// Construct one Huffman tree and assigns the code bit strings and lengths.
// Update the total bit length for the current block.
// IN assertion: the field freq is set for all tree elements.
// OUT assertions: the fields len and code are set to the optimal bit length
// and corresponding code. The length opt_len is updated; static_len is
// also updated if stree is not null. The field max_code is set.
internal void Build_tree(Deflate s)
{
using (MemoryHandle treeHandle = this.DynTree.AsMemory().Pin())
using (MemoryHandle streeHandle = this.StatDesc.StaticTreeValue.AsMemory().Pin())
{
ushort *tree = (ushort *)treeHandle.Pointer;
ushort *stree = (ushort *)streeHandle.Pointer;
int elems = this.StatDesc.Elems;
int n, m; // iterate over heap elements
var max_code = -1; // largest code with non zero frequency
int node; // new node being created
ushort *blCount = s.BlCountPointer;
int * heap = s.HeapPointer;
byte * depth = s.DepthPointer;
// Construct the initial heap, with least frequent element in
// heap[1]. The sons of heap[n] are heap[2*n] and heap[2*n+1].
// heap[0] is not used.
s.HeapLen = 0;
s.HeapMax = HEAPSIZE;
for (n = 0; n < elems; n++)
{
if (tree[n * 2] != 0)
{
heap[++s.HeapLen] = max_code = n;
depth[n] = 0;
}
else
{
tree[(n * 2) + 1] = 0;
}
}
// The pkzip format requires that at least one distance code exists,
// and that at least one bit should be sent even if there is only one
// possible code. So to avoid special checks later on we force at least
// two codes of non zero frequency.
while (s.HeapLen < 2)
{
node = heap[++s.HeapLen] = max_code < 2 ? ++max_code : 0;
tree[node * 2] = 1;
depth[node] = 0;
s.OptLen--;
if (stree != null)
{
s.StaticLen -= stree[(node * 2) + 1];
}
// node is 0 or 1 so it does not have extra bits
}
this.MaxCode = max_code;
// The elements heap[heap_len/2+1 .. heap_len] are leaves of the tree,
// establish sub-heaps of increasing lengths:
for (n = s.HeapLen / 2; n >= 1; n--)
{
s.Pqdownheap(tree, n);
}
// Construct the Huffman tree by repeatedly combining the least two
// frequent nodes.
node = elems; // next internal node of the tree
do
{
// n = node of least frequency
n = heap[1];
heap[1] = heap[s.HeapLen--];
s.Pqdownheap(tree, 1);
m = heap[1]; // m = node of next least frequency
heap[--s.HeapMax] = n; // keep the nodes sorted by frequency
heap[--s.HeapMax] = m;
// Create a new node father of n and m
tree[node * 2] = (ushort)(tree[n * 2] + tree[m * 2]);
depth[node] = (byte)(Math.Max(depth[n], depth[m]) + 1);
tree[(n * 2) + 1] = tree[(m * 2) + 1] = (ushort)node;
// and insert the new node in the heap
heap[1] = node++;
s.Pqdownheap(tree, 1);
}while (s.HeapLen >= 2);
heap[--s.HeapMax] = heap[1];
// At this point, the fields freq and dad are set. We can now
// generate the bit lengths.
this.Gen_bitlen(s);
// The field len is now set, we can generate the bit codes
Gen_codes(tree, max_code, blCount);
}
}
/// <summary>
/// Scan a literal or distance tree to determine the frequencies of the codes
/// in the bit length tree.
/// </summary>
/// <param name="s">The data compressor.</param>
/// <param name="tree">The tree to be scanned.</param>
/// <param name="max_code">And its largest code of non zero frequency</param>
public static void Scan_tree(Deflate s, DynamicTreeDesc tree, int max_code)
{
int n; // iterates over all tree elements
int prevlen = -1; // last emitted length
int curlen; // length of current code
int nextlen = tree[0].Len; // length of next code
ushort count = 0; // repeat count of the current code
int max_count = 7; // max repeat count
int min_count = 4; // min repeat count
DynamicTreeDesc blTree = s.DynBLTree;
if (nextlen == 0)
{
max_count = 138;
min_count = 3;
}
tree[max_code + 1].Len = ushort.MaxValue; // guard
for (n = 0; n <= max_code; n++)
{
curlen = nextlen;
nextlen = tree[n + 1].Len;
if (++count < max_count && curlen == nextlen)
{
continue;
}
else if (count < min_count)
{
blTree[curlen].Freq += count;
}
else if (curlen != 0)
{
if (curlen != prevlen)
{
blTree[curlen].Freq++;
}
blTree[REP36].Freq++;
}
else if (count <= 10)
{
blTree[REPZ310].Freq++;
}
else
{
blTree[REPZ11138].Freq++;
}
count = 0;
prevlen = curlen;
if (nextlen == 0)
{
max_count = 138;
min_count = 3;
}
else if (curlen == nextlen)
{
max_count = 6;
min_count = 3;
}
else
{
max_count = 7;
min_count = 4;
}
}
}
// Compute the optimal bit lengths for a tree and update the total bit length
// for the current block.
// IN assertion: the fields freq and dad are set, heap[heap_max] and
// above are the tree nodes sorted by increasing frequency.
// OUT assertions: the field len is set to the optimal bit length, the
// array bl_count contains the frequencies for each bit length.
// The length opt_len is updated; static_len is also updated if stree is
// not null.
private void Gen_bitlen(Deflate s)
{
using (MemoryHandle treeHandle = this.DynTree.AsMemory().Pin())
using (MemoryHandle streeHandle = this.StatDesc.StaticTreeValue.AsMemory().Pin())
using (MemoryHandle extraHandle = this.StatDesc.ExtraBits.AsMemory().Pin())
{
ushort *tree = (ushort *)treeHandle.Pointer;
ushort *stree = (ushort *)streeHandle.Pointer;
int * extra = (int *)extraHandle.Pointer;
int base_Renamed = this.StatDesc.ExtraBase;
int max_length = this.StatDesc.MaxLength;
int h; // heap index
int n, m; // iterate over the tree elements
int bits; // bit length
int xbits; // extra bits
ushort f; // frequency
int overflow = 0; // number of elements with bit length too large
ushort *blCount = s.BlCountPointer;
int * heap = s.HeapPointer;
for (bits = 0; bits <= MAXBITS; bits++)
{
blCount[bits] = 0;
}
// In a first pass, compute the optimal bit lengths (which may
// overflow in the case of the bit length tree).
tree[(heap[s.HeapMax] * 2) + 1] = 0; // root of the heap
for (h = s.HeapMax + 1; h < HEAPSIZE; h++)
{
n = heap[h];
bits = tree[(tree[(n * 2) + 1] * 2) + 1] + 1;
if (bits > max_length)
{
bits = max_length;
overflow++;
}
tree[(n * 2) + 1] = (ushort)bits;
// We overwrite tree[n*2+1] which is no longer needed
if (n > this.MaxCode)
{
continue; // not a leaf node
}
blCount[bits]++;
xbits = 0;
if (n >= base_Renamed)
{
xbits = extra[n - base_Renamed];
}
f = tree[n * 2];
s.OptLen += f * (bits + xbits);
if (stree != null)
{
s.StaticLen += f * (stree[(n * 2) + 1] + xbits);
}
}
if (overflow == 0)
{
return;
}
// This happens for example on obj2 and pic of the Calgary corpus
// Find the first bit length which could increase:
do
{
bits = max_length - 1;
while (blCount[bits] == 0)
{
bits--;
}
blCount[bits]--; // move one leaf down the tree
blCount[bits + 1] = (ushort)(blCount[bits + 1] + 2); // move one overflow item as its brother
blCount[max_length]--;
// The brother of the overflow item also moves one step up,
// but this does not affect bl_count[max_length]
overflow -= 2;
}while (overflow > 0);
for (bits = max_length; bits != 0; bits--)
{
n = blCount[bits];
while (n != 0)
{
m = heap[--h];
if (m > this.MaxCode)
{
continue;
}
if (tree[(m * 2) + 1] != bits)
{
s.OptLen = (int)(s.OptLen + ((bits - (long)tree[(m * 2) + 1]) * tree[m * 2]));
tree[(m * 2) + 1] = (ushort)bits;
}
n--;
}
}
}
}