57 lines
1.6 KiB
Go
57 lines
1.6 KiB
Go
// Copyright © 2018 Inanc Gumus
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// Learn Go Programming Course
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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//
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// For more tutorials : https://learngoprogramming.com
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// In-person training : https://www.linkedin.com/in/inancgumus/
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// Follow me on twitter: https://twitter.com/inancgumus
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package main
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import (
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"bytes"
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"fmt"
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"io"
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)
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// reader reads from `r` if the stream starts with a given signature.
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// otherwise it stops and returns with an error.
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type signatureReader struct {
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r io.Reader // reads from the response.Body (or from any reader)
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signature []byte // stream should start with this initial signature
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}
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// Read implements the io.Reader interface.
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func (sr *signatureReader) Read(b []byte) (n int, err error) {
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n, err = sr.r.Read(b)
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l := len(sr.signature)
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if l == 0 {
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// simply return if the signature has already been detected.
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return
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}
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// 1) buffer : **** -> b[:3] -> ***
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// signature: *** -> sr.signature[:3] -> ***
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// 2) buffer : ** -> b[:2] -> **
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// signature: **** -> sr.signature[:2] -> **
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if lb := len(b); lb < l {
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l = lb
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}
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if got, want := b[:l], sr.signature[:l]; !bytes.Equal(got, want) {
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err = fmt.Errorf("signature doesn't match, got: % x, want: % x", got, want)
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}
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// Assuming the `len(b)` is 4.
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// 1st Read(): pr.signature[0:4] -> first part
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// 2nd Read(): pr.signature[0:4] -> second part
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sr.signature = sr.signature[l:]
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return n, err
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}
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// create a reader for detecting only the png signatures.
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func pngReader(r io.Reader) io.Reader {
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return &signatureReader{
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r: r,
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signature: []byte("\x89PNG\r\n\x1a\n"),
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}
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}
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