// 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(DeflateManager s)
{
short[] tree = dyn_tree;
short[] stree = staticTree.treeCodes;
int elems = staticTree.elems;
int n, m; // iterate over heap elements
int max_code = -1; // largest code with non zero frequency
int node; // new node being created
// 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.heap_len = 0;
s.heap_max = HEAP_SIZE;
for (n = 0; n < elems; n++)
{
if (tree[n * 2] != 0)
{
s.heap[++s.heap_len] = max_code = n;
s.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.heap_len < 2)
{
node = s.heap[++s.heap_len] = (max_code < 2?++max_code:0);
tree[node * 2] = 1;
s.depth[node] = 0;
s.opt_len--;
if (stree != null)
s.static_len -= stree[node * 2 + 1];
// node is 0 or 1 so it does not have extra bits
}
this.max_code = 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.heap_len / 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 = s.heap[1];
s.heap[1] = s.heap[s.heap_len--];
s.pqdownheap(tree, 1);
m = s.heap[1]; // m = node of next least frequency
s.heap[--s.heap_max] = n; // keep the nodes sorted by frequency
s.heap[--s.heap_max] = m;
// Create a new node father of n and m
tree[node * 2] = unchecked((short) (tree[n * 2] + tree[m * 2]));
s.depth[node] = (sbyte) (System.Math.Max((byte) s.depth[n], (byte) s.depth[m]) + 1);
tree[n * 2 + 1] = tree[m * 2 + 1] = (short) node;
// and insert the new node in the heap
s.heap[1] = node++;
s.pqdownheap(tree, 1);
}
while (s.heap_len >= 2);
s.heap[--s.heap_max] = s.heap[1];
// At this point, the fields freq and dad are set. We can now
// generate the bit lengths.
gen_bitlen(s);
// The field len is now set, we can generate the bit codes
gen_codes(tree, max_code, s.bl_count);
}
