auto version string updater for Golang

autoup.go

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"io/ioutil"
	"os"
	"strconv"
	"strings"
)

func main() {
	verfile := "version.go"
	if len(os.Args) > 1 {
		verfile = os.Args[1]
	}

	ver, pos, err := searchVersion(verfile)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}

	if ver[0] != '"' || ver[len(ver)-1] != '"' {
		fmt.Fprintln(os.Stderr, "Version need string type")
		os.Exit(1)
	}
	ver = ver[1 : len(ver)-1] // "1.0" -> 1.0

	nums := strings.Split(ver, ".")
	lastnum, err := strconv.Atoi(nums[len(nums)-1])
	if err != nil {
		fmt.Fprintln(os.Stderr, "can't analyze version number")
		os.Exit(1)
	}

	lastnum++
	nums[len(nums)-1] = strconv.Itoa(lastnum)
	verup := strings.Join(nums, ".")

	text, err := ioutil.ReadFile(verfile)
	if err != nil {
		fmt.Fprintf(os.Stderr, "read failed %s: %v\n", verfile, err)
		os.Exit(1)
	}

	if len(ver) < len(verup) {
		text = append(text, byte(0))
		copy(text[pos.Offset+1:], text[pos.Offset:len(text)-1])
	}
	verup = `"` + verup + `"`
	copy(text[pos.Offset:pos.Offset+len(verup)], []byte(verup))

	if err := ioutil.WriteFile(verfile, text, os.ModePerm); err != nil {
		fmt.Fprintf(os.Stderr, "write failed %s: %v\n", verfile, err)
		os.Exit(1)
	}
}

func searchVersion(file string) (string, token.Position, error) {
	empos := token.Position{}
	fset := token.NewFileSet()

	f, err := parser.ParseFile(fset, file, nil, 0)
	if err != nil {
		return "", empos, fmt.Errorf("read failed %s", file)
	}

	for _, decl := range f.Decls {
		if td, ok := decl.(*ast.GenDecl); ok {
			for _, sp := range td.Specs {
				if valsp, ok := sp.(*ast.ValueSpec); ok {
					for i, n := range valsp.Names {
						if n.Name == "Version" {
							lit := valsp.Values[i].(*ast.BasicLit)
							pos := fset.Position(lit.Pos())
							return lit.Value, pos, nil
						}
					}
				}
			}
		}
	}

	return "", empos, fmt.Errorf("not found Version in %s", file)
}