png

A PNG reader: the chunk layer, the checksum, the filters, and the pixels underneath them — including its own inflate.

The axis: the byte level. Everything here is arithmetic on bytes somebody else laid out. A PNG is a signature, then a sequence of chunks, each one a big-endian length, a four-letter name, that many bytes, and a CRC of the name and the bytes together. The pixels are in the chunks named IDAT, concatenated and then deflated; what comes out is not the image but one filter byte per row followed by a row of differences, and the image is what is left after each row is reconstructed from the row above it and the pixel to its left.

Which is to say: three separate byte orders, four separate length units — bytes, samples, pixels, rows — and a checksum over each of the two layers. Being wrong by one anywhere in that produces an image, just not the right one. That is what the problem is for.

What it exercises

Naming what a file can be wrong about. A reader of somebody else’s format needs a lot of error cases, and naming them individually is the difference between “this file is broken” and a report that says where and what. That is an enum with a variant per failure, carried in a Result — the shape the language pushes you toward, and the one a byte-level reader wants anyway.

unit as a payload. This program and bytecode have nothing in common and both paid for the same missing thing: a fallible step that yields no value. Two independent programs reporting one absence is what moved it from a nuisance to a language change, and it is the clearest example in the set of why findings are written down rather than worked around.

The one piece of mathematics is the Paeth predictor, and the magnitude it needs comes from sysl.math rather than from a private helper this file writes. The integers are an open family, so Signed is a trait the compiler supplies membership for at every width — which is what stops a byte-level program from having to write its own abs for the width it happens to be using.

Worth noticing

The program carries its own fixtures rather than reading a file, so the whole thing is checkable with no filesystem and no os capability. That is not incidental to a guide program: the set has to keep passing across compiler changes, and a test that needs a file on disk is a test that fails for a reason unrelated to the language.

Source · Next: fft — a type the program defined, and floating point.