Add optimized single byte hash function.

Author: evoskuil

include/bitcoin/system/hash/sha/algorithm.hpp

@@ -77,7 +77,7 @@ class algorithm
     using iblocks_t = iterable<block_t>;
     using digests_t = std::vector<digest_t>;
 
-    /// Constants.
+    /// Count types.
     /// -----------------------------------------------------------------------
 
     /// count_t is uint64_t (sha160/256) or uint128_t (sha512).
@@ -92,12 +92,14 @@ class algorithm
 
     /// Single hashing.
     /// -----------------------------------------------------------------------
+
     template <size_t Size>
     static constexpr digest_t hash(const ablocks_t<Size>& blocks) NOEXCEPT;
     static constexpr digest_t hash(const block_t& block) NOEXCEPT;
     static constexpr digest_t hash(const half_t& half) NOEXCEPT;
     static constexpr digest_t hash(const half_t& left, const half_t& right) NOEXCEPT;
     static constexpr digest_t hash(const quart_t& left, const quart_t& right) NOEXCEPT;
+    static constexpr digest_t hash(uint8_t byte) NOEXCEPT;
     static digest_t hash(iblocks_t&& blocks) NOEXCEPT;
 
     /// Double hashing (sha256/512).
@@ -407,6 +409,7 @@ class algorithm
     static digest_t native_hash(const block_t& block) NOEXCEPT;
     static digest_t native_hash(const half_t& half) NOEXCEPT;
     static digest_t native_hash(const half_t& left, const half_t& right) NOEXCEPT;
+    static digest_t native_hash(uint8_t byte) NOEXCEPT;
 
     static digest_t native_double_hash(const block_t& block) NOEXCEPT;
     static digest_t native_double_hash(const half_t& half) NOEXCEPT;

include/bitcoin/system/impl/hash/sha/algorithm_native.ipp

@@ -344,6 +344,19 @@ native_hash(const half_t& left, const half_t& right) NOEXCEPT
     return native_finalize<one>(state);
 }
 
+TEMPLATE
+typename CLASS::digest_t CLASS::
+native_hash(uint8_t byte) NOEXCEPT
+{
+    constexpr auto pad = bit_hi<uint8_t>;
+
+    block_t block{};
+    block.at(0) = byte;
+    block.at(1) = pad;
+    block.at(63) = byte_bits;
+    return native_hash(block);
+}
+
 // Double hash functions start with BE data and end with BE digest_t.
 // ----------------------------------------------------------------------------
 

include/bitcoin/system/impl/hash/sha/algorithm_single.ipp

@@ -161,6 +161,39 @@ hash(const quart_t& left, const quart_t& right) NOEXCEPT
     }
 }
 
+TEMPLATE
+constexpr typename CLASS::digest_t CLASS::
+hash(uint8_t byte) NOEXCEPT
+{
+    const auto hasher = [](uint8_t byte) NOEXCEPT
+    {
+        // Shift the pad sentinel one byte to make room for the value.
+        // Byte swapped for the sake of simplicity and will be reversed below.
+        constexpr auto pad = shift_left<word_t>(bit_hi<uint8_t>, byte_bits);
+
+        auto state = H::get;
+        buffer_t buffer{};
+        buffer.at(0) = byteswap(bit_or<word_t>(pad, byte));
+        buffer.at(15) = byte_bits;
+        schedule(buffer);
+        compress(state, buffer);
+        return output(state);
+    };
+
+    if (std::is_constant_evaluated())
+    {
+        return hasher(byte);
+    }
+    else if constexpr (native && SHA::strength == 256)
+    {
+        return native_hash(byte);
+    }
+    else
+    {
+        return hasher(byte);
+    }
+}
+
 } // namespace sha
 } // namespace system
 } // namespace libbitcoin

test/hash/sha/sha256.cpp

@@ -201,6 +201,14 @@ BOOST_AUTO_TEST_CASE(sha256__hash__quart_blocks__expected)
     BOOST_CHECK_EQUAL(sha256::hash(sha256::quart_t{ 0 }, sha256::quart_t{ 0 }), expected);
 }
 
+BOOST_AUTO_TEST_CASE(sha256__hash__byte__expected)
+{
+    // github.com/mit-dci/rustreexo/blob/main/src/accumulator/node_hash.rs#L338
+    constexpr auto expected = base16_array("6e340b9cffb37a989ca544e6bb780a2c78901d3fb33738768511a30617afa01d");
+    static_assert(sha256::hash(0) == expected);
+    BOOST_CHECK_EQUAL(sha256::hash(0), expected);
+}
+
 // sha256::double_hash
 BOOST_AUTO_TEST_CASE(sha256__double_hash__full_block__expected)
 {