first sketch for a byte-accurate partial decoder

diff --git a/lib/lz4.c b/lib/lz4.c
index 35df7f5..6febb90 100644 (file)
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -483,9 +483,6 @@ typedef enum { clearedTable = 0, byPtr, byU32, byU16 } tableType_t;
 typedef enum { noDict = 0, withPrefix64k, usingExtDict, usingDictCtx } dict_directive;
 typedef enum { noDictIssue = 0, dictSmall } dictIssue_directive;
 
-typedef enum { endOnOutputSize = 0, endOnInputSize = 1 } endCondition_directive;
-typedef enum { full = 0, partial = 1 } earlyEnd_directive;
-
 
 /*-************************************
 *  Local Utils
@@ -684,9 +681,9 @@ LZ4_FORCE_INLINE int LZ4_compress_generic(
 
     /* the dictCtx currentOffset is indexed on the start of the dictionary,
      * while a dictionary in the current context precedes the currentOffset */
-    const BYTE* dictBase = dictDirective == usingDictCtx ?
-        dictionary + dictSize - dictCtx->currentOffset :
-        dictionary + dictSize - startIndex;
+    const BYTE* dictBase = (dictDirective == usingDictCtx) ?
+                            dictionary + dictSize - dictCtx->currentOffset :
+                            dictionary + dictSize - startIndex;
 
     BYTE* op = (BYTE*) dest;
     BYTE* const olimit = op + maxOutputSize;
@@ -1385,25 +1382,32 @@ int LZ4_saveDict (LZ4_stream_t* LZ4_dict, char* safeBuffer, int dictSize)
 
 
 
-/*-*****************************
-*  Decompression functions
-*******************************/
+/*-*******************************
+ *  Decompression functions
+ ********************************/
+
+typedef enum { endOnOutputSize = 0, endOnInputSize = 1 } endCondition_directive;
+typedef enum { decode_full_block = 0, partial_decode = 1 } earlyEnd_directive;
+
+#undef MIN
+#define MIN(a,b)    ( (a) < (b) ? (a) : (b) )
+
 /*! LZ4_decompress_generic() :
  *  This generic decompression function covers all use cases.
  *  It shall be instantiated several times, using different sets of directives.
  *  Note that it is important for performance that this function really get inlined,
  *  in order to remove useless branches during compilation optimization.
  */
-LZ4_FORCE_INLINE int
+LZ4_FORCE_INLINE
+int
 LZ4_decompress_generic(
                  const char* const src,
                  char* const dst,
                  int srcSize,
                  int outputSize,         /* If endOnInput==endOnInputSize, this value is `dstCapacity` */
 
-                 int endOnInput,         /* endOnOutputSize, endOnInputSize */
-                 int partialDecoding,    /* full, partial */
-                 int targetOutputSize,   /* only used if partialDecoding==partial */
+                 endCondition_directive endOnInput,     /* endOnOutputSize, endOnInputSize */
+                 earlyEnd_directive partialDecoding,    /* full, partial */
                  int dict,               /* noDict, withPrefix64k, usingExtDict */
                  const BYTE* const lowPrefix,  /* always <= dst, == dst when no prefix */
                  const BYTE* const dictStart,  /* only if dict==usingExtDict */
@@ -1416,7 +1420,6 @@ LZ4_decompress_generic(
     BYTE* op = (BYTE*) dst;
     BYTE* const oend = op + outputSize;
     BYTE* cpy;
-    BYTE* oexit = op + targetOutputSize;
 
     const BYTE* const dictEnd = (const BYTE*)dictStart + dictSize;
     const unsigned inc32table[8] = {0, 1, 2,  1,  0,  4, 4, 4};
@@ -1432,9 +1435,9 @@ LZ4_decompress_generic(
     DEBUGLOG(5, "LZ4_decompress_generic (srcSize:%i)", srcSize);
 
     /* Special cases */
-    if ((partialDecoding) && (oexit > oend-MFLIMIT)) oexit = oend-MFLIMIT;                      /* targetOutputSize too high => just decode everything */
+    assert(src != NULL);
     if ((endOnInput) && (unlikely(outputSize==0))) return ((srcSize==1) && (*ip==0)) ? 0 : -1;  /* Empty output buffer */
-    if ((!endOnInput) && (unlikely(outputSize==0))) return (*ip==0?1:-1);
+    if ((!endOnInput) && (unlikely(outputSize==0))) return (*ip==0 ? 1 : -1);
     if ((endOnInput) && unlikely(srcSize==0)) return -1;
 
     /* Main Loop : decode sequences */
@@ -1443,7 +1446,7 @@ LZ4_decompress_generic(
         size_t offset;
 
         unsigned const token = *ip++;
-        size_t length = token >> ML_BITS; /* literal length */
+        size_t length = token >> ML_BITS;  /* literal length */
 
         assert(!endOnInput || ip <= iend); /* ip < iend before the increment */
 
@@ -1468,6 +1471,7 @@ LZ4_decompress_generic(
             length = token & ML_MASK; /* match length */
             offset = LZ4_readLE16(ip); ip += 2;
             match = op - offset;
+            assert(match <= op); /* check overflow */
 
             /* Do not deal with overlapping matches. */
             if ( (length != ML_MASK)
@@ -1501,11 +1505,11 @@ LZ4_decompress_generic(
 
         /* copy literals */
         cpy = op+length;
-        if ( ((endOnInput) && ((cpy>(partialDecoding?oexit:oend-MFLIMIT)) || (ip+length>iend-(2+1+LASTLITERALS))) )
-            || ((!endOnInput) && (cpy>oend-WILDCOPYLENGTH)) )
+        if ( ((endOnInput) && ((cpy>oend-MFLIMIT) || (ip+length>iend-(2+1+LASTLITERALS))) )
+          || ((!endOnInput) && (cpy>oend-WILDCOPYLENGTH)) )
         {
             if (partialDecoding) {
-                if (cpy > oend) goto _output_error;                           /* Error : write attempt beyond end of output buffer */
+                if (cpy > oend) { cpy = oend; length = oend-op; }             /* Partial decoding : stop in the middle of literal segment */
                 if ((endOnInput) && (ip+length > iend)) goto _output_error;   /* Error : read attempt beyond end of input buffer */
             } else {
                 if ((!endOnInput) && (cpy != oend)) goto _output_error;       /* Error : block decoding must stop exactly there */
@@ -1514,10 +1518,15 @@ LZ4_decompress_generic(
             memcpy(op, ip, length);
             ip += length;
             op += length;
-            break;     /* Necessarily EOF, due to parsing restrictions */
+            if (!partialDecoding || (cpy == oend)) {
+                /* Necessarily EOF, due to parsing restrictions */
+                break;
+            }
+
+        } else {
+            LZ4_wildCopy(op, ip, cpy);
+            ip += length; op = cpy;
         }
-        LZ4_wildCopy(op, ip, cpy);
-        ip += length; op = cpy;
 
         /* get offset */
         offset = LZ4_readLE16(ip); ip+=2;
@@ -1541,21 +1550,24 @@ _copy_match:
         }
         length += MINMATCH;
 
-        /* check external dictionary */
+        /* match starting within external dictionary */
         if ((dict==usingExtDict) && (match < lowPrefix)) {
-            if (unlikely(op+length > oend-LASTLITERALS)) goto _output_error;   /* doesn't respect parsing restriction */
+            if (unlikely(op+length > oend-LASTLITERALS)) {
+                if (partialDecoding) length = MIN(length, (size_t)(oend-op));
+                else goto _output_error;   /* doesn't respect parsing restriction */
+            }
 
             if (length <= (size_t)(lowPrefix-match)) {
-                /* match can be copied as a single segment from external dictionary */
+                /* match fits entirely within external dictionary : just copy */
                 memmove(op, dictEnd - (lowPrefix-match), length);
                 op += length;
             } else {
-                /* match encompass external dictionary and current block */
-                size_t const copySize = (size_t)(lowPrefix-match);
+                /* match stretches into both external dictionary and current block */
+                size_t const copySize = (size_t)(lowPrefix - match);
                 size_t const restSize = length - copySize;
                 memcpy(op, dictEnd - copySize, copySize);
                 op += copySize;
-                if (restSize > (size_t)(op-lowPrefix)) {  /* overlap copy */
+                if (restSize > (size_t)(op - lowPrefix)) {  /* overlap copy */
                     BYTE* const endOfMatch = op + restSize;
                     const BYTE* copyFrom = lowPrefix;
                     while (op < endOfMatch) *op++ = *copyFrom++;
@@ -1568,6 +1580,22 @@ _copy_match:
 
         /* copy match within block */
         cpy = op + length;
+
+        /* specific : partial decode : does not respect end parsing restrictions */
+        assert(op<=oend);
+        if (partialDecoding && (cpy > oend-12)) {
+            size_t const mlen = MIN(length, (size_t)(oend-op));
+            const BYTE* const matchEnd = match + mlen;
+            BYTE* const copyEnd = op + mlen;
+            if (matchEnd > op) {   /* overlap copy */
+                while (op < copyEnd) *op++ = *match++;
+            } else {
+                memcpy(op, match, mlen);
+            }
+            op = copyEnd;
+            continue;
+        }
+
         if (unlikely(offset<8)) {
             op[0] = match[0];
             op[1] = match[1];
@@ -1576,23 +1604,26 @@ _copy_match:
             match += inc32table[offset];
             memcpy(op+4, match, 4);
             match -= dec64table[offset];
-        } else { memcpy(op, match, 8); match+=8; }
+        } else {
+            memcpy(op, match, 8);
+            match += 8;
+        }
         op += 8;
 
-        if (unlikely(cpy>oend-12)) {
-            BYTE* const oCopyLimit = oend-(WILDCOPYLENGTH-1);
+        if (unlikely(cpy > oend-12)) {
+            BYTE* const oCopyLimit = oend - (WILDCOPYLENGTH-1);
             if (cpy > oend-LASTLITERALS) goto _output_error;    /* Error : last LASTLITERALS bytes must be literals (uncompressed) */
             if (op < oCopyLimit) {
                 LZ4_wildCopy(op, match, oCopyLimit);
                 match += oCopyLimit - op;
                 op = oCopyLimit;
             }
-            while (op<cpy) *op++ = *match++;
+            while (op < cpy) *op++ = *match++;
         } else {
             memcpy(op, match, 8);
-            if (length>16) LZ4_wildCopy(op+8, match+8, cpy);
+            if (length > 16) LZ4_wildCopy(op+8, match+8, cpy);
         }
-        op = cpy;   /* correction */
+        op = cpy;   /* wildcopy correction */
     }
 
     /* end of decoding */
@@ -1613,23 +1644,24 @@ LZ4_FORCE_O2_GCC_PPC64LE
 int LZ4_decompress_safe(const char* source, char* dest, int compressedSize, int maxDecompressedSize)
 {
     return LZ4_decompress_generic(source, dest, compressedSize, maxDecompressedSize,
-                                  endOnInputSize, full, 0, noDict,
+                                  endOnInputSize, decode_full_block, noDict,
                                   (BYTE*)dest, NULL, 0);
 }
 
 LZ4_FORCE_O2_GCC_PPC64LE
-int LZ4_decompress_safe_partial(const char* source, char* dest, int compressedSize, int targetOutputSize, int maxDecompressedSize)
+int LZ4_decompress_safe_partial(const char* src, char* dst, int compressedSize, int targetOutputSize, int dstCapacity)
 {
-    return LZ4_decompress_generic(source, dest, compressedSize, maxDecompressedSize,
-                                  endOnInputSize, partial, targetOutputSize,
-                                  noDict, (BYTE*)dest, NULL, 0);
+    dstCapacity = MIN(targetOutputSize, dstCapacity);
+    return LZ4_decompress_generic(src, dst, compressedSize, dstCapacity,
+                                  endOnInputSize, partial_decode,
+                                  noDict, (BYTE*)dst, NULL, 0);
 }
 
 LZ4_FORCE_O2_GCC_PPC64LE
 int LZ4_decompress_fast(const char* source, char* dest, int originalSize)
 {
     return LZ4_decompress_generic(source, dest, 0, originalSize,
-                                  endOnOutputSize, full, 0, withPrefix64k,
+                                  endOnOutputSize, decode_full_block, withPrefix64k,
                                   (BYTE*)dest - 64 KB, NULL, 0);
 }
 
@@ -1639,7 +1671,7 @@ LZ4_FORCE_O2_GCC_PPC64LE /* Exported, an obsolete API function. */
 int LZ4_decompress_safe_withPrefix64k(const char* source, char* dest, int compressedSize, int maxOutputSize)
 {
     return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
-                                  endOnInputSize, full, 0, withPrefix64k,
+                                  endOnInputSize, decode_full_block, withPrefix64k,
                                   (BYTE*)dest - 64 KB, NULL, 0);
 }
 
@@ -1656,7 +1688,7 @@ static int LZ4_decompress_safe_withSmallPrefix(const char* source, char* dest, i
                                                size_t prefixSize)
 {
     return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
-                                  endOnInputSize, full, 0, noDict,
+                                  endOnInputSize, decode_full_block, noDict,
                                   (BYTE*)dest-prefixSize, NULL, 0);
 }
 
@@ -1666,7 +1698,7 @@ int LZ4_decompress_safe_forceExtDict(const char* source, char* dest,
                                      const void* dictStart, size_t dictSize)
 {
     return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
-                                  endOnInputSize, full, 0, usingExtDict,
+                                  endOnInputSize, decode_full_block, usingExtDict,
                                   (BYTE*)dest, (const BYTE*)dictStart, dictSize);
 }
 
@@ -1675,7 +1707,7 @@ static int LZ4_decompress_fast_extDict(const char* source, char* dest, int origi
                                        const void* dictStart, size_t dictSize)
 {
     return LZ4_decompress_generic(source, dest, 0, originalSize,
-                                  endOnOutputSize, full, 0, usingExtDict,
+                                  endOnOutputSize, decode_full_block, usingExtDict,
                                   (BYTE*)dest, (const BYTE*)dictStart, dictSize);
 }
 
@@ -1688,7 +1720,7 @@ int LZ4_decompress_safe_doubleDict(const char* source, char* dest, int compresse
                                    size_t prefixSize, const void* dictStart, size_t dictSize)
 {
     return LZ4_decompress_generic(source, dest, compressedSize, maxOutputSize,
-                                  endOnInputSize, full, 0, usingExtDict,
+                                  endOnInputSize, decode_full_block, usingExtDict,
                                   (BYTE*)dest-prefixSize, (const BYTE*)dictStart, dictSize);
 }
 
@@ -1697,7 +1729,7 @@ int LZ4_decompress_fast_doubleDict(const char* source, char* dest, int originalS
                                    size_t prefixSize, const void* dictStart, size_t dictSize)
 {
     return LZ4_decompress_generic(source, dest, 0, originalSize,
-                                  endOnOutputSize, full, 0, usingExtDict,
+                                  endOnOutputSize, decode_full_block, usingExtDict,
                                   (BYTE*)dest-prefixSize, (const BYTE*)dictStart, dictSize);
 }
 

diff --git a/lib/lz4.h b/lib/lz4.h
index 9d3890a..ce4d033 100644 (file)
--- a/lib/lz4.h
+++ b/lib/lz4.h
@@ -272,12 +272,12 @@ LZ4LIB_API int LZ4_loadDict (LZ4_stream_t* streamPtr, const char* dictionary, in
  * @return : size of compressed block
  *           or 0 if there is an error (typically, cannot fit into 'dst').
  *
- *  Note 1 : Each invocation to LZ4_compress_fast_continue() will generate a new block.
+ *  Note 1 : Each invocation to LZ4_compress_fast_continue() generates a new block.
  *           Each block has precise boundaries.
  *           It's not possible to append blocks together and expect a single invocation of LZ4_decompress_*() to decompress them together.
  *           Each block must be decompressed separately, calling LZ4_decompress_*() with associated metadata.
  *
- *  Note 2 : The previous 64KB of source data is assumed to remain present, unmodified, at same address in memory!
+ *  Note 2 : The previous 64KB of source data is __assumed__ to remain present, unmodified, at same address in memory!
  *
  *  Note 3 : When input is structured as a double-buffer, each buffer can have any size, including < 64 KB.
  *           Make sure that buffers are separated, by at least one byte.

diff --git a/tests/fullbench.c b/tests/fullbench.c
index 2818ea2..fd1202d 100644 (file)
--- a/tests/fullbench.c
+++ b/tests/fullbench.c
@@ -317,7 +317,9 @@ static int local_LZ4_decompress_safe_forceExtDict(const char* in, char* out, int
 
 static int local_LZ4_decompress_safe_partial(const char* in, char* out, int inSize, int outSize)
 {
-    return LZ4_decompress_safe_partial(in, out, inSize, outSize - 5, outSize);
+    int result = LZ4_decompress_safe_partial(in, out, inSize, outSize - 5, outSize);
+    if (result < 0) return result;
+    return outSize;
 }
 
 
@@ -462,9 +464,9 @@ int fullSpeedBench(const char** fileNamesTable, int nbFiles)
             case 12: compressionFunction = local_LZ4_compress_HC_extStateHC; compressorName = "LZ4_compress_HC_extStateHC"; break;
             case 14: compressionFunction = local_LZ4_compress_HC_continue; initFunction = local_LZ4_resetStreamHC; compressorName = "LZ4_compress_HC_continue"; break;
 #ifndef LZ4_DLL_IMPORT
-                       case 20: compressionFunction = local_LZ4_compress_forceDict; initFunction = local_LZ4_resetDictT; compressorName = "LZ4_compress_forceDict"; break;
+            case 20: compressionFunction = local_LZ4_compress_forceDict; initFunction = local_LZ4_resetDictT; compressorName = "LZ4_compress_forceDict"; break;
 #endif
-                       case 30: compressionFunction = local_LZ4F_compressFrame; compressorName = "LZ4F_compressFrame";
+            case 30: compressionFunction = local_LZ4F_compressFrame; compressorName = "LZ4F_compressFrame";
                         chunkP[0].origSize = (int)benchedSize; nbChunks=1;
                         break;
             case 40: compressionFunction = local_LZ4_saveDict; compressorName = "LZ4_saveDict";
@@ -542,6 +544,7 @@ int fullSpeedBench(const char** fileNamesTable, int nbFiles)
             const char* dName;
             int (*decompressionFunction)(const char*, char*, int, int);
             double bestTime = 100000000.;
+            int checkResult = 1;
 
             if ((g_decompressionAlgo != ALL_DECOMPRESSORS) && (g_decompressionAlgo != dAlgNb)) continue;
 
@@ -553,11 +556,11 @@ int fullSpeedBench(const char** fileNamesTable, int nbFiles)
             case 3: decompressionFunction = local_LZ4_decompress_fast_usingExtDict; dName = "LZ4_decompress_fast_using(Ext)Dict"; break;
             case 4: decompressionFunction = LZ4_decompress_safe; dName = "LZ4_decompress_safe"; break;
             case 6: decompressionFunction = local_LZ4_decompress_safe_usingDict; dName = "LZ4_decompress_safe_usingDict"; break;
-            case 7: decompressionFunction = local_LZ4_decompress_safe_partial; dName = "LZ4_decompress_safe_partial"; break;
+            case 7: decompressionFunction = local_LZ4_decompress_safe_partial; dName = "LZ4_decompress_safe_partial"; checkResult = 0; break;
 #ifndef LZ4_DLL_IMPORT
-                       case 8: decompressionFunction = local_LZ4_decompress_safe_forceExtDict; dName = "LZ4_decompress_safe_forceExtDict"; break;
+            case 8: decompressionFunction = local_LZ4_decompress_safe_forceExtDict; dName = "LZ4_decompress_safe_forceExtDict"; break;
 #endif
-                       case 9: decompressionFunction = local_LZ4F_decompress; dName = "LZ4F_decompress";
+            case 9: decompressionFunction = local_LZ4F_decompress; dName = "LZ4F_decompress";
                     errorCode = LZ4F_compressFrame(compressed_buff, compressedBuffSize, orig_buff, benchedSize, NULL);
                     if (LZ4F_isError(errorCode)) {
                         DISPLAY("Error while preparing compressed frame\n");
@@ -589,9 +592,13 @@ int fullSpeedBench(const char** fileNamesTable, int nbFiles)
                 clockTime = clock();
                 while(BMK_GetClockSpan(clockTime) < TIMELOOP) {
                     for (chunkNb=0; chunkNb<nbChunks; chunkNb++) {
-                        int decodedSize = decompressionFunction(chunkP[chunkNb].compressedBuffer, chunkP[chunkNb].origBuffer, chunkP[chunkNb].compressedSize, chunkP[chunkNb].origSize);
-                        if (chunkP[chunkNb].origSize != decodedSize) DISPLAY("ERROR ! %s() == %i != %i !! \n", dName, decodedSize, chunkP[chunkNb].origSize), exit(1);
-                    }
+                        int const decodedSize = decompressionFunction(chunkP[chunkNb].compressedBuffer, chunkP[chunkNb].origBuffer,
+                                                                      chunkP[chunkNb].compressedSize, chunkP[chunkNb].origSize);
+                        if (chunkP[chunkNb].origSize != decodedSize) {
+                            DISPLAY("ERROR ! %s() == %i != %i !! \n",
+                                    dName, decodedSize, chunkP[chunkNb].origSize);
+                            exit(1);
+                    }   }
                     nb_loops++;
                 }
                 clockTime = BMK_GetClockSpan(clockTime);
@@ -604,8 +611,11 @@ int fullSpeedBench(const char** fileNamesTable, int nbFiles)
 
                 /* CRC Checking */
                 crcDecoded = XXH32(orig_buff, (int)benchedSize, 0);
-                if (crcOriginal!=crcDecoded) { DISPLAY("\n!!! WARNING !!! %14s : Invalid Checksum : %x != %x\n", inFileName, (unsigned)crcOriginal, (unsigned)crcDecoded); exit(1); }
-            }
+                if (checkResult && (crcOriginal!=crcDecoded)) {
+                    DISPLAY("\n!!! WARNING !!! %14s : Invalid Checksum : %x != %x\n",
+                            inFileName, (unsigned)crcOriginal, (unsigned)crcDecoded);
+                    exit(1);
+            }   }
 
             DISPLAY("%2i-%-34.34s :%10i -> %7.1f MB/s\n", dAlgNb, dName, (int)benchedSize, (double)benchedSize / bestTime / 1000000);
         }

diff --git a/tests/fuzzer.c b/tests/fuzzer.c
index 6c79515..bdb7841 100644 (file)
--- a/tests/fuzzer.c
+++ b/tests/fuzzer.c
@@ -504,7 +504,7 @@ static int FUZ_test(U32 seed, U32 nbCycles, const U32 startCycle, const double c
 
         /* Test decoding with empty input */
         FUZ_DISPLAYTEST("LZ4_decompress_safe() with empty input");
-        LZ4_decompress_safe(NULL, decodedBuffer, 0, blockSize);
+        LZ4_decompress_safe(compressedBuffer, decodedBuffer, 0, blockSize);
 
         /* Test decoding with a one byte input */
         FUZ_DISPLAYTEST("LZ4_decompress_safe() with one byte input");
@@ -545,7 +545,6 @@ static int FUZ_test(U32 seed, U32 nbCycles, const U32 startCycle, const double c
         ret = LZ4_decompress_safe(compressedBuffer, decodedBuffer, compressedSize, blockSize+1);
         FUZ_CHECKTEST(ret<0, "LZ4_decompress_safe failed despite amply sufficient space");
         FUZ_CHECKTEST(ret!=blockSize, "LZ4_decompress_safe did not regenerate original data");
-        //FUZ_CHECKTEST(decodedBuffer[blockSize], "LZ4_decompress_safe wrote more than (unknown) target size");   // well, is that an issue ?
         FUZ_CHECKTEST(decodedBuffer[blockSize+1], "LZ4_decompress_safe overrun specified output buffer size");
         {   U32 const crcCheck = XXH32(decodedBuffer, blockSize, 0);
             FUZ_CHECKTEST(crcCheck!=crcOrig, "LZ4_decompress_safe corrupted decoded data");
@@ -579,15 +578,16 @@ static int FUZ_test(U32 seed, U32 nbCycles, const U32 startCycle, const double c
         FUZ_CHECKTEST(ret>=0, "LZ4_decompress_safe should have failed, due to input size being too large");
         FUZ_CHECKTEST(decodedBuffer[blockSize], "LZ4_decompress_safe overrun specified output buffer size");
 
-        // Test partial decoding with target output size being max/2 => must work
-        FUZ_DISPLAYTEST();
-        ret = LZ4_decompress_safe_partial(compressedBuffer, decodedBuffer, compressedSize, blockSize/2, blockSize);
-        FUZ_CHECKTEST(ret<0, "LZ4_decompress_safe_partial failed despite sufficient space");
-
-        // Test partial decoding with target output size being just below max => must work
-        FUZ_DISPLAYTEST();
-        ret = LZ4_decompress_safe_partial(compressedBuffer, decodedBuffer, compressedSize, blockSize-3, blockSize);
-        FUZ_CHECKTEST(ret<0, "LZ4_decompress_safe_partial failed despite sufficient space");
+        /* Test partial decoding => must work */
+        FUZ_DISPLAYTEST("test LZ4_decompress_safe_partial");
+        {   size_t const missingBytes = FUZ_rand(&randState) % blockSize;
+            int const targetSize = (int)(blockSize - missingBytes);
+            char const sentinel = compressedBuffer[targetSize] = block[targetSize] ^ 0x5A;
+            int const decResult = LZ4_decompress_safe_partial(compressedBuffer, decodedBuffer, compressedSize, targetSize, blockSize);
+            FUZ_CHECKTEST(decResult<0, "LZ4_decompress_safe_partial failed despite valid input data");
+            FUZ_CHECKTEST(decResult != targetSize, "LZ4_decompress_safe_partial did not regenerated required amount of data (%i < %i <= %i)", decResult, targetSize, blockSize);
+            FUZ_CHECKTEST(compressedBuffer[targetSize] != sentinel, "LZ4_decompress_safe_partial overwrite beyond requested size (though %i <= %i <= %i)", decResult, targetSize, blockSize);
+        }
 
         /* Test Compression with limited output size */