Time to run the whole decoder on a real file. Today you wire every decoder stage into one Decode call and turn an embedded baseline JPEG into pixels.
Decode an embedded baseline JPEG through the complete pipeline and read its dimensions and sample pixels.
This is the decoder’s finish line. Decode finally strings every chapter into one call: walk the markers, parse the DQT, DHT, and SOF0 segments, read the SOS header, decode the entropy scan into blocks of coefficients, then dequantize, inverse-DCT, level-shift, upsample chroma, and convert YCbCr to RGB, cropping to the declared size. The embedded file is a genuine baseline JPEG, and it comes out as an image of the right dimensions whose pixels are a flat mid-gray.
Because JPEG is lossy, the check is a tolerance, not exact bytes: a solid-gray source survives the transform and quantization almost perfectly, so every pixel lands within a couple of levels of (128,128,128), but pinning an exact value would be fragile. Every hard thing you built is exercised at once - the marker walk, the canonical Huffman decode with byte-stuffing, receive-and-extend, the run-length AC loop, the inverse DCT, and chroma upsampling - all cooperating to turn a real file into a picture. One capstone remains: proving the encoder closes the loop.
// Decode: walk markers -> parse DQT/DHT/SOF0 -> read SOS -> decode the// entropy scan (MCUs of blocks) -> dequantize + un-zig-zag -> inverse// DCT -> level shift -> upsample chroma -> YCbCr to RGB -> crop.// The embedded file is a valid baseline JPEG; assert dims + sample pixels.func Decode(b []byte) (*Image, error) { }