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fix: edge case in filling ALP encoded child on patches #939

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Merged
lwwmanning merged 1 commit into from
Sep 26, 2024
Merged

fix: edge case in filling ALP encoded child on patches #939

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51 changes: 24 additions & 27 deletions encodings/alp/src/alp.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,30 @@ 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());

if chunk_patch_count > 0 {
// we need to gather the patches for this chunk
// preallocate space for the patches (plus one because our loop may attempt to write one past the end)
patch_indices.reserve(chunk_patch_count + 1);
patch_values.reserve(chunk_patch_count + 1);

// record the patches in this chunk
let patch_indices_mut = patch_indices.spare_capacity_mut();
let patch_values_mut = patch_values.spare_capacity_mut();
let mut chunk_patch_index = 0;
for i in num_prev_encoded..encoded_output.len() {
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 - 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() && (num_prev_encoded + chunk_patch_count < encoded_output.len()) {
Expand All @@ -188,33 +212,6 @@ fn encode_chunk_unchecked<T: ALPFloat>(
}
}

// if there are no patches, we are done
if chunk_patch_count == 0 {
return;
}

// we need to gather the patches for this chunk
// preallocate space for the patches (plus one because our loop may attempt to write one past the end)
patch_indices.reserve(chunk_patch_count + 1);
patch_values.reserve(chunk_patch_count + 1);

// record the patches in this chunk
let patch_indices_mut = patch_indices.spare_capacity_mut();
let patch_values_mut = patch_values.spare_capacity_mut();
let mut chunk_patch_index = 0;
for i in num_prev_encoded..encoded_output.len() {
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 - 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);
}

// replace the patched values in the encoded array with the fill value
// for better downstream compression
if let Some(fill_value) = fill_value {
Expand Down