fix: MSB for big numbers

crates/starknet-types-core/src/hash/blake2s.rs

@@ -76,13 +76,11 @@ impl Blake2Felt252 {
                 unpacked_u32s.push(low);
             } else {
                 // big: 8 limbs, big‐endian order.
-                let start = unpacked_u32s.len();
-
                 for chunk in felt_as_be_bytes.chunks_exact(BYTES_PER_U32) {
                     unpacked_u32s.push(u32::from_be_bytes(chunk.try_into().unwrap()));
                 }
-                // set the MSB of the very first limb as the Cairo hint does with "+ 2**255".
-                unpacked_u32s[start] |= BIG_MARKER;
+                // set the MSB of the **entire 256-bit number**
+                unpacked_u32s[0] |= BIG_MARKER;
             }
         }
         unpacked_u32s
@@ -122,7 +120,7 @@ impl Blake2Felt252 {
             byte_stream.extend_from_slice(&word.to_le_bytes());
         }
 
-        // 3) Compute Blake2s-256 over the bytes and pack the first 224 bits into a Felt.
+        // 3) Compute Blake2s-256 over the bytes and pack the first 256 bits into a Felt.
         Self::blake2s_to_felt(&byte_stream)
     }
 }
@@ -133,80 +131,60 @@ mod tests {
     use crate::felt::Felt;
     use rstest::rstest;
 
-    /// Test two-limb encoding for a small Felt (< 2^63) into high and low 32-bit words.
     #[test]
     fn test_encode_felts_to_u32s_small() {
-        // Last 8 bytes of 0x1122334455667788 in big-endian are [0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88].
-        // High word = 0x11223344, low word = 0x55667788.
         let val = Felt::from_hex_unchecked("1122334455667788");
         let words = Blake2Felt252::encode_felts_to_u32s(&[val]);
         assert_eq!(words, vec![0x11223344, 0x55667788]);
     }
 
-    /// Test eight-limb encoding and MSB marker for a big Felt (>= 2^63).
     #[test]
     fn test_encode_felts_to_u32s_big() {
-        // The value 2^63 (0x8000000000000000) triggers the "big" path.
         let val = Felt::from_hex_unchecked("8000000000000000");
         let words = Blake2Felt252::encode_felts_to_u32s(&[val]);
 
-        // Expected limbs: split full 32-byte BE into eight 32-bit words.
-        // The seventh BE chunk [0x80,0x00,0x00,0x00] becomes 0x80000000 at index 6.
-        // Additionally, set the MSB marker (bit 31) on the first word.
         let mut expected = vec![0u32; 8];
-        expected[6] = 0x8000_0000;
-        expected[0] |= 1 << 31;
+        expected[0] |= 0x8000_0000;
+        expected[7] = 0x8000_0000;
         assert_eq!(words, expected);
     }
 
-    /// Test packing of a 32-byte little-endian buffer into a 256-bit Felt.
     #[test]
     fn test_pack_256_le_to_felt_basic() {
-        // Test with small values that won't trigger modular reduction.
         let mut buf = [0u8; 32];
         buf[0] = 0x01;
         buf[1] = 0x02;
         buf[2] = 0x03;
         buf[3] = 0x04;
-        // Leave the rest as zeros.
 
         let f = Blake2Felt252::pack_256_le_to_felt(&buf);
         let out = f.to_bytes_le();
 
-        // For small values, the first few bytes should match exactly.
         assert_eq!(out[0], 0x01);
         assert_eq!(out[1], 0x02);
         assert_eq!(out[2], 0x03);
         assert_eq!(out[3], 0x04);
 
-        // Test that the packing formula works correctly for a simple case.
         let expected = Felt::from(0x01)
             + Felt::from(0x02) * Felt::from(1u64 << 8)
             + Felt::from(0x03) * Felt::from(1u64 << 16)
             + Felt::from(0x04) * Felt::from(1u64 << 24);
         assert_eq!(f, expected);
 
-        // Test with a value that exceeds the field prime P to verify modular reduction.
-        // Create a 32-byte buffer with all 0xFF bytes, representing 2^256 - 1.
         let max_buf = [0xFF_u8; 32];
         let f_max = Blake2Felt252::pack_256_le_to_felt(&max_buf);
 
-        // The result should be (2^256 - 1) mod P.
-        // Since 2^256 = Felt::TWO.pow(256), we can compute this value directly.
-        // This tests that modular reduction works correctly when exceeding the field prime.
         let two_pow_256_minus_one = Felt::TWO.pow(256u32) - Felt::ONE;
         assert_eq!(f_max, two_pow_256_minus_one);
     }
 
-    /// Test that pack_256_le_to_felt panics when input is shorter than 32 bytes.
     #[test]
     #[should_panic(expected = "need at least 32 bytes to pack 8 words")]
     fn test_pack_256_le_to_felt_too_short() {
         let too_short = [0u8; 31];
         Blake2Felt252::pack_256_le_to_felt(&too_short);
     }
 
-    /// Test that hashing a single zero Felt produces the expected 256-bit Blake2s digest.
     #[test]
     fn test_hash_single_zero() {
         let zero = Felt::from_hex_unchecked("0");
@@ -217,7 +195,6 @@ mod tests {
         assert_eq!(hash, expected);
     }
 
-    /// Test that hashing an array of Felts [1, 2] produces the expected 256-bit Blake2s digest.
     #[test]
     fn test_hash_array_one_two() {
         let one = Felt::from_hex_unchecked("1");
@@ -229,8 +206,6 @@ mod tests {
         assert_eq!(hash, expected);
     }
 
-    /// Test the encode_felt252_data_and_calc_blake_hash function
-    /// with the same result as the Cairo v0.14 version.
     #[rstest]
     #[case::empty(vec![], "874258848688468311465623299960361657518391155660316941922502367727700287818")]
     #[case::boundary_under_2_63(vec![Felt::from((1u64 << 63) - 1)], "94160078030592802631039216199460125121854007413180444742120780261703604445")]