1 files changed, 94 insertions, 43 deletions

diff --git a/common/bitutil/compress.go b/common/bitutil/compress.go
index c6c139ab9..a806c0e8b 100644
--- a/common/bitutil/compress.go
+++ b/common/bitutil/compress.go
@@ -16,78 +16,129 @@
 
 package bitutil
 
-/*
-The compression algorithm implemented by CompressBytes and DecompressBytes is
-optimized for "sparse" input data which contains a lot of zero bytes. Decompression
-requires knowledge of the decompressed data length. Compression works as follows:
+import "errors"
 
-if data only contains zeroes,
-  CompressBytes(data) == nil
-otherwise if len(data) <= 1,
- CompressBytes(data) == data
-otherwise:
- CompressBytes(data) == append(CompressBytes(nonZeroBits(data)), nonZeroBytes(data)...)
-where
- nonZeroBits(data) is a bit vector with len(data) bits (MSB first):
-  nonZeroBits(data)[i/8] && (1 << (7-i%8)) != 0  if data[i] != 0
-  len(nonZeroBits(data)) == (len(data)+7)/8
- nonZeroBytes(data) contains the non-zero bytes of data in the same order
-*/
+var (
+	// ErrMissingData is returned from decompression if the byte referenced by
+	// the bitset header overflows the input data.
+	ErrMissingData = errors.New("missing bytes on input")
 
-// CompressBytes compresses the input byte slice
+	// ErrUnreferencedData is returned from decompression if not all bytes were used
+	// up from the input data after decompressing it.
+	ErrUnreferencedData = errors.New("extra bytes on input")
+
+	// ErrExceededTarget is returned from decompression if the bitset header has
+	// more bits defined than the number of target buffer space available.
+	ErrExceededTarget = errors.New("target data size exceeded")
+
+	// ErrZeroContent is returned from decompression if a data byte referenced in
+	// the bitset header is actually a zero byte.
+	ErrZeroContent = errors.New("zero byte in input content")
+)
+
+// The compression algorithm implemented by CompressBytes and DecompressBytes is
+// optimized for sparse input data which contains a lot of zero bytes. Decompression
+// requires knowledge of the decompressed data length.
+//
+// Compression works as follows:
+//
+//   if data only contains zeroes,
+//       CompressBytes(data) == nil
+//   otherwise if len(data) <= 1,
+//       CompressBytes(data) == data
+//   otherwise:
+//       CompressBytes(data) == append(CompressBytes(nonZeroBitset(data)), nonZeroBytes(data)...)
+//       where
+//         nonZeroBitset(data) is a bit vector with len(data) bits (MSB first):
+//             nonZeroBitset(data)[i/8] && (1 << (7-i%8)) != 0  if data[i] != 0
+//             len(nonZeroBitset(data)) == (len(data)+7)/8
+//         nonZeroBytes(data) contains the non-zero bytes of data in the same order
+
+// CompressBytes compresses the input byte slice according to the sparse bitset
+// representation algorithm.
 func CompressBytes(data []byte) []byte {
+	// Empty slices get compressed to nil
 	if len(data) == 0 {
 		return nil
 	}
+	// One byte slices compress to nil or retain the single byte
 	if len(data) == 1 {
 		if data[0] == 0 {
 			return nil
-		} else {
-			return data
 		}
+		return data
 	}
-
-	bitsLen := (len(data) + 7) / 8
-	nonZeroBits := make([]byte, bitsLen)
+	// Calculate the bitset of set bytes, and gather the non-zero bytes
+	nonZeroBitset := make([]byte, (len(data)+7)/8)
 	nonZeroBytes := make([]byte, 0, len(data))
+
 	for i, b := range data {
 		if b != 0 {
 			nonZeroBytes = append(nonZeroBytes, b)
-			nonZeroBits[i/8] |= 1 << byte(7-i%8)
+			nonZeroBitset[i/8] |= 1 << byte(7-i%8)
 		}
 	}
 	if len(nonZeroBytes) == 0 {
 		return nil
 	}
-	return append(CompressBytes(nonZeroBits), nonZeroBytes...)
+	return append(CompressBytes(nonZeroBitset), nonZeroBytes...)
 }
 
-// DecompressBytes decompresses data with a known target size.
-// In addition to the decompressed output, the function returns the length of
-// compressed input data corresponding to the output. The input slice may be longer.
-// If the input slice is too short, (nil, -1) is returned.
-func DecompressBytes(data []byte, targetLen int) ([]byte, int) {
-	decomp := make([]byte, targetLen)
-	if len(data) == 0 {
-		return decomp, 0
+// DecompressBytes decompresses data with a known target size. In addition to the
+// decompressed output, the function returns the length of compressed input data
+// corresponding to the output as the input slice may be longer.
+func DecompressBytes(data []byte, target int) ([]byte, error) {
+	out, size, err := decompressBytes(data, target)
+	if err != nil {
+		return nil, err
 	}
-	if targetLen == 1 {
-		return data[0:1], 1
+	if size != len(data) {
+		return nil, ErrUnreferencedData
 	}
+	return out, nil
+}
 
-	bitsLen := (targetLen + 7) / 8
-	nonZeroBits, ptr := DecompressBytes(data, bitsLen)
-	if ptr < 0 {
-		return nil, -1
+// decompressBytes decompresses data with a known target size. In addition to the
+// decompressed output, the function returns the length of compressed input data
+// corresponding to the output as the input slice may be longer.
+func decompressBytes(data []byte, target int) ([]byte, int, error) {
+	// Sanity check 0 targets to avoid infinite recursion
+	if target == 0 {
+		return nil, 0, nil
+	}
+	// Handle the zero and single byte corner cases
+	decomp := make([]byte, target)
+	if len(data) == 0 {
+		return decomp, 0, nil
+	}
+	if target == 1 {
+		decomp[0] = data[0] // copy to avoid referencing the input slice
+		if data[0] != 0 {
+			return decomp, 1, nil
+		}
+		return decomp, 0, nil
+	}
+	// Decompress the bitset of set bytes and distribute the non zero bytes
+	nonZeroBitset, ptr, err := decompressBytes(data, (target+7)/8)
+	if err != nil {
+		return nil, ptr, err
 	}
-	for i, _ := range decomp {
-		if nonZeroBits[i/8]&(1<<byte(7-i%8)) != 0 {
-			if ptr == len(data) {
-				return nil, -1
+	for i := 0; i < 8*len(nonZeroBitset); i++ {
+		if nonZeroBitset[i/8]&(1<<byte(7-i%8)) != 0 {
+			// Make sure we have enough data to push into the correct slot
+			if ptr >= len(data) {
+				return nil, 0, ErrMissingData
+			}
+			if i >= len(decomp) {
+				return nil, 0, ErrExceededTarget
+			}
+			// Make sure the data is valid and push into the slot
+			if data[ptr] == 0 {
+				return nil, 0, ErrZeroContent
 			}
 			decomp[i] = data[ptr]
 			ptr++
 		}
 	}
-	return decomp, ptr
+	return decomp, ptr, nil
 }