Make Everything constexpr

[itzmeanjan]

Make whole implementation of SHA3 standard constexpr ( more @ https://en.cppreference.com/w/cpp/language/constexpr ) so that one can evaluate SHA3-{224, 256, 384, 512} and SHAKE-{128, 256} hash function/ xof on statically defined input data during compilation time itself.

#include "sha3_256.hpp"
#include <gtest/gtest.h>

constexpr std::array<uint8_t, sha3_256::DIGEST_LEN>
eval_sha3_256()
{
  // Statically defined input.
  std::array<uint8_t, sha3_256::DIGEST_LEN * 2> data{};
  std::iota(data.begin(), data.end(), 0);

  // To be computed output.
  std::array<uint8_t, sha3_256::DIGEST_LEN> md{};

  sha3_256::sha3_256_t hasher;
  hasher.absorb(data);
  hasher.finalize();
  hasher.digest(md);

  return md;
}

TEST(Sha3Hashing, CompileTimeEvalSha3_256)
{
  // Input  = 000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f
  // Output = c8ad478f4e1dd9d47dfc3b985708d92db1f8db48fe9cddd459e63c321f490402

  constexpr auto md = eval_sha3_256();
  static_assert(md ==
                  std::array<uint8_t, sha3_256::DIGEST_LEN>{
                    200, 173, 71, 143, 78, 29,  217, 212, 125, 252, 59,
                    152, 87,  8,  217, 45, 177, 248, 219, 72,  254, 156,
                    221, 212, 89, 230, 60, 50,  31,  73,  4,   2 },
                "Must compute Sha3-256 hash during compile-time !");
}