wgtmac commented on code in PR #557: URL: https://github.com/apache/parquet-format/pull/557#discussion_r3627263618
########## Encodings.md: ########## @@ -391,3 +391,423 @@ After applying the transformation, the data has the following representation: ``` Bytes AA 00 A3 BB 11 B4 CC 22 C5 DD 33 D6 ``` + +<a name="ALP"></a> +### Adaptive Lossless floating-Point: (ALP = 10) + +Supported Types: FLOAT, DOUBLE + +This encoding is adapted from the paper +["ALP: Adaptive Lossless floating-Point Compression"](https://dl.acm.org/doi/10.1145/3626717) +by Afroozeh and Boncz (SIGMOD 2024). + +ALP works by converting floating-point values to integers using decimal scaling +(controlled by an *exponent* `e` and *factor* `f`), then applying Frame of +Reference (FOR) encoding and bit-packing. Values that cannot be losslessly +converted are stored separately as *exceptions*. The encoding achieves high +compression for decimal-like floating-point data (e.g., monetary values, sensor +readings) while remaining fully lossless. Each value is encoded independently, +enabling random access to individual values and parallel encode/decode. + +#### Overview + +ALP encoding consists of a page-level header followed by an offset array and one +or more encoded vectors (batches of values). Each vector contains up to +`vector_size` elements (default 1024). + +``` ++-------------+-----------------------------+--------------------------------------+ +| Header | Offset Array | Vector Data | +| (7 bytes) | (num_vectors * 4 bytes) | (variable) | ++-------------+------+------+-----+---------+----------+----------+-----+----------+ +| Page Header | off0 | off1 | ... | off N-1 | Vector 0 | Vector 1 | ... | Vec N-1 | +| (7 bytes) | (4B) | (4B) | | (4B) |(variable)|(variable)| |(variable)| ++-------------+------+------+-----+---------+----------+----------+-----+----------+ +``` + +The compression pipeline below describes *one* way to produce a conforming +vector. It is informative, not normative: an encoder may use any strategy as long +as it emits the byte layout defined in [Page Layout](#page-layout). Only that +byte layout and the [Decoding](#decoding) procedure are normative. + +``` + Input: float/double array + | + v + +----------------------------------------------------------+ + | 1. CHOOSE PARAMETERS | + | Select (exponent, factor) pair for this vector | + +----------------------------------------------------------+ + | + v + +----------------------------------------------------------+ + | 2. DECIMAL ENCODING | + | encoded[i] = fast_round(value[i] * 10^e * 10^(-f)) | + | Detect exceptions where decode(encode(v)) != v | + +----------------------------------------------------------+ + | + v + +----------------------------------------------------------+ + | 3. FRAME OF REFERENCE (FOR) | + | min_val = min(encoded[:]) | + | delta[i] = encoded[i] - min_val | + +----------------------------------------------------------+ + | + v + +----------------------------------------------------------+ + | 4. BIT PACKING | + | bit_width = ceil(log2(max_delta + 1)) | + | Pack each delta into bit_width bits | + +----------------------------------------------------------+ + | + v + Output: Serialized vector bytes +``` + +The `fast_round` used in step 2 is defined precisely in +[Fast Rounding](#fast-rounding) below, including the exact magic-number values +and the sign branching required for negative values. + +#### Page Layout + +##### Header (7 bytes) + +All multi-byte values are stored in little-endian order. + +``` + Byte: 0 1 2 3 4 5 6 + +----------------+---------------+--------------+----+----+----+----+ + | compression | integer | log_vector | num_elements | + | _mode | _encoding | _size | (int32 LE) | + +----------------+---------------+--------------+----+----+----+----+ +``` + +| Offset | Field | Size | Type | Description | +|--------|-------|------|------|-------------| +| 0 | compression_mode | 1 byte | uint8 | Compression mode (0 = ALP). Reserved for future variants (e.g., ALP-RD). | +| 1 | integer_encoding | 1 byte | uint8 | Integer encoding (must be 0 = FOR + bit-packing) | +| 2 | log_vector_size | 1 byte | uint8 | log2(vector\_size). Must be in the inclusive range \[3, 15\]. Recommended default: 10 (vector size 1024) | +| 3 | num_elements | 4 bytes | int32 | Total number of floating-point values in the page | + +The number of vectors is `ceil(num_elements / vector_size)`. The last vector may +contain fewer than `vector_size` elements. + +**Note:** The number of elements per vector is NOT stored in the header — it is +derived: `vector_size` for all vectors except the last, which may be smaller. + +##### Offset Array + +Immediately following the header is an array of `num_vectors` little-endian uint32 +values. Each offset gives the byte position of the corresponding vector's data, +measured from the start of the offset array itself. + +The first offset always equals `num_vectors * 4` (pointing just past the offset array). +Each subsequent offset equals the previous offset plus the stored size of the +previous vector. No padding is inserted between vectors. + +A vector's absolute byte position within the page is Review Comment: This is incorrect when pages are compressed or have rep/def levels. I would suggest removing this sentence. If we want to emphasize random access of ALP, perhaps we should replace `page_data_start` with a more precise start point after decompressed. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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