sha256.h has the exported (visible from outside) methods and functions. To compute SHA256, you should #include that.

• sha256_sse4.cpp
• sha256_sse41.cpp
• sha256_avx2.cpp
• sha256_shani.cpp

contain SHA256 transform functions (used in SHA256.Write), each specialized for different processors with different instruction sets. Above is the list in order, most of the computers support SSE4, but only a few support SHA-NI instruction set, which is the fastest way a new processor can calculate SHA256.

sha256.cpp contains other methods that aren't a bottleneck, functions that check which instruction sets are supported, tests, as well as a pure-C++ transform function for older and non-x86 computers.

hash.h favors polymorphism via C++ templates.

Examples:

• hash std::vector<something> vch to std::vector<something> vchHash

• hash std::vector<something> in to std::vector<something> WitnessV0ScriptHash

• hash uint8_t * nonce.begin(), uint8_t * hash.begin(), unsigned char pubkey[], std::vector<unsigned char> vchSig to std::vector<unsigned char> entry

• hash op_true to witness_program