A DEFLATE stream is a sequence of blocks, each opening with three bits that say whether it is the last block and how it is encoded. Today you read that header, the fork in the road for the whole inflater.
Read a block's final-flag bit and its two-bit type from the bit reader.
DEFLATE data is one or more blocks. Each block starts with a 1-bit BFINAL flag - is this the last block? - followed by a 2-bit BTYPE that selects the encoding: 0 for a stored (uncompressed) block, 1 for a fixed-Huffman block, 2 for a dynamic-Huffman block (3 is reserved and invalid). Reading these three bits is how the inflater decides which decoder to run next, and the BFINAL flag is how it knows when to stop looping over blocks.
Note the bit order in action: from byte 0x01 (0000_0001), the first bit read is the low bit, 1, so BFINAL is set; the next two bits are 00, so BTYPE is a stored block. From 0x04 (0000_0100) the first bit is 0 and the next two are 10 read low-first, giving BTYPE 2, a dynamic block. Three bits decided everything; the rest of the chapter and the next build out the three decoders they point to.
// BFINAL is 1 bit, BTYPE is the next 2 bits (0=stored, 1=fixed, 2=dynamic)func readBlockHeader(r *BitReader) (final bool, btype uint32) {final = r.ReadBit() == 1btype = r.ReadBits(2)return}