state: Implement difficulty calculation for pre Byzantium rev

Author: rodiazet

test/state/ethash_difficulty.cpp

@@ -28,6 +28,29 @@ int64_t get_bomb_delay(evmc_revision rev) noexcept
         return 9'700'000;
     }
 }
+
+int64_t calculate_difficulty_pre_byzantium(int64_t parent_difficulty, int64_t parent_timestamp,
+    int64_t current_timestamp, int64_t block_number, evmc_revision rev)
+{
+    // According to https://eips.ethereum.org/EIPS/eip-2
+    const auto period_count = block_number / 100'000;
+    const auto offset = parent_difficulty / 2048;
+
+    auto diff = parent_difficulty;
+
+    if (rev < EVMC_HOMESTEAD)
+        diff += offset * (current_timestamp - parent_timestamp < 13 ? 1 : -1);
+    else
+        diff += offset * std::max(1 - (current_timestamp - parent_timestamp) / 10, int64_t{-99});
+
+    if (period_count > 2)
+        diff += 2 << (block_number / 100'000 - 3);
+    else if (period_count == 2)
+        diff += 1;
+
+    return diff;
+}
+
 }  // namespace
 
 int64_t calculate_difficulty(int64_t parent_difficulty, bool parent_has_ommers,
@@ -39,9 +62,9 @@ int64_t calculate_difficulty(int64_t parent_difficulty, bool parent_has_ommers,
     if (rev >= EVMC_PARIS)
         return 0;  // No difficulty after the Merge.
 
-    // TODO: Implement for older revisions
     if (rev < EVMC_BYZANTIUM)
-        return 0x020000;
+        return calculate_difficulty_pre_byzantium(
+            parent_difficulty, parent_timestamp, current_timestamp, block_number, rev);
 
     static constexpr auto min_difficulty = int64_t{1} << 17;
 

test/unittests/state_difficulty_test.cpp

@@ -22,6 +22,66 @@ struct DifficultyTest  // NOLINT(clang-analyzer-optin.performance.Padding)
 /// Example difficulty tests from
 /// https://github.com/ethereum/tests/blob/develop/DifficultyTests.
 static constexpr DifficultyTest tests[] = {
+    {
+        EVMC_FRONTIER,
+        "DifficultyTest1",
+        0x0186a0,
+        0x6a8d5758858f3fb6,
+        0x75311a08b,
+        0x6a8007579a9bec39,
+        0x75311a08a,
+        false,
+    },
+    {
+        EVMC_FRONTIER,
+        "DifficultyTest1040",
+        0x10c8e0,
+        0x3857fe2e57047922,
+        0x3558049dc,
+        0x385f0a0f98f79614,
+        0x3558049c8,
+        true,
+    },
+    {
+        EVMC_FRONTIER,
+        "DifficultyTest1031",
+        0x030d40,
+        0x7b435a6e9d83b81e,
+        0x2442b7295,
+        0x7b52c4c7366a856d,
+        0x2442b7281,
+        true,
+    },
+    {
+        EVMC_HOMESTEAD,
+        "DifficultyTest1",
+        0x0186a0,
+        0x6ab7534e3bcfec27,
+        0x68450ae0d,
+        0x6aa9fe0e7a00ac12,
+        0x68450ae0c,
+        false,
+    },
+    {
+        EVMC_HOMESTEAD,
+        "DifficultyTest1040",
+        0x10c8e0,
+        0x024bf6f60ecc847d,
+        0x7b389fe96,
+        0x024c407e1e905487,
+        0x7b389fe82,
+        true,
+    },
+    {
+        EVMC_HOMESTEAD,
+        "DifficultyTest1031",
+        0x030d40,
+        0x39699ae4587a0b05,
+        0x3b907b4f4,
+        0x3970c8fd78291026,
+        0x3b907b4e0,
+        true,
+    },
     {
         EVMC_BYZANTIUM,
         "DifficultyTest1",