perf allow for dirty padding of decompression output (#1100)

std/compress/lzss/snark.go

@@ -18,7 +18,12 @@ import (
 // it is recommended to pack the dictionary using compress.Pack and take a MiMC checksum of it.
 // d will consist of bytes
 // It returns the length of d as a frontend.Variable; if the decompressed stream doesn't fit in d, dLength will be "-1"
-func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variable, d, dict []frontend.Variable) (dLength frontend.Variable, err error) {
+func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variable, d, dict []frontend.Variable, options ...DecompressionOption) (dLength frontend.Variable, err error) {
+
+	var aux decompressionAux
+	for _, opt := range options {
+		opt(&aux)
+	}
 
 	api.AssertIsLessOrEqual(cLength, len(c)) // sanity check
 
@@ -100,8 +105,11 @@ func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variab
 
 		// write to output
 		outVal := api.Select(copying, toCopy, curr)
-		// TODO previously the last byte of the output kept getting repeated. That can be worked with. If there was a reason to save some 600K constraints in the zkEVM decompressor, take this out again
-		d[outI] = plonk.EvaluateExpression(api, outVal, eof, 1, 0, -1, 0) // write zeros past eof
+		if aux.noZeroPaddingOutput {
+			d[outI] = outVal
+		} else {
+			d[outI] = plonk.EvaluateExpression(api, outVal, eof, 1, 0, -1, 0) // write zeros past eof
+		}
 		// WARNING: curr modified by MulAcc
 		outTable.Insert(d[outI])
 
@@ -114,11 +122,8 @@ func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variab
 		// TODO Try removing this check and requiring the user to pad the input with nonzeros
 		// TODO Change inner to mulacc once https://github.com/Consensys/gnark/pull/859 is merged
 		// inI = inI + inIDelta * (1 - eof)
-		if eof == 0 {
-			inI = api.Add(inI, inIDelta)
-		} else {
-			inI = api.Add(inI, plonk.EvaluateExpression(api, inIDelta, eof, 1, 0, -1, 0)) // if eof, stay put
-		}
+
+		inI = api.Add(inI, plonk.EvaluateExpression(api, inIDelta, eof, 1, 0, -1, 0)) // if eof, stay put
 
 		eofNow := rangeChecker.IsLessThan(8, api.Sub(cLength, inI)) // less than a byte left; meaning we are at the end of the input
 
@@ -179,3 +184,18 @@ func RegisterHints() {
 	hint.RegisterHint(internal.BreakUpBytesIntoHalfHint)
 	hint.RegisterHint(compress.UnpackIntoBytesHint)
 }
+
+// options and other auxiliary input
+type decompressionAux struct {
+	noZeroPaddingOutput bool
+}
+
+type DecompressionOption func(*decompressionAux)
+
+// WithoutZeroPaddingOutput disables the feature where all decompressor output past the end is zeroed out
+// It saves one constraint per byte of output but necessitates more assignment work
+// If using this option, the output will be padded by the first byte of the input past the end
+// If further the input is not padded, the output still will be padded with zeros
+func WithoutZeroPaddingOutput(aux *decompressionAux) {
+	aux.noZeroPaddingOutput = true
+}

std/compress/lzss/snark_test.go

@@ -366,3 +366,38 @@ func TestBuildDecompress1KBto9KB(t *testing.T) {
 	assert.NoError(t, err)
 	fmt.Println(cs.GetNbConstraints())
 }
+
+func TestNoZeroPaddingOutput(t *testing.T) {
+	assignment := testNoZeroPaddingOutputCircuit{
+		C:    []frontend.Variable{0, 1, 0, 2, 3, 0, 0, 0},
+		D:    []frontend.Variable{2, 3, 3},
+		CLen: 4,
+		DLen: 1,
+	}
+	circuit := testNoZeroPaddingOutputCircuit{
+		C: make([]frontend.Variable, len(assignment.C)),
+		D: make([]frontend.Variable, len(assignment.D)),
+	}
+
+	RegisterHints()
+	test.NewAssert(t).CheckCircuit(&circuit, test.WithValidAssignment(&assignment), test.WithBackends(backend.PLONK), test.WithCurves(ecc.BLS12_377))
+}
+
+type testNoZeroPaddingOutputCircuit struct {
+	CLen, DLen frontend.Variable
+	C, D       []frontend.Variable
+}
+
+func (c *testNoZeroPaddingOutputCircuit) Define(api frontend.API) error {
+	dict := []frontend.Variable{254, 255}
+	d := make([]frontend.Variable, len(c.D))
+	dLen, err := Decompress(api, c.C, c.CLen, d, dict, WithoutZeroPaddingOutput)
+	if err != nil {
+		return err
+	}
+	api.AssertIsEqual(c.DLen, dLen)
+	for i := range c.D {
+		api.AssertIsEqual(c.D[i], d[i])
+	}
+	return nil
+}