Encodes a value against its previous version.
This is the mechanism that replaced content-defined chunking. A rewritten field - a body, a JSON blob of notifications, a serialized settings array - is almost entirely the value it replaces, but a byte-level splitter cannot see that because the change is interior. Handing the previous version to the compressor as its dictionary can: measured on 5,967-byte notification blobs with one flag flipped and one entry appended, zstd -19 against the previous version reached 63.70x and 94 bytes per object, against 4.49x and 1,330 bytes standalone - 93.0% smaller. At level 3 it is 51.93x, so the win survives the fast setting used on the flush path.
The previous version is already in memory during a capture as the entity's original, and is retrievable from the store by digest during a restore, so neither path pays to fetch it twice.
Two rules bound the dependency this creates. A chain deeper than the policy allows is anchored instead. And an encoding that does not beat the standalone form by DeltaCodec::MIN_GAIN is discarded in favour of the anchor, so a codec without dictionary support produces correct output without a branch at any call site.
$codec : \Drupal\strata\Codec\CompressionCodecInterface
$policy : \Drupal\strata\Delta\ChainDepthPolicy
__construct(\Drupal\strata\Codec\CompressionCodecInterface $codec, \Drupal\strata\Delta\ChainDepthPolicy $policy, int|null $level = null) : mixed
Constructs a codec.
| \Drupal\strata\Codec\CompressionCodecInterface | $codec | The compressor. When it reports no dictionary support every frame is an anchor, so CodecRegistry prefers a dictionary-capable codec. |
| \Drupal\strata\Delta\ChainDepthPolicy | $policy | The chain-depth policy. |
| int|null | $level | Compression level, or NULL for the codec's default. |
encode(string $current, string|null $previous = null, string|null $parentHash = null, int $parentDepth = 0) : \Drupal\strata\Delta\DeltaFrame
Encodes a value, against its previous version when that is worthwhile.
| string | $current | The value to encode. |
| string|null | $previous | The previous version of the same subject, or NULL when there is none. |
| string|null | $parentHash | Digest of $previous, or NULL to compute it. Pass it when the caller already has it. |
| int | $parentDepth | Chain depth of the frame holding $previous; zero when that frame is an anchor. |
When the compressor fails.
A delta frame when one is available, cheaper and within the depth policy; otherwise an anchor.
decode(\Drupal\strata\Delta\DeltaFrame $frame, string|null $previous = null) : string
Decodes a frame back to its value.
| \Drupal\strata\Delta\DeltaFrame | $frame | The frame to decode. |
| string|null | $previous | The parent value, required for a delta frame and ignored for an anchor. |
When a delta frame is handed no parent, the wrong parent, or the decode is truncated.
The decoded value.
decodeChain(list<\Drupal\strata\Delta\DeltaFrame> $chain) : string
Walks a chain of successive versions to the value the last frame encodes.
The whole chain is decoded in order because a delta is meaningless without the link before it. It must begin with an anchor: a chain starting at a delta cannot be decoded without a parent the caller has not supplied.
An anchor part-way through is NOT an error and must not be treated as one. Two ordinary things produce it - the depth cap forcing a re-anchor, and DeltaCodec::encode() discarding a delta that failed to beat its own anchor by DeltaCodec::MIN_GAIN, which happens whenever a value is small enough that a chain link cannot pay for itself. Such an anchor RESTARTS the chain: every frame before it becomes unnecessary and depth counting begins again.
A depth that does not follow its predecessor within the current run is a corruption symptom and is named in the error.
| list<\Drupal\strata\Delta\DeltaFrame> | $chain | Frames from an anchor to the target, in order. |
When the chain is empty, does not start at an anchor, has a depth discontinuity within a run, or any link fails to decode.
The value the last frame encodes.
verify(\Drupal\strata\Delta\DeltaFrame $frame, string $plain) : string
Catches a decode that came back the wrong length.
A truncated decompression is indistinguishable from correct output until much later, so the recorded plain length is checked before the bytes leave this class.
| \Drupal\strata\Delta\DeltaFrame | $frame | The frame that was decoded. |
| string | $plain | The decoded bytes. |
When the decoded length does not match what the frame recorded.
The same bytes, once they are known to be complete.