You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: extract.go
    path of file: ./repos/go-starter-kit/vendor/golang.org/x/text/cmd/gotext
    
    the code of the file until where you have to start completion: // Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"go/ast"
	"go/build"
	"go/constant"
	"go/format"
	"go/parser"
	"go/types"
	"io/ioutil"
	"os"
	"path"
	"path/filepath"
	"strings"

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

// TODO:
// - merge information into existing files
// - handle different file formats (PO, XLIFF)
// - handle features (gender, plural)
// - message rewriting

var cmdExtract = &Command{
	Run:       runExtract,
	UsageLine: "extract <package>*",
	Short:     "extract strings to be translated from code",
}

func runExtract(cmd *Command, args []string) error {
	if len(args) == 0 {
		args = []string{"."}
	}

	conf := loader.Config{
		Build:      &build.Default,
		ParserMode: parser.ParseComments,
	}

	// Use the initial packages from the command line.
	args, err := conf.FromArgs(args, false)
	if err != nil {
		return err
	}

	// Load, parse and type-check the whole program.
	iprog, err := conf.Load()
	if err != nil {
		return err
	}

	// print returns Go syntax for the specified node.
	print := func(n ast.Node) string {
		var buf bytes.Buffer
		format.Node(&buf, conf.Fset, n)
		return buf.String()
	}

	var translations []Translation

	for _, info := range iprog.InitialPackages() {
		for _, f := range info.Files {
			// Associate comments with nodes.
			cmap := ast.NewCommentMap(iprog.Fset, f, f.Comments)
			getComment := func(n ast.Node) string {
				cs := cmap.Filter(n).Comments()
				if len(cs) > 0 {
					return strings.TrimSpace(cs[0].Text())
				}
				return ""
			}

			// Find function calls.
			ast
    
func runExtract(cmd *Command, args []string) error {
	if len(args) == 0 {
		args = []string{"."}
	}

	conf := loader.Config{
		Build:      &build.Default,
		ParserMode: parser.ParseComments,
	}

	// Use the initial packages from the command line.
	args, err := conf.FromArgs(args, false)
	if err != nil {
		return err
	}

	// Load, parse and type-check the whole program.
	iprog, err := conf.Load()
	if err != nil {
		return err
	}

	// print returns Go syntax for the specified node.
	print := func(n ast.Node) string {
		var buf bytes.Buffer
		format.Node(&buf, conf.Fset, n)
		return buf.String()
	}

	var translations []Translation

	for _, info := range iprog.InitialPackages() {
		for _, f := range info.Files {
			// Associate comments with nodes.
			cmap := ast.NewCommentMap(iprog.Fset, f, f.Comments)
			getComment := func(n ast.Node) string {
				cs := cmap.Filter(n).Comments()
				if len(cs) > 0 {
					return strings.TrimSpace(cs[0].Text())
				}
				return ""
			}

			// Find function calls.
			ast.Inspect(f, func(n ast.Node) bool {
				call, ok := n.(*ast.CallExpr)
				if !ok {
					return true
				}

				// Skip calls of functions other than
				// (*message.Printer).{Sp,Fp,P}rintf.
				sel, ok := call.Fun.(*ast.SelectorExpr)
				if !ok {
					return true
				}
				meth := info.Selections[sel]
				if meth == nil || meth.Kind() != types.MethodVal {
					return true
				}
				// TODO: remove cheap hack and check if the type either
				// implements some interface or is specifically of type
				// "golang.org/x/text/message".Printer.
				m, ok := extractFuncs[path.Base(meth.Recv().String())]
				if !ok {
					return true
				}

				// argn is the index of the format string.
				argn, ok := m[meth.Obj().Name()]
				if !ok || argn >= len(call.Args) {
					return true
				}

				// Skip calls with non-constant format string.
				fmtstr := info.Types[call.Args[argn]].Value
				if fmtstr == nil || fmtstr.Kind() != constant.String {
					return true
				}

				posn := conf.Fset.Position(call.Lparen)
				filepos := fmt.Sprintf("%s:%d:%d", filepath.Base(posn.Filename), posn.Line, posn.Column)

				// TODO: identify the type of the format argument. If it is not
				// a string, multiple keys may be defined.
				var key []string

				// TODO: replace substitutions (%v) with a translator friendly
				// notation. For instance:
				//     "%d files remaining" -> "{numFiles} files remaining", or
				//     "%d files remaining" -> "{arg1} files remaining"
				// Alternatively, this could be done at a later stage.
				msg := constant.StringVal(fmtstr)

				// Construct a Translation unit.
				c := Translation{
					Key:              key,
					Position:         filepath.Join(info.Pkg.Path(), filepos),
					Original:         Text{Msg: msg},
					ExtractedComment: getComment(call.Args[0]),
					// TODO(fix): this doesn't get the before comment.
					// Comment: getComment(call),
				}

				for i, arg := range call.Args[argn+1:] {
					var val string
					if v := info.Types[arg].Value; v != nil {
						val = v.ExactString()
					}
					posn := conf.Fset.Position(arg.Pos())
					filepos := fmt.Sprintf("%s:%d:%d", filepath.Base(posn.Filename), posn.Line, posn.Column)
					c.Args = append(c.Args, Argument{
						ID:             i + 1,
						Type:           info.Types[arg].Type.String(),
						UnderlyingType: info.Types[arg].Type.Underlying().String(),
						Expr:           print(arg),
						Value:          val,
						Comment:        getComment(arg),
						Position:       filepath.Join(info.Pkg.Path(), filepos),
						// TODO report whether it implements
						// interfaces plural.Interface,
						// gender.Interface.
					})
				}

				translations = append(translations, c)
				return true
			})
		}
	}

	data, err := json.MarshalIndent(translations, "", "    ")
	if err != nil {
		return err
	}
	for _, tag := range getLangs() {
		// TODO: merge with existing files, don't overwrite.
		os.MkdirAll(*dir, 0744)
		file := filepath.Join(*dir, fmt.Sprintf("gotext_%v.out.json", tag))
		if err := ioutil.WriteFile(file, data, 0744); err != nil {
			return fmt.Errorf("could not create file: %v", err)
		}
	}
	return nil
}