fix fill bug

encodings/alp/src/alp.rs

@@ -179,6 +179,18 @@ fn encode_chunk_unchecked<T: ALPFloat>(
     let chunk_patch_count = chunk_patch_count; // immutable hereafter
     assert_eq!(encoded_output.len(), num_prev_encoded + chunk.len());
 
+    // find the first successfully encoded value (i.e., not patched)
+    // this is our fill value for missing values
+    if fill_value.is_none() && (num_prev_encoded + chunk_patch_count < encoded_output.len()) {
+        assert_eq!(num_prev_encoded, num_prev_patches);
+        for i in num_prev_encoded..encoded_output.len() {
+            if i >= patch_indices.len() || patch_indices[i] != i as u64 {
+                *fill_value = Some(encoded_output[i]);
+                break;
+            }
+        }
+    }
+
     // if there are no patches, we are done
     if chunk_patch_count == 0 {
         return;
@@ -197,27 +209,15 @@ fn encode_chunk_unchecked<T: ALPFloat>(
         let decoded = T::decode_single(encoded_output[i], exp);
         // write() is only safe to call more than once because the values are primitive (i.e., Drop is a no-op)
         patch_indices_mut[chunk_patch_index].write(i as u64);
-        patch_values_mut[chunk_patch_index].write(chunk[i]);
-        chunk_patch_index += (decoded != chunk[i]) as usize;
+        patch_values_mut[chunk_patch_index].write(chunk[i - num_prev_encoded]);
+        chunk_patch_index += (decoded != chunk[i - num_prev_encoded]) as usize;
     }
     assert_eq!(chunk_patch_index, chunk_patch_count);
     unsafe {
         patch_indices.set_len(num_prev_patches + chunk_patch_count);
         patch_values.set_len(num_prev_patches + chunk_patch_count);
     }
 
-    // find the first successfully encoded value (i.e., not patched)
-    // this is our fill value for missing values
-    if fill_value.is_none() {
-        assert_eq!(num_prev_encoded, num_prev_patches);
-        for i in num_prev_encoded..encoded_output.len() {
-            if i >= patch_indices.len() || patch_indices[i] != i as u64 {
-                *fill_value = Some(encoded_output[i]);
-                break;
-            }
-        }
-    }
-
     // replace the patched values in the encoded array with the fill value
     // for better downstream compression
     if let Some(fill_value) = fill_value {