simplify chunks_vectored implementation

src/cursor/mod.rs

@@ -201,6 +201,10 @@ impl<T: AsRef<BufList>> Buf for Cursor<T> {
         total.saturating_sub(self.data.pos) as usize
     }
 
+    fn has_remaining(&self) -> bool {
+        self.data.num_bytes(self.inner.as_ref()) > self.data.pos
+    }
+
     fn chunk(&self) -> &[u8] {
         self.data.fill_buf_impl(self.inner.as_ref())
     }
@@ -210,40 +214,30 @@ impl<T: AsRef<BufList>> Buf for Cursor<T> {
     }
 
     fn chunks_vectored<'iovs>(&'iovs self, iovs: &mut [IoSlice<'iovs>]) -> usize {
-        if iovs.is_empty() {
+        let list = self.inner.as_ref();
+
+        if iovs.is_empty() || !self.has_remaining() {
             return 0;
         }
 
-        let list = self.inner.as_ref();
-        let mut filled = 0;
-        let mut current_chunk = self.data.chunk;
-        let mut current_pos = self.data.pos;
-
-        // Iterate through chunks starting from the current position
-        while filled < iovs.len() && current_chunk < list.num_chunks() {
-            if let Some(chunk) = list.get_chunk(current_chunk) {
-                let chunk_start_pos = list.get_start_pos()[current_chunk];
-                let offset_in_chunk = (current_pos - chunk_start_pos) as usize;
-
-                if offset_in_chunk < chunk.len() {
-                    let chunk_slice = &chunk.as_ref()[offset_in_chunk..];
-                    iovs[filled] = IoSlice::new(chunk_slice);
-                    filled += 1;
-                }
+        let current_chunk = self.data.chunk;
+        let chunk_start_pos = list.get_start_pos()[current_chunk];
+        let offset_in_chunk = (self.data.pos - chunk_start_pos) as usize;
 
-                current_chunk += 1;
-                // Move to the start of the next chunk
-                if let Some(&next_start_pos) = list.get_start_pos().get(current_chunk) {
-                    current_pos = next_start_pos;
-                } else {
-                    break;
-                }
-            } else {
-                break;
-            }
+        iovs[0] = IoSlice::new(
+            &list.get_chunk(current_chunk).expect("chunk is in range")[offset_in_chunk..],
+        );
+        // Fill up the remaining iovs with as many slices as possible.
+        let to_fill = (iovs.len()).min(list.num_chunks() - current_chunk);
+        for (i, iov) in iovs.iter_mut().enumerate().take(to_fill).skip(1) {
+            *iov = IoSlice::new(
+                &list
+                    .get_chunk(current_chunk + i)
+                    .expect("chunk is in range")[..],
+            );
         }
 
-        filled
+        to_fill
     }
 }
 

src/cursor/tests.rs

@@ -57,7 +57,6 @@ enum CursorOp {
     // return value separately.
     Consume(prop::sample::Index),
     // Buf trait operations
-    BufRemaining,
     BufChunk,
     BufAdvance(prop::sample::Index),
     BufChunksVectored(prop::sample::Index),
@@ -192,18 +191,6 @@ impl CursorOp {
                 buf_list.consume(amt);
                 oracle.consume(amt);
             }
-            Self::BufRemaining => {
-                eprintln!("buf_remaining");
-
-                let buf_list_remaining = buf_list.remaining();
-                let oracle_remaining = oracle.remaining();
-                ensure!(
-                    buf_list_remaining == oracle_remaining,
-                    "remaining didn't match: buf_list {} == oracle {}",
-                    buf_list_remaining,
-                    oracle_remaining
-                );
-            }
             Self::BufChunk => {
                 eprintln!("buf_chunk");
 
@@ -297,7 +284,8 @@ impl CursorOp {
                     ensure!(
                         !buf_list_bytes.is_empty(),
                         "chunks_vectored should return some data \
-                         when remaining > 0 and num_iovs > 0"
+                         when remaining = {buf_list_remaining} > 0 \
+                         and num_iovs = {num_iovs} > 0"
                     );
                     ensure!(
                         !oracle_bytes.is_empty(),
@@ -312,6 +300,14 @@ impl CursorOp {
                         "buf_list chunks_vectored data should match beginning \
                          of oracle data"
                     );
+
+                    // Verify that all iovs up to buf_list_filled are non-empty.
+                    for (i, iov) in buf_list_iovs[..buf_list_filled].iter().enumerate() {
+                        ensure!(
+                            !iov.is_empty(),
+                            "buf_list iov at index {i} should be non-empty",
+                        );
+                    }
                 } else if buf_list_remaining == 0 {
                     // If no bytes remaining, should return no data
                     ensure!(
@@ -449,6 +445,25 @@ impl CursorOp {
             }
         }
 
+        // Check general properties: remaining and has_remaining are the same.
+        let buf_list_remaining = buf_list.remaining();
+        let oracle_remaining = oracle.remaining();
+        ensure!(
+            buf_list_remaining == oracle_remaining,
+            "remaining didn't match: buf_list {} == oracle {}",
+            buf_list_remaining,
+            oracle_remaining
+        );
+
+        let buf_list_has_remaining = buf_list.has_remaining();
+        let oracle_has_remaining = oracle.has_remaining();
+        ensure!(
+            buf_list_has_remaining == oracle_has_remaining,
+            "has_remaining didn't match: buf_list {} == oracle {}",
+            buf_list_has_remaining,
+            oracle_has_remaining
+        );
+
         // Also check that the position is the same.
         let buf_list_position = buf_list.position();
         ensure!(