chore: undo recent change

src/hash/rescue/rpo/mod.rs

@@ -86,9 +86,16 @@ impl Hasher for Rpo256 {
         // every time the rate range is filled, a permutation is performed. if the final value of
         // `rate_pos` is not zero, then the chunks count wasn't enough to fill the state range,
         // and an additional permutation must be performed.
+        let mut current_element = 0_usize;
+        // handle the case of an empty `bytes`
+        let last_element = if num_field_elem == 0 {
+            current_element
+        } else {
+            num_field_elem - 1
+        };
         let rate_pos = bytes.chunks(BINARY_CHUNK_SIZE).fold(0, |rate_pos, chunk| {
             // copy the chunk into the buffer
-            if chunk.len() == BINARY_CHUNK_SIZE {
+            if current_element != last_element {
                 buf[..BINARY_CHUNK_SIZE].copy_from_slice(chunk);
             } else {
                 // on the last iteration, we pad `buf` with a 1 followed by as many 0's as are
@@ -97,6 +104,7 @@ impl Hasher for Rpo256 {
                 buf[..chunk.len()].copy_from_slice(chunk);
                 buf[chunk.len()] = 1;
             }
+            current_element += 1;
 
             // set the current rate element to the input. since we take at most 7 bytes, we are
             // guaranteed that the inputs data will fit into a single field element.

src/hash/rescue/rpx/mod.rs

@@ -91,9 +91,16 @@ impl Hasher for Rpx256 {
         // every time the rate range is filled, a permutation is performed. if the final value of
         // `rate_pos` is not zero, then the chunks count wasn't enough to fill the state range,
         // and an additional permutation must be performed.
+        let mut current_element = 0_usize;
+        // handle the case of an empty `bytes`
+        let last_element = if num_field_elem == 0 {
+            current_element
+        } else {
+            num_field_elem - 1
+        };
         let rate_pos = bytes.chunks(BINARY_CHUNK_SIZE).fold(0, |rate_pos, chunk| {
             // copy the chunk into the buffer
-            if chunk.len() == BINARY_CHUNK_SIZE {
+            if current_element != last_element {
                 buf[..BINARY_CHUNK_SIZE].copy_from_slice(chunk);
             } else {
                 // on the last iteration, we pad `buf` with a 1 followed by as many 0's as are
@@ -102,6 +109,7 @@ impl Hasher for Rpx256 {
                 buf[..chunk.len()].copy_from_slice(chunk);
                 buf[chunk.len()] = 1;
             }
+            current_element += 1;
 
             // set the current rate element to the input. since we take at most 7 bytes, we are
             // guaranteed that the inputs data will fit into a single field element.