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Merged
pull merged 2 commits into from
Apr 7, 2026
Merged

[pull] master from ladislav-zezula:master #81

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d76f421
Fixed undefined behavior in THuffmannTree
ladislav-zezula Apr 6, 2026
df675be
Merge pull request #425 from ladislav-zezula/LZ_UndefinedBehaviorFix
ladislav-zezula Apr 6, 2026
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16 changes: 8 additions & 8 deletions src/huffman/huff.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -424,7 +424,7 @@ void THTreeItem::RemoveItem()

THuffmannTree::THuffmannTree(bool bCompression)
{
pFirst = pLast = LIST_HEAD();
ListHead.pNext = ListHead.pPrev = LIST_HEAD();
MinValidValue = 1;
ItemsUsed = 0;
bIsCmp0 = 0;
Expand Down Expand Up @@ -525,7 +525,7 @@ unsigned int THuffmannTree::FixupItemPosByWeight(THTreeItem * pNewItem, unsigned
if(pNewItem->Weight < MaxWeight)
{
// Find an item that has higher weight than this one
pHigherItem = FindHigherOrEqualItem(pLast, pNewItem->Weight);
pHigherItem = FindHigherOrEqualItem(ListHead.pPrev, pNewItem->Weight);

// Remove the item and put it to the new position
pNewItem->RemoveItem();
Expand Down Expand Up @@ -578,7 +578,7 @@ bool THuffmannTree::BuildTree(unsigned int CompressionType)

// Now we need to build the tree. We start at the last entry
// and go backwards to the first one
pChildLo = pLast;
pChildLo = ListHead.pPrev;

// Work as long as both children are valid
// pChildHi is child with higher weight, pChildLo is the one with lower weight
Expand Down Expand Up @@ -659,7 +659,7 @@ void THuffmannTree::IncWeightsAndRebalance(THTreeItem * pItem)

bool THuffmannTree::InsertNewBranchAndRebalance(unsigned int Value1, unsigned int Value2)
{
THTreeItem * pLastItem = pLast;
THTreeItem * pLastItem = ListHead.pPrev;
THTreeItem * pChildHi;
THTreeItem * pChildLo;

Expand Down Expand Up @@ -738,11 +738,11 @@ unsigned int THuffmannTree::DecodeOneByte(TInputStream * is)
else
{
// Just a sanity check
if(pFirst == LIST_HEAD())
if(ListHead.pNext == LIST_HEAD())
return HUFF_DECOMPRESS_ERROR;

// We don't have the quick-link item, we need to parse the tree from its root
pItem = pFirst;
pItem = ListHead.pNext;
}

// Step down the tree until we find a terminal item
Expand Down Expand Up @@ -827,7 +827,7 @@ unsigned int THuffmannTree::Compress(TOutputStream * os, void * pvInBuffer, int
// Store the loaded byte into output stream
os->PutBits(InputByte, 8);

if(!InsertNewBranchAndRebalance(pLast->DecompressedValue, InputByte))
if(!InsertNewBranchAndRebalance(ListHead.pPrev->DecompressedValue, InputByte))
return 0;

if(bIsCmp0)
Expand Down Expand Up @@ -892,7 +892,7 @@ unsigned int THuffmannTree::Decompress(void * pvOutBuffer, unsigned int cbOutLen
if(!is->Get8Bits(DecompressedValue))
return 0;

if(!InsertNewBranchAndRebalance(pLast->DecompressedValue, DecompressedValue))
if(!InsertNewBranchAndRebalance(ListHead.pPrev->DecompressedValue, DecompressedValue))
return 0;

if(bIsCmp0 == 0)
Expand Down
11 changes: 6 additions & 5 deletions src/huffman/huff.h
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Expand Up @@ -55,8 +55,9 @@ class TOutputStream
unsigned int BitCount; // Number of bits in the bit buffer
};

// A virtual tree item that represents the head of the item list
#define LIST_HEAD() ((THTreeItem *)(&pFirst))
// The list head sentinel - an actual THTreeItem embedded in THuffmannTree.
// Its pNext serves as "pFirst" (highest weight) and pPrev as "pLast" (lowest weight).
#define LIST_HEAD() (&ListHead)

enum TInsertPoint
{
Expand Down Expand Up @@ -129,9 +130,9 @@ class THuffmannTree
THTreeItem ItemBuffer[HUFF_ITEM_COUNT]; // Buffer for tree items. No memory allocation is needed
unsigned int ItemsUsed; // Number of tree items used from ItemBuffer

// Head of the linear item list
THTreeItem * pFirst; // Pointer to the highest weight item
THTreeItem * pLast; // Pointer to the lowest weight item
// Sentinel node for the linear item list (doubly-linked, circular).
// ListHead.pNext = highest weight item, ListHead.pPrev = lowest weight item
THTreeItem ListHead;

THTreeItem * ItemsByByte[0x102]; // Array of item pointers, one for each possible byte value
TQuickLink QuickLinks[LINK_ITEM_COUNT]; // Array of quick-link items
Expand Down
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