Implement std::hash support for evmc_bytes32

Uses FNV1a hash with 64-bit result.

include/evmc/helpers.hpp

@@ -16,6 +16,7 @@
 #include <evmc/evmc.h>
 
 #include <cstring>
+#include <functional>
 
 /// The comparator for std::map<evmc_address, ...>.
 bool operator<(const evmc_address& a, const evmc_address& b)
@@ -41,4 +42,39 @@ bool operator==(const evmc_bytes32& a, const evmc_bytes32& b)
     return std::memcmp(a.bytes, b.bytes, sizeof(a)) == 0;
 }
 
+/// FNV1a hash function with 64-bit result.
+static inline uint64_t fnv1a_64(const uint8_t* ptr, size_t len)
+{
+    constexpr uint64_t prime = 1099511628211ULL;
+    constexpr uint64_t offset_basis = 14695981039346656037ULL;
+
+    uint64_t ret = offset_basis;
+    for (size_t i = 0; i < len; i++)
+    {
+        ret ^= ptr[i];
+        ret *= prime;
+    }
+    return ret;
+}
+
+static_assert(sizeof(size_t) == sizeof(uint64_t), "size_t is not 64-bit wide");
+
+namespace std
+{
+/// Hash operator template needed for std::ordered_set and others using hashes.
+template <>
+struct hash<evmc_bytes32>
+{
+    /// The argument type.
+    typedef evmc_bytes32 argument_type;
+    /// The result type.
+    typedef std::size_t result_type;
+    /// Hash operator using FNV1a.
+    result_type operator()(argument_type const& s) const noexcept
+    {
+        return fnv1a_64(s.bytes, sizeof(s.bytes));
+    }
+};
+}  // namespace std
+
 /** @} */