fix: Decompressor to return -1 when output doesn't fit (#1022)

* fix: Decompressor to return -1 when output doesn't fit

* docs explain "outBuffer too short" logic

* fix make staticcheck happy

std/compress/lzss/snark.go

@@ -15,7 +15,7 @@ import (
 // it is on the caller to ensure that the dictionary is correct; in particular it must consist of bytes. Decompress does not check this.
 // 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
+// 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, level lzss.Level) (dLength frontend.Variable, err error) {
 
 	// size-related "constants"
@@ -62,7 +62,7 @@ func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variab
 	copyLen := frontend.Variable(0) // remaining length of the current copy
 	copyLen01 := frontend.Variable(1)
 	eof := frontend.Variable(0)
-	dLength = 0
+	dLength = -1 // if the following loop ends before hitting eof, we will get the "error" value -1 for dLength
 
 	for outI := range d {
 
@@ -114,7 +114,9 @@ func Decompress(api frontend.API, c []frontend.Variable, cLength frontend.Variab
 
 		eofNow := rangeChecker.IsLessThan(byteNbWords, api.Sub(cLength, inI)) // less than a byte left; meaning we are at the end of the input
 
-		dLength = api.Add(dLength, api.Mul(api.Sub(eofNow, eof), outI+1)) // if eof, don't advance dLength
+		// if eof, don't advance dLength
+		// if eof was JUST hit, dLength += outI + 2; so dLength = -1 + outI + 2 = outI + 1 which is the current output length
+		dLength = api.Add(dLength, api.Mul(api.Sub(eofNow, eof), outI+2))
 		eof = eofNow
 
 	}

std/compress/lzss/snark_test.go

@@ -103,6 +103,31 @@ func Test3c2943withHeader(t *testing.T) {
 	test.NewAssert(t).CheckCircuit(circuit, test.WithValidAssignment(assignment), test.WithBackends(backend.PLONK), test.WithCurves(ecc.BLS12_377))
 }
 
+func TestOutBufTooShort(t *testing.T) {
+	const truncationAmount = 3
+	d := []byte{1, 2, 3, 4, 5, 6, 7, 8, 9}
+	compressor, err := lzss.NewCompressor(nil, lzss.BestCompression)
+	require.NoError(t, err)
+	c, err := compressor.Compress(d)
+	require.NoError(t, err)
+
+	circuit := decompressionLengthTestCircuit{
+		C: make([]frontend.Variable, len(c)+inputExtraBytes),
+		D: make([]frontend.Variable, len(d)-truncationAmount), // not enough room
+
+	}
+
+	assignment := decompressionLengthTestCircuit{
+		C:               test_vector_utils.ToVariableSlice(append(c, make([]byte, inputExtraBytes)...)),
+		CLength:         len(c),
+		D:               test_vector_utils.ToVariableSlice(d[:len(d)-truncationAmount]),
+		ExpectedDLength: -1,
+	}
+
+	RegisterHints()
+	test.NewAssert(t).CheckCircuit(&circuit, test.WithValidAssignment(&assignment), test.WithCurves(ecc.BLS12_377))
+}
+
 // Fuzz test the decompression
 func Fuzz(f *testing.F) { // TODO This is always skipped
 	f.Fuzz(func(t *testing.T, input, dict []byte) {
@@ -179,3 +204,19 @@ func getDictionary() []byte {
 	}
 	return d
 }
+
+type decompressionLengthTestCircuit struct {
+	C, D            []frontend.Variable
+	CLength         frontend.Variable
+	ExpectedDLength frontend.Variable
+}
+
+func (c *decompressionLengthTestCircuit) Define(api frontend.API) error {
+	dict := test_vector_utils.ToVariableSlice(lzss.AugmentDict(nil))
+	if dLength, err := Decompress(api, c.C, c.CLength, c.D, dict, lzss.BestCompression); err != nil {
+		return err
+	} else {
+		api.AssertIsEqual(dLength, c.ExpectedDLength)
+		return nil
+	}
+}