From b88df6b1b058a1a4884e8c7c80ce8b7e0839eb30 Mon Sep 17 00:00:00 2001
From: Yann Collet <cyan@fb.com>
Date: Wed, 29 Mar 2017 12:51:08 -0700
Subject: [PATCH] Improved comments on LZ4F_getFrameInfo()

and added LZ4F_resetCompressionContext()
---
 lib/lz4frame.c        |  6 ++++++
 lib/lz4frame.h        | 14 +++++++++-----
 lib/lz4frame_static.h |  9 +++++++++
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/lib/lz4frame.c b/lib/lz4frame.c
index 518194d..1184a0f 100644
--- a/lib/lz4frame.c
+++ b/lib/lz4frame.c
@@ -792,6 +792,12 @@ typedef enum {
     dstage_skipSkippable
 } dStage_t;
 
+LZ4F_errorCode_t LZ4F_resetDecompressionContext(LZ4F_dctx* dctx)
+{
+    dctx->dStage = dstage_getHeader;
+    return 0;
+}
+
 
 /*! LZ4F_headerSize() :
 *   @return : size of frame header
diff --git a/lib/lz4frame.h b/lib/lz4frame.h
index 7c33464..76b4e69 100644
--- a/lib/lz4frame.h
+++ b/lib/lz4frame.h
@@ -300,7 +300,7 @@ typedef struct {
  * That is, it should be == 0 if decompression has been completed fully and correctly.
  */
 LZ4FLIB_API LZ4F_errorCode_t LZ4F_createDecompressionContext(LZ4F_dctx** dctxPtr, unsigned version);
-LZ4FLIB_API LZ4F_errorCode_t LZ4F_freeDecompressionContext(LZ4F_dctx* const dctx);
+LZ4FLIB_API LZ4F_errorCode_t LZ4F_freeDecompressionContext(LZ4F_dctx* dctx);
 
 
 /*-***********************************
@@ -309,12 +309,15 @@ LZ4FLIB_API LZ4F_errorCode_t LZ4F_freeDecompressionContext(LZ4F_dctx* const dctx
 
 /*! LZ4F_getFrameInfo() :
  * This function extracts frame parameters (such as max blockSize, frame checksum, etc.).
- * Its usage is optional. The objective is to provide relevant information for allocation purposes.
+ * Its usage is optional. Extracted information can be useful for allocation purposes, typically.
  * This function works in 2 situations :
  *   - At the beginning of a new frame, in which case it will decode this information from `srcBuffer`, and start the decoding process.
- *     Amount of input data provided must be large enough to successfully decode the frame header.
- *     A header size is variable, but is guaranteed to be <= LZ4F_HEADER_SIZE_MAX bytes. It's possible to provide more input data than this minimum.
- *   - After decoding has been started. In which case, no input is read, frame parameters are extracted from dctx.
+ *     Input size must be large enough to successfully decode the entire frame header.
+ *     Frame header size is variable, but is guaranteed to be <= LZ4F_HEADER_SIZE_MAX bytes.
+ *     It's allowed to provide more input data than this minimum.
+ *   - After decoding has been started.
+ *     In which case, no input is read, frame parameters are extracted from dctx.
+ *     If decoding has just started, but not yet extracted information from header, LZ4F_getFrameInfo() will fail.
  * The number of bytes consumed from srcBuffer will be updated within *srcSizePtr (necessarily <= original value).
  * Decompression must resume from (srcBuffer + *srcSizePtr).
  * @return : an hint about how many srcSize bytes LZ4F_decompress() expects for next call,
@@ -348,6 +351,7 @@ LZ4FLIB_API size_t LZ4F_getFrameInfo(LZ4F_dctx* dctx,
  * If decompression failed, @return is an error code, which can be tested using LZ4F_isError().
  *
  * After a frame is fully decoded, dctx can be used again to decompress another frame.
+ * After a decompression error, use LZ4F_resetDecompressionContext() before re-using dctx, to return to clean state.
  */
 LZ4FLIB_API size_t LZ4F_decompress(LZ4F_dctx* dctx,
                                    void* dstBuffer, size_t* dstSizePtr,
diff --git a/lib/lz4frame_static.h b/lib/lz4frame_static.h
index d3bae82..8ea496d 100644
--- a/lib/lz4frame_static.h
+++ b/lib/lz4frame_static.h
@@ -50,6 +50,15 @@ extern "C" {
 #include "lz4frame.h"
 
 
+/* ---   Experimental functions   --- */
+/* LZ4F_resetDecompressionContext() :
+ * LZ4F_decompress() does not guarantee to leave dctx in clean state in case of errors.
+ * In order to re-use a dctx after a decompression error,
+ * use LZ4F_resetDecompressionContext() first.
+ * dctx will be able to start decompression on a new frame */
+LZ4FLIB_API LZ4F_errorCode_t LZ4F_resetDecompressionContext(LZ4F_dctx* dctx);
+
+
 /* ---   Error List   --- */
 #define LZ4F_LIST_ERRORS(ITEM) \
         ITEM(OK_NoError) \
-- 
2.7.4