Discussion
Upon IDAT blocks (whilst a flag's set & other properties are initialized) another helper reads & validates a couple bytes describing how the data's compressed (GRUB's support is quite limited) before repeatedly reading a few bits (until one of them's set or an error occurred) branching over 2 of them skipping a few bytes afterwords.
For inflate-stored subblocks it reads a 2byte little-endian length skipping a couple more bytes & copies over that many bytes verbatim.
2/?
For inflate-fixed subblocks it inits a decompression tree to a recomputed constant value before a invoking a shared helper with inflate-dynamic subblocks. Inflate-dynamic subblocks meanwhile parses the file's decoding-tree.
This shared helper repeatedly consults the tree decompress a value from the file to branch over. If its less than 256 it appends it to a ringbuffer & outputs it verbatim. If it is 256 it stops decompressing these bytes.
3/4?
And for anything larger it subtracts off 257, retrieves the length from an array possibly summing in a few bytes according to another array, if successful calculates an offset, & copies data over from the ringbuffer byte-by-byte.
A helper for outputting these bytes performs some validation, increments some counters, & branches over the filtering mode to apply the appropriate the corresponding formula which ensures the image Deflate-compresses well.
4/5
There's a suite of helpers to aid constructing the trie consulted for decompressing data (called a "Huffman Tree", but my understanding is that Huffman's innovation applies to compression not decompression).
There's some hardcoded data.
For IHDR blocks it parses out various structural metadata fields & allocates the bitmap.
For PLTE blocks it reads all the data into a 2D palette array.
There's a helper for reading individual bits. With others for 8bit or 16bit values.
5/5 Fin on GRUB Core!
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