feat(test): add gas formula egde cases

Author: LouisTsai-Csie

tests/osaka/eip7883_modexp_gas_increase/test_modexp_thresholds.py

@@ -292,3 +292,90 @@ def test_modexp_entry_points(
         gas_limit=tx_gas_limit,
     )
     state_test(pre=pre, tx=tx, post={})
+
+
+def create_modexp_variable_gas_test_cases():
+    """
+    Create test cases for ModExp variable gas cost testing.
+
+    Returns:
+        List of pytest.param objects for the test cases
+
+    """
+    # Test case definitions: (base, exponent, modulus, expected_result, test_id)
+    test_cases = [
+        ("", "", "", "", "Z0"),
+        ("01" * 16, "00" * 16, "02" * 16, "00" * 15 + "01", "S0"),
+        ("01" * 16, "00" * 15 + "03", "02" * 16, "01" * 16, "S1"),
+        ("01" * 32, "FF" * 32, "02" * 32, "01" * 32, "S2"),
+        ("01" * 16, "00" * 40, "02" * 16, "00" * 15 + "01", "S3"),
+        ("01" * 16, "00" * 39 + "01", "02" * 16, "01" * 16, "S4"),
+        ("01" * 24, "00", "02" * 8, "00" * 7 + "01", "S5"),
+        ("01" * 8, "01", "02" * 24, "00" * 16 + "01" * 8, "S6"),
+        ("01" * 40, "00" * 16, "02" * 40, "00" * 39 + "01", "L0"),
+        ("01" * 40, "FF" * 32, "02" * 40, "01" * 40, "L1"),
+        ("01" * 40, "00" * 40, "02" * 40, "00" * 39 + "01", "L2"),
+        ("01" * 40, "00" * 39 + "01", "02" * 40, "01" * 40, "L3"),
+        ("01" * 48, "01", "02" * 16, "01" * 16, "L4"),
+        ("01" * 16, "00" * 40, "02" * 48, "00" * 47 + "01", "L5"),
+    ]
+
+    # Gas calculation parameters:
+    #
+    # Please refer to EIP-7883 for details of each function in the gas calculation.
+    # Link: https://eips.ethereum.org/EIPS/eip-7883
+    #
+    # - calculate_multiplication_complexity:
+    #   - Comp: if max_length <= 32 bytes, it is Small (S), otherwise it is Large (L)
+    #   - Rel (Length Relation): base < modulus (<), base = modulus (=), base > modulus (>)
+    #
+    # - calculate_iteration_count
+    #   - Iter (Iteration Case):
+    #     - A: exp≤32 and exp=0
+    #     - B: exp≤32 and exp≠0
+    #     - C: exp>32 and low256=0
+    #     - D: exp>32 and low256≠0
+    #
+    # - calculate_gas_cost
+    #   - Clamp: True if raw gas < 500 (clamped to 500), False if raw gas ≥ 500 (no clamping)
+
+    # Test case coverage table:
+    # ┌─────┬──────┬─────┬──────┬───────┬─────────┬─────────────────────────────────────────────┐
+    # │ ID  │ Comp │ Rel │ Iter │ Clamp │   Gas   │ Description                                 │
+    # ├─────┼──────┼─────┼──────┼───────┼─────────┼─────────────────────────────────────────────┤
+    # │ Z0  │  -   │  -  │  -   │  -    │   500   │ Zero case - empty inputs                    │
+    # │ S0  │  S   │  =  │  A   │ True  │   500   │ Small, equal, zero exponent, clamped        │
+    # │ S1  │  S   │  =  │  B   │ True  │   500   │ Small, equal, small exp, clamped            │
+    # │ S2  │  S   │  =  │  B   │ False │  4080   │ Small, equal, large exp, unclamped          │
+    # │ S3  │  S   │  =  │  C   │ False │  2032   │ Small, equal, large exp+zero low256         │
+    # │ S4  │  S   │  =  │  D   │ False │  2048   │ Small, equal, large exp+non-zero low256     │
+    # │ S5  │  S   │  >  │  A   │ True  │   500   │ Small, base>mod, zero exp, clamped          │
+    # │ S6  │  S   │  <  │  B   │ True  │   500   │ Small, base<mod, small exp, clamped         │
+    # │ L0  │  L   │  =  │  A   │ True  │   500   │ Large, equal, zero exp, clamped             │
+    # │ L1  │  L   │  =  │  B   │ False │ 12750   │ Large, equal, large exp, unclamped          │
+    # │ L2  │  L   │  =  │  C   │ False │  6350   │ Large, equal, large exp+zero low256         │
+    # │ L3  │  L   │  =  │  D   │ False │  6400   │ Large, equal, large exp+non-zero low256     │
+    # │ L4  │  L   │  >  │  B   │ True  │   500   │ Large, base>mod, small exp, clamped         │
+    # │ L5  │  L   │  <  │  C   │ False │  9144   │ Large, base<mod, large exp+zero low256      │
+    # └─────┴──────┴─────┴──────┴───────┴─────────┴─────────────────────────────────────────────┘
+
+    for base, exponent, modulus, expected_result, test_id in test_cases:
+        yield pytest.param(
+            ModExpInput(base=base, exponent=exponent, modulus=modulus),
+            bytes.fromhex(expected_result),
+            id=test_id,
+        )
+
+
+@pytest.mark.parametrize(
+    "modexp_input,modexp_expected",
+    create_modexp_variable_gas_test_cases(),
+)
+def test_modexp_variable_gas_cost(
+    state_test: StateTestFiller,
+    pre: Alloc,
+    tx: Transaction,
+    post: Dict,
+):
+    """Test ModExp variable gas cost."""
+    state_test(pre=pre, tx=tx, post=post)