Lesson 01 · The eight-byte signature - Build a PNG Encoder and Decoder

The eight-byte signature

Every PNG file begins with the same eight magic bytes. Today you build the smallest useful thing a decoder can do - recognize that a byte stream even claims to be a PNG - so the codec has a front door from day one.

The goal

Report whether a byte slice begins with the exact PNG signature.

Start here - the target

TO DO

Scenario: Recognizing the PNG magic bytes

Giventhe byte sequence 137, 80, 78, 71, 13, 10, 26, 10 followed by any other bytes

Whenthe signature check runs on it

Thenit reports true

Anda sequence whose first byte is 138 instead of 137, or one shorter than eight bytes, reports false

Background

Every PNG file opens with the same eight-byte signature: 89 50 4E 47 0D 0A 1A 0A. It is carefully chosen. The high bit of the first byte (0x89) catches transmission over channels that strip bit 7; the ASCII PNG (50 4E 47) is human-readable in a hex dump; and the 0D 0A, 1A, 0D 0A bytes detect the kinds of newline mangling and file truncation that plagued early binary formats.

You will not decode anything yet. You are building the one gate every decoder needs first: does this stream even claim to be a PNG? Anything that does not begin with these exact bytes is rejected before a single chunk is read. Pin the edges now - the wrong first byte fails, and a stream shorter than eight bytes fails rather than reading past its end.

Make it work

// the fixed 8-byte PNG signature
var Signature = []byte{0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A}
func HasSignature(b []byte) bool {
  // true only if b starts with all eight signature bytes
}

CheckpointDONE

Your codec recognizes a PNG by its signature and rejects anything else. Commit and stop here.