Header: modules/predictors/diff/diff.h
Class: fz::DifferenceStage<T, TOut = T>
Category: Predictor / transform (lossless)

What it does

Forward (compression): first-order differences — output[i] = input[i] - input[i-1], output[0] = input[0].
Inverse (decompression): cumulative sum.

When TOut != T, the stage writes the forward deltas in negabinary form into the unsigned output type, and the inverse path decodes negabinary before the prefix sum. This is equivalent to DifferenceStage<T> followed by a NegabinaryStage, but fused into one kernel.

Output is the same byte size as input (sizeof(T) == sizeof(TOut) is enforced).

Template parameters

Parameter	Constraint
`T`	Numeric type (input / output when `TOut == T`, see available instantiations)
`TOut`	Defaults to `T`. When different: unsigned counterpart of signed `T` (negabinary fusion)

Available instantiations

Single-parameter (no negabinary fusion):

DifferenceStage<float>
DifferenceStage<double>
DifferenceStage<uint8_t>
DifferenceStage<uint16_t>
DifferenceStage<uint32_t>
DifferenceStage<int32_t>
DifferenceStage<int64_t>

Negabinary-fused pairs (<signed, unsigned>):

DifferenceStage<int8_t, uint8_t>
DifferenceStage<int16_t, uint16_t>
DifferenceStage<int32_t, uint32_t>
DifferenceStage<int64_t, uint64_t>

Using any other combination will result in a linker error. Common choices: DifferenceStage<int32_t, uint32_t> (after quantizer codes), or DifferenceStage<int32_t> (plain delta coding).

Stage settings

Setting	Purpose	Notes
`setChunkSize(bytes)`	Reset delta at chunk boundaries	0 = no chunking

diff->setChunkSize(16384); // bytes; 0 = no chunking

Chunking

setChunkSize(bytes) makes differences and cumulative sums reset at each chunk boundary. Each chunk is independent: output[chunk_start] = input[chunk_start] (previous = 0 implied).

This is required for the PFPL pipeline where 16 KB chunks flow independently through BitshuffleStage and RZEStage. Chunk size must be a positive multiple of sizeof(T). Default is 0 (no chunking — whole array is one context).

Common instantiations

Instantiation	Use case
`DifferenceStage<int32_t, uint32_t>`	After `QuantizerStage` codes (negabinary fused)
`DifferenceStage<int32_t>`	After `QuantizerStage` codes (plain delta, no negabinary)
`DifferenceStage<float>`	Delta coding of raw float data

Typical pipeline

auto* quant = p.addStage<QuantizerStage<float, uint32_t>>();
auto* diff  = p.addStage<DifferenceStage<int32_t, uint32_t>>();
auto* bshuf = p.addStage<BitshuffleStage>();
 
diff->setChunkSize(16384);
bshuf->setElementWidth(sizeof(uint32_t));
 
p.connect(diff,  quant, "codes");
p.connect(bshuf, diff);
p.finalize();