golang：从文件中读取字符串，整数和字节

Question

I have to read a PPM file with its data encrypted (metadata is not encrypted), using Go, and the file format is given to me containing:

• The "P3" magic number (read as string)

• Image width (read as integer)

• Image height (read as integer)

• Maximum color value (read as integer)

Then, I need to read the rest of the file is the encrypted bits, which I have to read as a single byte array/slice.

e.g.:

P6
480 360
255
�š��GHFHFI�GHFHFG~EG~EG~E
...
HFD{BR�Rz�y|�vxyyhlf%8&NFzx

What is a good way to read string and integers (the 4 initial metadata values) and the rest (encrypted part) as bytes from the file? It can be the most efficient, but the cleanest (less lines) is preferred.

Answer 1

You can use bufio.Reader to read the first 3 lines using the ReadLine or ReadString method and reading the rest of the file using the Read method.

After you've read the first 3 lines you can use the strings package to split the second line, and then the strconv package to parse the strings containing numbers as integers.

For example:

r := bufio.NewReader(file)
line1, err := r.ReadString('
')
if err != nil {
    panic(err)
}
// repeat to read line 2 and 3

size := strings.Split(line2, " ")
width, err := strconv.Atoi(size[0])
if err != nil {
    panic(err)
}
height, err := strconv.Atoi(size[1])
if err != nil {
    panic(err)
}
// repeat with line 3

Update:

As mentioned in the comments by Adrian you can use bufio.Scanner together with the bufio.ScanWord SplitFunc to scan for the metadata.

s := bufio.NewScanner(r)
s.Split(bufio.ScanWords)

var count int
for s.Scan() && count < 4 {
    switch count {
    case 0:
        magic = s.Text()
    case 1:
        if width, err = strconv.Atoi(s.Text()); err != nil {
            return
        }
    case 2:
        if height, err = strconv.Atoi(s.Text()); err != nil {
            return
        }
    case 3:
        if color, err = strconv.Atoi(s.Text()); err != nil {
            return
        }
    }
    count++
}

https://play.golang.org/p/-rOJb_WlFf