us sort the constants into increasing order.
	// We take care in the Less method to sort in signed or unsigned order,
	// as appropriate.
	type byValue []Value

	func (b byValue) Len() int { return len(b) }
	func (b byValue) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
	func (b byValue) Less(i, j int) bool {
	if b[i].signed {
	return int64(b[i].value) < int64(b[j].value)
	}
	return b[i].value < b[j].value
	}

	// genDecl processes one declaration clause.
	func (f *File) genDecl(node ast.Node) bool {
	decl, ok := node.(*ast.GenDecl)
	if !ok \|\| decl.Tok != token.CONST {
	// We only care about const declarations.
	return true
	}
	// The name of the type of the constants we are declaring.
	// Can change if this is a multi-element declaration.
	typ := ""
	// Loop over the elements of the declaration. Each element is a ValueSpec:
	// a list of names possibly followed by a type, possibly followed by values.
	// If the type and value are both missing, we carry down the type (and value,
	// but the "go/types" package takes care of that).
	for _, spec := range decl.Specs {
	vspec := spec.(*ast.ValueSpec) // Guaranteed to succeed as this is CONST.
	if vspec.Type == nil && len(vspec.Values) > 0 {
	// "X = 1". With no type but a value. If the constant is untyped,
	// skip this vspec and reset the remembered type.
	typ = ""

	// If this is a simple type conversion, remember the type.
	// We don't mind if this is actually a call; a qualified call won't
	// be matched (that will be SelectorExpr, not Ident), and only unusual
	// situations will result in a function call that appears to be
	// a type conversion.
	ce, ok := vspec.Values[0].(*ast.CallExpr)
	if !ok {
	continue
	}
	id, ok := ce.Fun.(*ast.Ident)
	if !ok {
	continue
	}
	typ = id.Name
	}
	if vspec.Type != nil {
	// "X T". We have a type. Remember it.
	ident, ok := vspec.Type.(*ast.Ident)
	if !ok {
	continue
	}
	typ = ident.Name
	}
	if typ != f.typeName {
	// This is not the type we're looking for.
	continue
	}
	// We now have a list of names (from one line of source code) all being
	// declared with the desired type.
	// Grab their names and actual values and store them in f.values.
	for _, name := range vspec.Names {
	if name.Name == "_" {
	continue
	}
	// This dance lets the type checker find the values for us. It's a
	// bit tricky: look up the object declared by the name, find its
	// types.Const, and extract its value.
	obj, ok := f.pkg.defs[name]
	if !ok {
	log.Fatalf("no value for constant %s", name)
	}
	info := obj.Type().Underlying().(*types.Basic).Info()
	if info&types.IsInteger == 0 {
	log.Fatalf("can't handle non-integer constant type %s", typ)
	}
	value := obj.(*types.Const).Val() // Guaranteed to succeed as this is CONST.
	if value.Kind() != constant.Int {
	log.Fatalf("can't happen: constant is not an integer %s", name)
	}
	i64, isInt := constant.Int64Val(value)
	u64, isUint := constant.Uint64Val(value)
	if !isInt && !isUint {
	log.Fatalf("internal error: value of %s is not an integer: %s", name, value.String())
	}
	if !isInt {
	u64 = uint64(i64)
	}
	v := Value{
	originalName: name.Name,
	value: u64,
	signed: info&types.IsUnsigned == 0,
	str: value.String(),
	}
	if c := vspec.Comment; f.lineComment && c != nil && len(c.List) == 1 {
	v.name = strings.TrimSpace(c.Text())
	} else {
	v.name = strings.TrimPrefix(v.originalName, f.trimPrefix)
	}
	f.values = append(f.values, v)
	}
	}
	return false
	}

	// Helpers

	// usize returns the number of bits of the smallest unsigned integer
	// type that will hold n. Used to create the smallest possible slice of
	// integers to use as indexes into the concatenated strings.
	func usize(n int) int {
	switch {
	case n < 1<<8:
	return 8
	case n < 1<<16:
	return 16
	default:
	// 2^32 is enough constants for anyone.
	return 32
	}
	}

	// declareIndexAndNameVars declares the index slices and concatenated names
	// strings representing the runs of values.
	func (g *Generator) declareIndexAndNameVars(runs [][]Value, typeName string) {
	var indexes, names []string
	for i, run := range runs {
	index, name := g.createIndexAndNameDecl(run, typeName, fmt.Sprintf("_%d", i))
	if len(run) != 1 {
	indexes = append(indexes, index)
	}
	names = append(names, name)
	}
	g.Printf("const (\n")
	for _, name := range names {
	g.Printf("\t%s\n", name)
	}
	g.Printf(")\n\n")

	if len(indexes) > 0 {
	g.Printf("var (")
	for _, index := range indexes {
	g.Printf("\t%s\n", index)
	}
	g.Printf(")\n\n")
	}
	}

	// declareIndexAndNameVar is the single-run version of declareIndexAndNameVars
	func (g *Generator) declareIndexAndNameVar(run []Value, typeName string) {
	index, name := g.createIndexAndNameDecl(run, typeName, "")
	g.Printf("const %s\n", name)
	g.Printf("var %s\n", index)
	}

	// createIndexAndNameDecl returns the pair of declarations for the run. The caller will add "const" and "var".
	func (g *Generator) createIndexAndNameDecl(run []Value, typeName string, suffix string) (string, string) {
	b := new(bytes.Buffer)
	indexes := make([]int, len(run))
	for i := range run {
	b.WriteString(run[i].name)
	indexes[i] = b.Len()
	}
	nameConst := fmt.Sprintf("_%s_name%s = %q", typeName, suffix, b.String())
	nameLen := b.Len()
	b.Reset()
	fmt.Fprintf(b, "_%s_index%s = [...]uint%d{0, ", typeName, suffix, usize(nameLen))
	for i, v := range indexes {
	if i > 0 {
	fmt.Fprintf(b, ", ")
	}
	fmt.Fprintf(b, "%d", v)
	}
	fmt.Fprintf(b, "}")
	return b.String(), nameConst
	}

	// declareNameVars declares the concatenated names string representing all the values in the runs.
	func (g *Generator) declareNameVars(runs [][]Value, typeName string, suffix string) {
	g.Printf("const _%s_name%s = \"", typeName, suffix)
	for _, run := range runs {
	for i := range run {
	g.Printf("%s", run[i].name)
	}
	}
	g.Printf("\"\n")
	}

	// buildOneRun generates the variables and String method for a single run of contiguous values.
	func (g *Generator) buildOneRun(runs [][]Value, typeName string) {
	values := runs[0]
	g.Printf("\n")
	g.declareIndexAndNameVar(values, typeName)
	// The generated code is simple enough to write as a Printf format.
	lessThanZero := ""
	if values[0].signed {
	lessThanZero = "i < 0 \|\| "
	}
	if values[0].value == 0 { // Signed or unsigned, 0 is still 0.
	g.Printf(stringOneRun, typeName, usize(len(values)), lessThanZero)
	} else {
	g.Printf(stringOneRunWithOffset, typeName, values[0].String(), usize(len(values)), lessThanZero)
	}
	}

	// Arguments to format are:
	// [1]: type name
	// [2]: size of index element (8 for uint8 etc.)
	// [3]: less than zero check (for signed types)
	const stringOneRun = `func (i %[1]s) String() string {
	if %[3]si >= %[1]s(len(_%[1]s_index)-1) {
	return "%[1]s(" + strconv.FormatInt(int64(i), 10) + ")"
	}
	return _%[1]s_name[_%[1]s_index[i]:_%[1]s_index[i+1]]
	}
	`

	// Arguments to format are:
	// [1]: type name
	// [2]: lowest defined value for type, as a string
	// [3]: size of index element (8 for uint8 etc.)
	// [4]: less than zero check (for signed types)
	/*
	*/
	const stringOneRunWithOffset = `func (i %[1]s) String() string {
	i -= %[2]s
	if %[4]si >= %[1]s(len(_%[1]s_index)-1) {
	return "%[1]s(" + strconv.FormatInt(int64(i + %[2]s), 10) + ")"
	}
	return _%[1]s_name[_%[1]s_index[i] : _%[1]s_index[i+1]]
	}
	`

	// buildMultipleRuns generates the variables and String method for multiple runs of contiguous values.
	// For this pattern, a single Printf format won't do.
	func (g *Generator) buildMultipleRuns(runs [][]Value, typeName string) {
	g.Printf("\n")
	g.declareIndexAndNameVars(runs, typeName)
	g.Printf("func (i %s) String() string {\n", typeName)
	g.Printf("\tswitch {\n")
	for i, values := range runs {
	if len(values) == 1 {
	g.Printf("\tcase i == %s:\n", &values[0])
	g.Printf("\t\treturn _%s_name_%d\n", typeName, i)
	continue
	}
	if values[0].value == 0 && !values[0].signed {
	// For an unsigned lower bound of 0, "0 <= i" would be redundant.
	g.Printf("\tcase i <= %s:\n", &values[len(values)-1])
	} else {
	g.Printf("\tcase %s <= i && i <= %s:\n", &values[0], &values[len(values)-1])
	}
	if values[0].value != 0 {
	g.Printf("\t\ti -= %s\n", &values[0])
	}
	g.Printf("\t\treturn _%s_name_%d[_%s_index_%d[i]:_%s_index_%d[i+1]]\n",
	typeName, i, typeName, i, typeName, i)
	}
	g.Printf("\tdefault:\n")
	g.Printf("\t\treturn \"%s(\" + strconv.FormatInt(int64(i), 10) + \")\"\n", typeName)
	g.Printf("\t}\n")
	g.Printf("}\n")
	}

	// buildMap handles the case where the space is so sparse a map is a reasonable fallback.
	// It's a rare situation but has simple code.
	func (g *Generator) buildMap(runs [][]Value, typeName string) {
	g.Printf("\n")
	g.declareNameVars(runs, typeName, "")
	g.Printf("\nvar _%s_map = map[%s]string{\n", typeName, typeName)
	n := 0
	for _, values := range runs {
	for _, value := range values {
	g.Printf("\t%s: _%s_name[%d:%d],\n", &value, typeName, n, n+len(value.name))
	n += len(value.name)
	}
	}
	g.Printf("}\n\n")
	g.Printf(stringMap, typeName)
	}

	// Argument to format is the type name.
	const stringMap = `func (i %[1]s) String() string {
	if str, ok := _%[1]s_map[i]; ok {
	return str
	}
	return "%[1]s(" + strconv.FormatInt(int64(i), 10) + ")"
	}
	`