OpenSolaris_b135/cmd/spell/huff.c
/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License, Version 1.0 only
* (the "License"). You may not use this file except in compliance
* with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2005 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/* Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T */
/* All Rights Reserved */
#pragma ident "%Z%%M% %I% %E% SMI"
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#define BYTE 8
#define QW 1 /* width of bas-q digit in bits */
/*
* this stuff should be local and hidden; it was made
* accessible outside for dirty reasons: 20% faster spell
*/
#include "huff.h"
struct huff huffcode;
/*
* Infinite Huffman code
*
* Let the messages be exponentially distributed with ratio r:
* P {message k} = r^k*(1-r), k = 0, 1, ...
* Let the messages be coded in base q, and suppose
* r^n = 1/q
* If each decade(base q) contains n codes, then
* the messages assigned to each decade will be q times
* as probable as the next. Moreover the code for the tail of
* the distribution after truncating one decade should look
* just like the original, but longer by one leading digit q-1.
* q(z+n) = z + (q-1)q^w
* where z is first code of decade, w is width of code, in shortest
* full decade. Examples, base 2:
* r^1 = 1/2 r^5 = 1/2
* 0 0110
* 10 0111
* 110 1000
* 1110 1001
* ... 1010
* 10110
* w = 1, z = 0 w = 4, z = 0110
* Rewriting slightly
* (q-1)z + q*n = (q-1)q^w
* whence z is a multiple of q and n is a multiple of q-1. Let
* z = cq, n = d(q-1)
* We pick w to be the least integer such that
* d = n/(q-1) <= q^(w-1)
* Then solve for c
* c = q^(w-1) - d
* If c is not zero, the first decade may be preceded by
* even shorter (w-1)-digit codes 0, 1, ..., c-1. Thus
* the example code with r^5 = 1/2 becomes
* 000
* 001
* 010
* 0110
* 0111
* 1000
* 1001
* 1010
* 10110
* ...
* 110110
* ...
* The expected number of base-q digits in a codeword is then
* w - 1 + r^c/(1-r^n)
* The present routines require q to be a power of 2
*/
/*
* There is a lot of hanky-panky with left justification against
* sign instead of simple left justification because
* unsigned long is not available
*/
#define L (BYTE*(sizeof (long))-1) /* length of signless long */
#define MASK (~((unsigned long)1<<L)) /* mask out sign */
/*
* decode the prefix of word y (which is left justified against sign)
* place mesage number into place pointed to by kp
* return length (in bits) of decoded prefix or 0 if code is out of
* range
*/
int
decode(long y, long *pk)
{
int l;
long v;
if (y < cs) {
*pk = y >> (long)(L+QW-w);
return (w-QW);
}
for (l = w, v = v0; y >= qcs;
y = ((unsigned long)y << QW) & MASK, v += n)
if ((l += QW) > L)
return (0);
*pk = v + (y>>(long)(L-w));
return (l);
}
/*
* encode message k and put result (right justified) into
* place pointed to by py.
* return length (in bits) of result,
* or 0 if code is too long
*/
int
encode(long k, long *py)
{
int l;
long y;
if (k < c) {
*py = k;
return (w-QW);
}
for (k -= c, y = 1, l = w; k >= n; k -= n, y <<= QW)
if ((l += QW) > L)
return (0);
*py = ((y-1)<<w) + cq + k;
return (l);
}
/*
* Initialization code, given expected value of k
* E(k) = r/(1-r) = a
* and given base width b
* return expected length of coded messages
*/
static struct qlog {
long p;
double u;
} z;
static struct qlog
qlog(double x, double y, long p, double u) /* find smallest p so x^p<=y */
{
if (u/x <= y) {
z.p = 0;
z.u = 1;
} else {
z = qlog(x, y, p+p, u*u);
if (u*z.u/x > y) {
z.p += p;
z.u *= u;
}
}
return (z);
}
double
huff(float a)
{
int i, q;
long d, j;
double r = a/(1.0 + a);
double rc, rq;
for (i = 0, q = 1, rq = r; i < QW; i++, q *= 2, rq *= rq)
continue;
rq /= r; /* rq = r^(q-1) */
(void) qlog(rq, 1./q, 1L, rq);
d = z.p;
n = d*(q-1);
if (n != d * (q - 1))
abort(); /* time to make n long */
for (w = QW, j = 1; j < d; w += QW, j *= q)
continue;
c = j - d;
cq = c*q;
cs = cq<<(L-w);
qcs = (((long)(q-1)<<w) + cq) << (L-QW-w);
v0 = c - cq;
for (i = 0, rc = 1; i < c; i++, rc *= r) /* rc = r^c */
continue;
return (w + QW*(rc/(1-z.u) - 1));
}
void
whuff(void)
{
(void) fwrite((char *) & huffcode, sizeof (huffcode), 1, stdout);
}
int
rhuff(FILE *f)
{
return (read(fileno(f), (char *)&huffcode, sizeof (huffcode)) ==
sizeof (huffcode));
}