Astro

Conformance Statement for `pkg/ldc`, CCSDS 121.0-B-3

A2.1 GENERAL INFORMATION

A2.1.1 Identification of PICS

Field	Value
Date of Statement (DD/MM/YYYY)	23/08/2026
PICS Serial Number	ASTRO-LDC-PICS-001
System Conformance Statement Cross-Reference	This document

A2.1.2 Identification of Implementation Under Test (IUT)

Field	Value
Implementation Name	astro/pkg/ldc
Implementation Version	See `go.mod` / latest commit on `main`
Special Configuration	None
Other Information	Go library implementing the Rice adaptive entropy coder and its preprocessor, encoder and decoder both. Integer arithmetic throughout; no floating point appears anywhere in the package. Byte-slice in, byte-slice out, packetization is the caller's.

A2.1.3 Identification of Supplier

Field	Value
Supplier	Ravi Suhag
Contact Point for Queries	GitHub, github.com/ravisuhag/astro
Implementation Name(s) and Version(s)	astro/pkg/ldc (Go package)
System Name(s)	Astro

A2.1.4 Identification of Specification

Field	Value
Specification	CCSDS 121.0-B-3 (Lossless Data Compression, Blue Book, Issue 3, August 2020)
Companion report	CCSDS 120.0-G-4 (Lossless Data Compression, Green Book, Issue 4, November 2021)
Have any exceptions been required?	Yes [X] No [ ]

NOTE, Non-supported capabilities are identified in section A2.3. All of them are options the standard leaves to the application or defines for transport layers this package does not touch.

NOTE. This implementation is validated against the official CCSDS 121.0-B-2 test vectors published by the SLS Data Compression working group (as mirrored in libaec's data/121B2TestData). All 72 AllOptions and LowEntropyOptions vectors (36 in each set, covering resolutions 1 through 32) encode byte-identically and decode back to the exact samples; they are vendored in pkg/ldc/testdata/ and every one runs as TestVectors_* in vectors_test.go. The ExtendedParameters set is excluded for the reason given in section A2.3.

A2.2 REQUIREMENTS LIST

Table A-1: Adaptive entropy coder, parameters

Item	Description	Reference	Status	Values Allowed	Support	Notes
LDC-1	Block size J	3.1.6	M	8, 16, 32, 64	Yes	`Params.BlockSize`. Any other value is `ErrInvalidBlockSize`.
LDC-2	Sample resolution n	3.1.6	M	1 to 32 bits	Yes	`Params.Resolution`.
LDC-3	Unsigned sample range	3.1.6, 4.4	M	0 to 2^n-1	Yes
LDC-4	Signed sample range	3.1.6, 4.4	M	-2^(n-1) to 2^(n-1)-1	Yes	`Params.Signed`. Two's complement, sign extended from n bits. Requires the unit-delay predictor, see the interpretation note in A2.3.
LDC-5	Option identifier attached to every coded data set	3.1.1, 5.2.1.3	M	-	Yes	Always written, even when a subset of options is in use.

Table A-2: Code options

Item	Description	Reference	Status	Support	Notes
LDC-6	Fundamental sequence option	3.2	M	Yes	Table 3-1 transcribed as a test. Implemented as the split-sample option with k=0, which clause 3.3.2 says it is.
LDC-7	Split-sample options	3.3	M	Yes	Every k the resolution allows: up to 5, 13 or 29 in the three basic columns of table 5-1.
LDC-8	Split-sample field order	3.3.3	M	Yes	All FS codewords for the block first, then all split bits. Not interleaved. The spec is explicit and the obvious implementation is wrong.
LDC-9	Second-extension option	3.4	M	Yes	The transform of clause 3.4.1, with δ_1 = 0 substituted on a reference block.
LDC-10	Second-extension overflow	3.4.2	M	Yes	At 32-bit resolution the transform can exceed a 64-bit integer. The option reports itself unusable rather than wrapping. Clause 3.4.2's note that it "is only designed to be a useful option when all of the transformed symbols are small" is why this can never lose data.
LDC-11	Zero-block option	3.5	M	Yes	Including the ROS codeword and the 64-block segments of clause 3.5.2.
LDC-12	Zero-block run codewords	3.5.3, table 3-2	M	Yes	Table transcribed in full, including the ROS codeword displaced between four and five.
LDC-13	Zero-block spans multiple blocks	3.1.4, 3.5.1	M	Yes	One coded data set covers the whole run.
LDC-14	All-zeros with a reference sample	3.5.1, 3.7.2	M	Yes	A reference block counts as all zeros when the J-1 samples after the reference are zero, whatever the reference itself is.
LDC-15	No-compression option	3.6	M	Yes

Table A-3: Code selection

Item	Description	Reference	Status	Support	Notes
LDC-16	Select the option minimizing encoded bits, identifier included	3.7.1, 3.7.3	M	Yes	Every option is priced without being emitted, which matters: an FS codeword at 32-bit resolution can be four billion bits long.
LDC-17	Zero-block always selected for all-zeros runs	3.7.2	M	Yes	Not priced against the others; imposed.
LDC-18	Tie-breaking order	3.7.4	M	Yes	No compression, then second extension, then smallest k. Pinned by test, because it is not the order an implementer would guess.

Table A-4: Preprocessor

Item	Description	Reference	Status	Support	Notes
LDC-19	Preprocessor may be omitted	4.1	O	Yes	`PredictorNone`.
LDC-20	Unit-delay predictor	4.2.5	M	Yes	The only predictor the standard specifies.
LDC-21	First sample of a reference interval predicts itself	4.2.5	M	Yes	So its prediction error is zero.
LDC-22	Bypass predictor	4.2.3	O	Yes	Predicts zero, keeps the mapper.
LDC-23	Application-specific predictor	4.2.4	O	No	The standard names it and does not define it. A file header requesting it is refused with `ErrUnsupportedPredictor`.
LDC-24	Reference samples required only for previous-sample predictors	4.2.6	M	Yes	Inserted for the unit-delay predictor, and for nothing else.
LDC-25	Reference sample is the first of a block, uncoded, leading the CDS	4.2.6, 5.2.2	M	Yes
LDC-26	Reference sample interval r	4.3	M	Yes	1 to 4096 blocks. Bounds the zero-block segments even when no reference samples are used.
LDC-27	Prediction error mapper	4.4	M	Yes	The three-branch equation, with θ = min(x̂ - xmin, xmax - x̂). Verified against the worked table of Green Book clause 3.3.3, including the two rows past θ.
LDC-28	Mapper is a bijection	4.4	M	Yes	Checked exhaustively at low resolution across every predictor value.

Table A-5: Coded data set formats

Item	Description	Reference	Status	Support	Notes
LDC-29	Option identification key	5.2.1, table 5-1	M	Yes	All five resolution columns, transcribed in full as a test.
LDC-30	Basic code option set	5.2.1.1	M	Yes	3, 4 and 5-bit identifiers by resolution.
LDC-31	Restricted code option set	5.2.1.1	O	Yes	1 and 2-bit identifiers, allowed only at n <= 4. Requesting it above that is `ErrRestrictedNotAllowed`.
LDC-32	CDS format, FS and split-sample	5.2.3	M	Yes
LDC-33	CDS format, no compression	5.2.4	M	Yes
LDC-34	CDS format, zero block	5.2.5	M	Yes
LDC-35	CDS format, second extension	5.2.6	M	Yes	J/2 symbols, per clause 3.4.1 and figure 5-4. The prose of clause 5.2.6 says "2J transformed pairs", which contradicts both and is read as a typo.
LDC-36	Bit order MSB first	1.5.2	M	Yes

Table A-6: File format

Item	Description	Reference	Status	Support	Notes
LDC-37	File header, 12 octets	7.2.2, table 7-1	O	Yes	Every field, encoded and decoded.
LDC-38	Output word size B	7.2.1.2	O	Yes	1 to 8 octets.
LDC-39	Reserved fields are zero	table 7-1	M	Yes	All three checked on decode; a set bit is `ErrReservedFieldSet`.
LDC-40	Data Sense	table 7-1	M	Yes	'0' two's complement, '1' positive. The one field that reads the opposite way from the rest, and pinned by its own test for that reason.
LDC-41	Application-specific mapper	table 7-1	O	No	Refused with `ErrUnsupportedMapper`.
LDC-42	File body is concatenated CDSes	7.2.3.1	O	Yes
LDC-43	Zero fill to the word boundary	7.2.3.2	O	Yes

Table A-7: Packet formats

Item	Description	Reference	Status	Support	Notes
LDC-44	CDSes inserted into space packets	5.3	O	No	Out of scope by design: this package emits coded data sets and the caller packetizes, with `pkg/spp`.
LDC-45	Compression identification packet	6	O	No	The standard marks section 6 optional. The file header of section 7 covers the same need and is implemented instead.

A2.3 EXCEPTIONS AND LIMITATIONS

Non-Supported Items

Item	Description	Reason
LDC-23	Application-specific predictor	clause 4.2.4 names it and leaves it undefined: "such a predictor is unique to the application and is not specified in this Recommended Standard." There is nothing to implement. A file header requesting it is refused rather than silently decoded with the wrong predictor.
LDC-41	Application-specific mapper	Same reasoning, from table 7-1.
LDC-44	Insertion into space packets	The caller composes coded data sets into packets. Keeping the two apart is what lets this package be used with the file format, with packets, or with neither.
LDC-45	Compression identification packet	Section 6 is optional and duplicates what the file header carries.
-	Reference-interval byte alignment	Some encoders pad the coded stream to a byte boundary at the end of each reference sample interval, an application framing choice the standard leaves open rather than a numbered requirement (libaec exposes it as its `-p` option). This decoder reads the coded data set as one continuous bit stream and cannot consume such streams. Seen in the official 121.0-B-2 `ExtendedParameters/sar32bit.j16.r256.rz` vector, which is why that set is not in `testdata/`.

Interpretations

Two places where the standard's text admits more than one reading, and the reading taken here:

Where Reading taken

Signed samples and the predictor (table 7-1, Data Sense) Table 7-1 makes the positive Data Sense "mandatory if preprocessor is bypassed or preprocessor absent". Read narrowly, that constrains only a section-7 file header field; read broadly, it says signed samples are meaningful only under the unit-delay predictor. This implementation takes the broad, conservative reading and enforces it in Params.Validate everywhere, not just in the file path: Signed with any predictor other than unit delay is refused with ErrUnsupportedPredictor. The narrow reading would let signed samples through with the bypass predictor outside the file format, and would then produce parameter sets a section-7 header cannot describe. Refusing keeps every compressible stream expressible as a file.

Second-extension CDS symbol count (clause 5.2.6) The prose says "2J transformed pairs", which contradicts clause 3.4.1 and figure 5-4 (J/2 symbols). Read as a typo; J/2 is implemented. Also recorded at LDC-35.

Where	Reading taken
Signed samples and the predictor (table 7-1, Data Sense)	Table 7-1 makes the positive Data Sense "mandatory if preprocessor is bypassed or preprocessor absent". Read narrowly, that constrains only a section-7 file header field; read broadly, it says signed samples are meaningful only under the unit-delay predictor. This implementation takes the broad, conservative reading and enforces it in `Params.Validate` everywhere, not just in the file path: `Signed` with any predictor other than unit delay is refused with `ErrUnsupportedPredictor`. The narrow reading would let signed samples through with the bypass predictor outside the file format, and would then produce parameter sets a section-7 header cannot describe. Refusing keeps every compressible stream expressible as a file.
Second-extension CDS symbol count (clause 5.2.6)	The prose says "2J transformed pairs", which contradicts clause 3.4.1 and figure 5-4 (J/2 symbols). Read as a typo; J/2 is implemented. Also recorded at LDC-35.

Implementation-Defined Limits

None of these limits are in the standard. The first two exist because a decoder must not be able to be told to exhaust memory; the third is a documented limitation of the one entry point that decodes without a sample count.

Limit	Value	Why
Decodable sample count	2^28 samples	The header's Number of Samples field is 48 bits, so a twelve-octet file can claim 2^48 samples, a terabyte of output, and the decoder would size a slice from it before reading a coded bit.
FS codeword length	Bounded by the resolution	A run of zero octets in a corrupt stream would otherwise be read as an enormous sample value. `ReadFS` takes a limit and refuses past it.
Fill skipped by the unbounded `Decompress`	7 bits	`Decompress` (no sample count) treats a trailing all-zero run of fewer than eight bits as clause 7.2.3.2 fill, which covers a B=1 file body exactly. A file written with an output word size B > 1 octet can carry up to 8B-1 fill bits, and without the count that tail cannot be told from a truncated coded data set, so `Decompress` fails with an error rather than guessing. `DecompressCount` and `DecompressFile` know the count and skip any fill. Pinned by `TestDecompressRefusesLongWordFill`.

Fully Supported Mandatory Items

Every mandatory item is supported. All five code options are implemented in both directions, along with both specified predictors, the mapper, the coded data set formats, and the option selection rule including its tie-breaks.

Area	Items	Implementation
Parameters	LDC-1-5	`params.go`
Bit packing	LDC-36	`bits.go`
Code options	LDC-6-15	`options.go`
Selection	LDC-16-18	`encoder.go`
Preprocessor	LDC-19-28	`preprocessor.go`
CDS formats	LDC-29-35	`encoder.go`, `decoder.go`
File format	LDC-37-43	`file.go`

Verification

Source	What it pins
Blue Book table 3-1	Fundamental sequence codewords
Blue Book table 3-2	Zero-block run codewords, including ROS
Blue Book table 5-1	Every option identifier at every resolution
Blue Book table 7-1	File header, field by field
Blue Book clause 3.4.1	The second-extension transform
Blue Book clause 3.7.4	The tie-breaking order
Green Book clause 3.3.3	The preprocessor and mapper, as a worked table
Green Book figure 3-4	Zero-block segmentation and ROS

Annex A of the Green Book names a fuller vector set at cwe.ccsds.org/sls/docs/sls-dc/BB121B3TestData. That location requires a CCSDS login and returned HTTP 403, so it has not been run against this implementation. Anyone with access is encouraged to.

Lossless Data Compression

Conformance Statement for pkg/ldc, CCSDS 121.0-B-3