enum.go

package main

import (
	"bytes"
	"flag"
	"fmt"
	"go/ast"
	"go/constant"
	"go/format"
	"go/token"
	"go/types"
	"log"
	"os"
	"path/filepath"
	"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")
)

// Usage is a replacement usage function for the flags package.
func Usage() {
	fmt.Fprintf(os.Stderr, "Usage of enum:\n")
	fmt.Fprintf(os.Stderr, "Flags:\n")
	flag.PrintDefaults()
}

func main() {
	log.SetFlags(0)
	log.SetPrefix("enum: ")
	flag.Usage = Usage
	flag.Parse()
	if len(*typeNames) == 0 {
		flag.Usage()
		os.Exit(2)
	}
	types := strings.Split(*typeNames, ",")

	// 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{}
	// 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 {
		dir = filepath.Dir(args[0])
	}

	g.parsePackage(args)

	// 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")

	// 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 := os.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.
}

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.

}

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) {
	cfg := &packages.Config{
		Mode:  packages.NeedSyntax,
		Tests: false,
	}
	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,
		}
	}
}

// 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 {
		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("\nimport (\n")
	g.Printf("\t\"database/sql/driver\"\n")
	g.Printf("\t\"fmt\"\n")
	g.Printf(")\n")

	g.Printf("func (%s %s) String() string {\n", strings.ToLower(typeName), typeName)
	g.Printf("\tswitch %s {\n", strings.ToLower(typeName))
	for _, v := range values {
		g.Printf("\tcase %s:\n", v.originalName)
		g.Printf("\t\treturn \"%s\"\n", v.value)
	}
	g.Printf("}\n")
	g.Printf("\treturn \"\"\n")
	g.Printf("}\n\n")

	g.Printf("func (%s %s) Value() (driver.Value, error) {\n", strings.ToLower(typeName), typeName)
	g.Printf("\treturn string(%s), nil\n", strings.ToLower(typeName))
	g.Printf("}\n\n")

	g.Printf("func (%s *%s) Scan(v interface{}) error {\n", strings.ToLower(typeName), typeName)
	g.Printf("\tvalue, ok := v.(string)\n")
	g.Printf("\tif ok {\n")
	g.Printf("\t\t*%s = %s(value)\n", strings.ToLower(typeName), typeName)
	g.Printf("\t\treturn nil\n")
	g.Printf("\t}\n")
	g.Printf("\treturn fmt.Errorf(\"can not convert %%v to %s\", v)\n", typeName)
	g.Printf("}\n\n")

}

// 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.
	value        string // Will be converted to string when needed.
}

// 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.IsString == 0 {
				log.Fatalf("can't handle non-string constant type %s", typ)
			}
			value := obj.(*types.Const).Val() // Guaranteed to succeed as this is CONST.
			if value.Kind() != constant.String {
				log.Fatalf("can't happen: constant is not an string %s", name)
			}
			v := Value{
				originalName: name.Name,
				value:        constant.StringVal(value),
			}
			f.values = append(f.values, v)
		}
	}
	return false
}