/*
 * $Id: peer_userhash.c,v 1.2 2006/05/16 04:52:26 hno Exp $
 *
 * DEBUG: section 44    Peer user hash based selection
 * AUTHOR: Henrik Nordstrom
 * BASED ON: carp.c by Eric Stern
 *
 * SQUID Web Proxy Cache          http://www.squid-cache.org/
 * ----------------------------------------------------------
 *
 *  Squid is the result of efforts by numerous individuals from
 *  the Internet community; see the CONTRIBUTORS file for full
 *  details.   Many organizations have provided support for Squid's
 *  development; see the SPONSORS file for full details.  Squid is
 *  Copyrighted (C) 2001 by the Regents of the University of
 *  California; see the COPYRIGHT file for full details.  Squid
 *  incorporates software developed and/or copyrighted by other
 *  sources; see the CREDITS file for full details.
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *  
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *  
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA.
 *
 */

#include "squid.h"

#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> ((32-(n)))))

static int n_userhash_peers = 0;
static peer **userhash_peers = NULL;

static int
peerSortWeight(const void *a, const void *b)
{
    const peer *const *p1 = a, *const *p2 = b;
    return (*p1)->weight - (*p2)->weight;
}

void
peerUserHashInit(void)
{
    int W = 0;
    int K;
    int k;
    double P_last, X_last, Xn;
    peer *p;
    peer **P;
    char *t;
    /* Clean up */
    for (k = 0; k < n_userhash_peers; k++) {
	cbdataUnlock(userhash_peers[k]);
    }
    safe_free(userhash_peers);
    n_userhash_peers = 0;
    /* find out which peers we have */
    for (p = Config.peers; p; p = p->next) {
	if (!p->options.userhash)
	    continue;
	assert(p->type == PEER_PARENT);
	if (p->weight == 0)
	    continue;
	n_userhash_peers++;
	W += p->weight;
    }
    if (n_userhash_peers == 0)
	return;
    userhash_peers = xcalloc(n_userhash_peers, sizeof(*userhash_peers));
    /* Build a list of the found peers and calculate hashes and load factors */
    for (P = userhash_peers, p = Config.peers; p; p = p->next) {
	if (!p->options.userhash)
	    continue;
	if (p->weight == 0)
	    continue;
	/* calculate this peers hash */
	p->userhash.hash = 0;
	for (t = p->host; *t != 0; t++)
	    p->userhash.hash += ROTATE_LEFT(p->userhash.hash, 19) + (unsigned int) *t;
	p->userhash.hash += p->userhash.hash * 0x62531965;
	p->userhash.hash = ROTATE_LEFT(p->userhash.hash, 21);
	/* and load factor */
	p->userhash.load_factor = ((double) p->weight) / (double) W;
	if (floor(p->userhash.load_factor * 1000.0) == 0.0)
	    p->userhash.load_factor = 0.0;
	/* add it to our list of peers */
	*P++ = p;
	cbdataLock(p);
    }
    /* Sort our list on weight */
    qsort(userhash_peers, n_userhash_peers, sizeof(*userhash_peers), peerSortWeight);
    /* Calculate the load factor multipliers X_k
     *
     * X_1 = pow ((K*p_1), (1/K))
     * X_k = ([K-k+1] * [P_k - P_{k-1}])/(X_1 * X_2 * ... * X_{k-1})
     * X_k += pow ((X_{k-1}, {K-k+1})
     * X_k = pow (X_k, {1/(K-k+1)})
     * simplified to have X_1 part of the loop
     */
    K = n_userhash_peers;
    P_last = 0.0;		/* Empty P_0 */
    Xn = 1.0;			/* Empty starting point of X_1 * X_2 * ... * X_{x-1} */
    X_last = 0.0;		/* Empty X_0, nullifies the first pow statement */
    for (k = 1; k <= K; k++) {
	double Kk1 = (double) (K - k + 1);
	p = userhash_peers[k - 1];
	p->userhash.load_multiplier = (Kk1 * (p->userhash.load_factor - P_last)) / Xn;
	p->userhash.load_multiplier += pow(X_last, Kk1);
	p->userhash.load_multiplier = pow(p->userhash.load_multiplier, 1.0 / Kk1);
	Xn *= p->userhash.load_multiplier;
	X_last = p->userhash.load_multiplier;
	P_last = p->userhash.load_factor;
    }
}

peer *
peerUserHashSelectParent(request_t * request)
{
    int k;
    const char *c;
    peer *p = NULL;
    peer *tp;
    unsigned int user_hash = 0;
    unsigned int combined_hash;
    double score;
    double high_score = 0;
    char *key = NULL;

    if (request->auth_user_request)
	key = authenticateUserRequestUsername(request->auth_user_request);
    if (!key)
	return NULL;

    /* calculate hash key */
    debug(39, 2) ("peerUserHashSelectParent: Calculating hash for %s\n", key);
    for (c = key; *c != 0; c++)
	user_hash += ROTATE_LEFT(user_hash, 19) + *c;
    /* select peer */
    for (k = 0; k < n_userhash_peers; k++) {
	tp = userhash_peers[k];
	combined_hash = (user_hash ^ tp->userhash.hash);
	combined_hash += combined_hash * 0x62531965;
	combined_hash = ROTATE_LEFT(combined_hash, 21);
	score = combined_hash * tp->userhash.load_multiplier;
	debug(39, 3) ("peerUserHashSelectParent: %s combined_hash %u score %.0f\n",
	    tp->host, combined_hash, score);
	if ((score > high_score) && peerHTTPOkay(tp, request)) {
	    p = tp;
	    high_score = score;
	}
    }
    if (p)
	debug(39, 2) ("peerUserHashSelectParent: selected %s\n", p->host);
    return p;
}