5.6 seeking a position within a file 查找文件中的位置

package main

import (
	"errors"
	"fmt"
	"os"
)

const lineLegth = 25

func main() {

	f, e := os.OpenFile("flatfile.txt", os.O_RDWR|os.O_CREATE, os.ModePerm)
	if e != nil {
		panic(e)
	}
	defer f.Close()

	fmt.Println(readRecords(2, "last", f))
	if err := writeRecord(2, "first", "Radomir", f); err != nil {
		panic(err)
	}
	fmt.Println(readRecords(2, "first", f))
	if err := writeRecord(10, "first", "Andrew", f); err != nil {
		panic(err)
	}
	fmt.Println(readRecords(10, "first", f))
	fmt.Println(readLine(2, f))
}

func readLine(line int, f *os.File) (string, error) {
	lineBuffer := make([]byte, 24)
	f.Seek(int64(line*lineLegth), 0)
	_, err := f.Read(lineBuffer)
	return string(lineBuffer), err
}

func writeRecord(line int, column, dataStr string, f *os.File) error {
	definedLen := 10
	position := int64(line * lineLegth)
	switch column {
	case "id":
		definedLen = 4
	case "first":
		position += 4
	case "last":
		position += 14
	default:
		return errors.New("Column not defined")
	}

	if len([]byte(dataStr)) > definedLen {
		return fmt.Errorf("Maximum length for '%s' is %d", column, definedLen)
	}

	data := make([]byte, definedLen)
	for i := range data {
		data[i] = '.'
	}
	copy(data, []byte(dataStr))
	_, err := f.WriteAt(data, position)
	return err
}

func readRecords(line int, column string, f *os.File) (string, error) {
	lineBuffer := make([]byte, 24)
	f.ReadAt(lineBuffer, int64(line*lineLegth))
	var retVal string
	switch column {
	case "id":
		return string(lineBuffer[:3]), nil
	case "first":
		return string(lineBuffer[4:13]), nil
	case "last":
		return string(lineBuffer[14:23]), nil
	}

	return retVal, errors.New("Column not defined")
}

/*
 322e 2e4 <nil>
Radomir.. <nil>
Andrew... <nil>
2e2eRadomir... 322e 2e4e <nil>
 */

 
 /* file content
 3132 332e 4a75 6e2e 2e2e 2e2e 2e2e 576f
6e67 2e2e 2e2eRadomir... 322e 2e4e 6f76
616b 2e2e 2e2e 2e4a 7572 6765 6e2e 2e2e
2e0a 3130 2e2e 5261 646f 6d69 722e 2e2e
536f 686c 6963 682e 2e2e 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000Andrew.... 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 0000
0000 0000 0000 0000 0000 0000 0000 416e
6472 6577 2e2e 2e2e
 
 
 */