zstd_decompress.c is a self-contained implementation of a decoder according to the Zstandard format specification written in C99. While it does not implement as many features as the reference decoder, such as the streaming API or content checksums, it is written to be easy to follow and understand, to help understand how the Zstandard format works. It's laid out to match the format specification, so it can be used to understand how confusing segments could be implemented. It also contains implementations of Huffman and FSE table decoding.
harness.c provides a simple test harness around the decoder:
harness <input-file> <output-file> [dictionary]