LZ4F_compressBound(0) provides upper bound for LZ4F_flush() and LZ4F_compressEnd() [#290, suggested by @vtermanis]

2 files changed, 23 insertions, 15 deletions

diff --git a/lib/lz4frame.c b/lib/lz4frame.c
index 3c2b788..626ac98 100644
--- a/lib/lz4frame.c
+++ b/lib/lz4frame.c
@@ -158,7 +158,7 @@ static void LZ4F_writeLE64 (void* dst, U64 value64)
 #define LZ4F_BLOCKSIZEID_DEFAULT LZ4F_max64KB
 
 static const size_t minFHSize = 7;
-static const size_t maxFHSize = 15;   /* max Frame Header Size */
+static const size_t maxFHSize = LZ4F_HEADER_SIZE_MAX;   /* 15 */
 static const size_t BHSize = 4;
 
 
@@ -254,20 +254,27 @@ static LZ4F_blockSizeID_t LZ4F_optimalBSID(const LZ4F_blockSizeID_t requestedBSI
     return requestedBSID;
 }
 
+/* LZ4F_compressBound() :
+ * Provides dstCapacity given a srcSize to guarantee operation success in worst case situations.
+ * prefsPtr is optional : you can provide NULL as argument, preferences will be set to cover worst case scenario.
+ * Result is always the same for a srcSize and prefsPtr, so it can be trusted to size reusable buffers.
+ * When srcSize==0, LZ4F_compressBound() provides an upper bound for LZ4F_flush() and LZ4F_compressEnd() operations.
+ */
 static size_t LZ4F_compressBound_internal(size_t srcSize, const LZ4F_preferences_t* preferencesPtr, size_t alreadyBuffered)
 {
     LZ4F_preferences_t prefsNull;
     memset(&prefsNull, 0, sizeof(prefsNull));
     prefsNull.frameInfo.contentChecksumFlag = LZ4F_contentChecksumEnabled;   /* worst case */
     {   const LZ4F_preferences_t* const prefsPtr = (preferencesPtr==NULL) ? &prefsNull : preferencesPtr;
+        U32 const flush = prefsPtr->autoFlush | (srcSize==0);
         LZ4F_blockSizeID_t const bid = prefsPtr->frameInfo.blockSizeID;
         size_t const blockSize = LZ4F_getBlockSize(bid);
         size_t const maxBuffered = blockSize - 1;
         size_t const bufferedSize = MIN(alreadyBuffered, maxBuffered);
         size_t const maxSrcSize = srcSize + bufferedSize;
         unsigned const nbFullBlocks = (unsigned)(maxSrcSize / blockSize);
-        size_t const partialBlockSize = srcSize & (blockSize-1);
-        size_t const lastBlockSize = prefsPtr->autoFlush ? partialBlockSize : 0;
+        size_t const partialBlockSize = (srcSize - (srcSize==0)) & (blockSize-1);   /* 0 => -1 == MAX => blockSize-1 */
+        size_t const lastBlockSize = flush ? partialBlockSize : 0;
         unsigned const nbBlocks = nbFullBlocks + (lastBlockSize>0);
 
         size_t const blockHeaderSize = 4;   /* default, without block CRC option (which cannot be generated with current API) */
@@ -398,7 +405,7 @@ LZ4F_errorCode_t LZ4F_freeCompressionContext(LZ4F_compressionContext_t LZ4F_comp
 
 /*! LZ4F_compressBegin() :
  * will write the frame header into dstBuffer.
- * dstBuffer must be large enough to accommodate a header (dstCapacity). Maximum header size is LZ4F_MAXHEADERFRAME_SIZE bytes.
+ * dstBuffer must be large enough to accommodate a header (dstCapacity). Maximum header size is LZ4F_HEADER_SIZE_MAX bytes.
  * @return : number of bytes written into dstBuffer for the header
  *           or an error code (can be tested using LZ4F_isError())
  */
diff --git a/lib/lz4frame.h b/lib/lz4frame.h
index 3104d2e..a4a4ce6 100644
--- a/lib/lz4frame.h
+++ b/lib/lz4frame.h
@@ -206,10 +206,11 @@ LZ4FLIB_API LZ4F_errorCode_t LZ4F_freeCompressionContext(LZ4F_cctx* cctx);
 
 /* Compression */
 
+#define LZ4F_HEADER_SIZE_MAX 15
 LZ4FLIB_API size_t LZ4F_compressBegin(LZ4F_cctx* cctx, void* dstBuffer, size_t dstCapacity, const LZ4F_preferences_t* prefsPtr);
 /* LZ4F_compressBegin() :
  * will write the frame header into dstBuffer.
- * dstBuffer must be large enough to accommodate a header. Maximum header size is 15 bytes.
+ * dstCapacity must be large enough to store the header. Maximum header size is LZ4F_HEADER_SIZE_MAX bytes.
  * `prefsPtr` is optional : you can provide NULL as argument, all preferences will then be set to default.
  * @return : number of bytes written into dstBuffer for the header
  *           or an error code (which can be tested using LZ4F_isError())
@@ -217,20 +218,20 @@ LZ4FLIB_API size_t LZ4F_compressBegin(LZ4F_cctx* cctx, void* dstBuffer, size_t d
 
 LZ4FLIB_API size_t LZ4F_compressBound(size_t srcSize, const LZ4F_preferences_t* prefsPtr);
 /* LZ4F_compressBound() :
- * Provides the minimum size of Dst buffer given srcSize to handle worst case situations.
- * Different preferences can produce different results.
- * prefsPtr is optional : you can provide NULL as argument, all preferences will then be set to cover worst case.
- * This function includes frame termination cost (4 bytes, or 8 if frame checksum is enabled)
+ * Provides dstCapacity given a srcSize to guarantee operation success in worst case situations.
+ * prefsPtr is optional : you can provide NULL as argument, preferences will be set to cover worst case scenario.
+ * Result is always the same for a srcSize and prefsPtr, so it can be trusted to size reusable buffers.
+ * When srcSize==0, LZ4F_compressBound() provides an upper bound for LZ4F_flush() and LZ4F_compressEnd() operations.
  */
 
 LZ4FLIB_API size_t LZ4F_compressUpdate(LZ4F_cctx* cctx, void* dstBuffer, size_t dstCapacity, const void* srcBuffer, size_t srcSize, const LZ4F_compressOptions_t* cOptPtr);
 /* LZ4F_compressUpdate() :
  * LZ4F_compressUpdate() can be called repetitively to compress as much data as necessary.
- * An important rule is that dstBuffer MUST be large enough (dstCapacity) to ensure compression completion even in worst case.
- * This value is provided by using LZ4F_compressBound().
+ * An important rule is that dstCapacity MUST be large enough to ensure operation success even in worst case situations.
+ * This value is provided by LZ4F_compressBound().
  * If this condition is not respected, LZ4F_compress() will fail (result is an errorCode).
  * LZ4F_compressUpdate() doesn't guarantee error recovery. When an error occurs, compression context must be freed or resized.
- * `cOptPtr` is optional : it's possible to provide NULL, all options will be set to default.
+ * `cOptPtr` is optional : NULL can be provided, in which case all options are set to default.
  * @return : number of bytes written into `dstBuffer` (it can be zero, meaning input data was just buffered).
  *           or an error code if it fails (which can be tested using LZ4F_isError())
  */
@@ -245,12 +246,12 @@ LZ4FLIB_API size_t LZ4F_flush(LZ4F_cctx* cctx, void* dstBuffer, size_t dstCapaci
  *           or an error code if it fails (which can be tested using LZ4F_isError())
  */
 
-LZ4FLIB_API size_t LZ4F_compressEnd(LZ4F_cctx* cctx, void* dstBuffer, size_t dstMaxSize, const LZ4F_compressOptions_t* cOptPtr);
+LZ4FLIB_API size_t LZ4F_compressEnd(LZ4F_cctx* cctx, void* dstBuffer, size_t dstCapacity, const LZ4F_compressOptions_t* cOptPtr);
 /* LZ4F_compressEnd() :
- * To properly finish the compressed frame, invoke LZ4F_compressEnd().
+ * To properly finish an LZ4 frame, invoke LZ4F_compressEnd().
  * It will flush whatever data remained within `cctx` (like LZ4_flush())
  * and properly finalize the frame, with an endMark and a checksum.
- * `cOptPtr` is optional : it's possible to provide NULL, all options will be set to default.
+ * `cOptPtr` is optional : NULL can be provided, in which case all options will be set to default.
  * @return : number of bytes written into dstBuffer (necessarily >= 4 (endMark), or 8 if optional frame checksum is enabled)
  *           or an error code if it fails (which can be tested using LZ4F_isError())
  * A successful call to LZ4F_compressEnd() makes `cctx` available again for another compression task.