Read string, integers and bytes from file

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.

Are the integers in a string representation, or are they binary? What size are they, or how are they delimited? If they're binary, are the big endian and little endian? — Mr_Pink
– Mr_Pink, Commented May 4, 2017 at 14:00
I think the guys from spakin/netpbm have a pretty neat way of extracting the metadata part for netpbm files using bufio (see their github-repo) — skymon
– skymon, Commented May 4, 2017 at 14:29
@JimB the example I gave is extracted from a real file. The metadata is as written in the example, "P6\n480 360\n255\n", and the rest (data) is just bytes. It's all encoded in utf-8. — Gabriel Calixto
– Gabriel Calixto, Commented May 4, 2017 at 17:27

Community · Accepted Answer · 2017-05-23 11:47:11Z

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('\n') 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

I tried this approach before, but the problem is: in the PPM file format, any whitespace can be used between the metadata themselves and between metadata and image data. All the metadata can be written in a single line ("P3 480 360 255") or any combination of whitespaces, not necessarily newline, and I'd like to cover all cases.
A Scanner would provide the flexibility you need if the delimiters are not consistent: golang.org/pkg/bufio/#Scanner
@GabrielCalixto I've added an example using bufio.Scanner as pointed out by @Adrian.
Thanks, guys! Actually, I need to read the first 4 infos as 1 string and 3 integers, which the current answer does, but I also need to read the rest of the file (the image pixel data) as a single byte array/slice, or maybe read byte per byte. In addition, I cannot ignore whitespace included in the pixel data, since it may be an encrypted byte, so I need to read every byte after the first 4 symbols, I cannot split all the file in lines or words, since it'll remove the whitespace included in the data.

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