protected void jpeg_gen_optimal_table(JHUFF_TBL htbl, long[] freq)
{
const int MAX_CLEN = 32; /* assumed maximum initial code length */
byte[] bits = new byte[MAX_CLEN + 1]; /* bits[k] = # of symbols with code length k */
int[] codesize = new int[257]; /* codesize[k] = code length of symbol k */
int[] others = new int[257]; /* next symbol in current branch of tree */
int c1, c2;
int p, i, j;
long v;
/* This algorithm is explained in section K.2 of the JPEG standard */
for (i = 0; i < 257; i++)
others[i] = -1; /* init links to empty */
freq[256] = 1; /* make sure 256 has a nonzero count */
/* Including the pseudo-symbol 256 in the Huffman procedure guarantees
* that no real symbol is given code-value of all ones, because 256
* will be placed last in the largest codeword category.
*/
/* Huffman's basic algorithm to assign optimal code lengths to symbols */
for (;;)
{
/* Find the smallest nonzero frequency, set c1 = its symbol */
/* In case of ties, take the larger symbol number */
c1 = -1;
v = 1000000000L;
for (i = 0; i <= 256; i++)
{
if (freq[i] != 0 && freq[i] <= v)
{
v = freq[i];
c1 = i;
}
}
/* Find the next smallest nonzero frequency, set c2 = its symbol */
/* In case of ties, take the larger symbol number */
c2 = -1;
v = 1000000000L;
for (i = 0; i <= 256; i++)
{
if (freq[i] != 0 && freq[i] <= v && i != c1)
{
v = freq[i];
c2 = i;
}
}
/* Done if we've merged everything into one frequency */
if (c2 < 0)
break;
/* Else merge the two counts/trees */
freq[c1] += freq[c2];
freq[c2] = 0;
/* Increment the codesize of everything in c1's tree branch */
codesize[c1]++;
while (others[c1] >= 0)
{
c1 = others[c1];
codesize[c1]++;
}
others[c1] = c2; /* chain c2 onto c1's tree branch */
/* Increment the codesize of everything in c2's tree branch */
codesize[c2]++;
while (others[c2] >= 0)
{
c2 = others[c2];
codesize[c2]++;
}
}
/* Now count the number of symbols of each code length */
for (i = 0; i <= 256; i++)
{
if (codesize[i] != 0)
{
/* The JPEG standard seems to think that this can't happen, */
/* but I'm paranoid... */
if (codesize[i] > MAX_CLEN)
m_cinfo.ERREXIT(J_MESSAGE_CODE.JERR_HUFF_CLEN_OVERFLOW);
bits[codesize[i]]++;
}
}
/* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
* Huffman procedure assigned any such lengths, we must adjust the coding.
* Here is what the JPEG spec says about how this next bit works:
* Since symbols are paired for the longest Huffman code, the symbols are
* removed from this length category two at a time. The prefix for the pair
* (which is one bit shorter) is allocated to one of the pair; then,
* skipping the BITS entry for that prefix length, a code word from the next
* shortest nonzero BITS entry is converted into a prefix for two code words
* one bit longer.
*/
for (i = MAX_CLEN; i > 16; i--)
{
while (bits[i] > 0)
{
j = i - 2; /* find length of new prefix to be used */
while (bits[j] == 0)
j--;
bits[i] -= 2; /* remove two symbols */
bits[i - 1]++; /* one goes in this length */
bits[j + 1] += 2; /* two new symbols in this length */
bits[j]--; /* symbol of this length is now a prefix */
}
}
/* Remove the count for the pseudo-symbol 256 from the largest codelength */
while (bits[i] == 0) /* find largest codelength still in use */
i--;
bits[i]--;
/* Return final symbol counts (only for lengths 0..16) */
Buffer.BlockCopy(bits, 0, htbl.Bits, 0, htbl.Bits.Length);
/* Return a list of the symbols sorted by code length */
/* It's not real clear to me why we don't need to consider the codelength
* changes made above, but the JPEG spec seems to think this works.
*/
p = 0;
for (i = 1; i <= MAX_CLEN; i++)
{
for (j = 0; j <= 255; j++)
{
if (codesize[j] == i)
{
htbl.Huffval[p] = (byte)j;
p++;
}
}
}
/* Set sent_table false so updated table will be written to JPEG file. */
htbl.Sent_table = false;
}
}
