ngram_search_fwdflat.c

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/* -*- c-basic-offset: 4; indent-tabs-mode: nil -*- */
/* ====================================================================
 * Copyright (c) 2008 Carnegie Mellon University.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer. 
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * This work was supported in part by funding from the Defense Advanced 
 * Research Projects Agency and the National Science Foundation of the 
 * United States of America, and the CMU Sphinx Speech Consortium.
 *
 * THIS SOFTWARE IS PROVIDED BY CARNEGIE MELLON UNIVERSITY ``AS IS'' AND 
 * ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, 
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY
 * NOR ITS EMPLOYEES BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 
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 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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 * ====================================================================
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 */

/* System headers. */
#include <string.h>
#include <assert.h>

/* SphinxBase headers. */
#include <sphinxbase/ckd_alloc.h>
#include <sphinxbase/listelem_alloc.h>
#include <sphinxbase/err.h>

/* Local headers. */
#include "ngram_search.h"
#include "ps_lattice_internal.h"

/* Turn this on to dump channels for debugging */
#define __CHAN_DUMP__           0
#if __CHAN_DUMP__
#define chan_v_eval(chan) hmm_dump_vit_eval(&(chan)->hmm, stderr)
#else
#define chan_v_eval(chan) hmm_vit_eval(&(chan)->hmm)
#endif

static void
ngram_fwdflat_expand_all(ngram_search_t *ngs)
{
    int n_words, i;

    /* For all "real words" (not fillers or <s>/</s>) in the dictionary,
     *
     * 1) Add the ones which are in the LM to the fwdflat wordlist
     * 2) And to the expansion list (since we are expanding all)
     */
    ngs->n_expand_words = 0;
    n_words = ps_search_n_words(ngs);
    bitvec_clear_all(ngs->expand_word_flag, ps_search_n_words(ngs));
    for (i = 0; i < n_words; ++i) {
        if (!ngram_model_set_known_wid(ngs->lmset,
                                       dict_basewid(ps_search_dict(ngs),i)))
            continue;
        ngs->fwdflat_wordlist[ngs->n_expand_words] = i;
        ngs->expand_word_list[ngs->n_expand_words] = i;
        bitvec_set(ngs->expand_word_flag, i);
        ngs->n_expand_words++;
    }
    E_INFO("Utterance vocabulary contains %d words\n", ngs->n_expand_words);
    ngs->expand_word_list[ngs->n_expand_words] = -1;
    ngs->fwdflat_wordlist[ngs->n_expand_words] = -1;
}

static void
ngram_fwdflat_allocate_1ph(ngram_search_t *ngs)
{
    dict_t *dict = ps_search_dict(ngs);
    int n_words = ps_search_n_words(ngs);
    int i, w;

    /* Allocate single-phone words, since they won't have
     * been allocated for us by fwdtree initialization. */
    ngs->n_1ph_words = 0;
    for (w = 0; w < n_words; w++) {
        if (dict_is_single_phone(dict, w))
            ++ngs->n_1ph_words;
    }
    ngs->single_phone_wid = ckd_calloc(ngs->n_1ph_words,
                                       sizeof(*ngs->single_phone_wid));
    ngs->rhmm_1ph = ckd_calloc(ngs->n_1ph_words, sizeof(*ngs->rhmm_1ph));
    i = 0;
    for (w = 0; w < n_words; w++) {
        if (!dict_is_single_phone(dict, w))
            continue;

        /* DICT2PID location */
        ngs->rhmm_1ph[i].ciphone = dict_first_phone(dict, w);
        ngs->rhmm_1ph[i].ci2phone = bin_mdef_silphone(ps_search_acmod(ngs)->mdef);
        hmm_init(ngs->hmmctx, &ngs->rhmm_1ph[i].hmm, TRUE,
                 /* ssid */ bin_mdef_pid2ssid(ps_search_acmod(ngs)->mdef,
                                              ngs->rhmm_1ph[i].ciphone),
                 /* tmatid */ bin_mdef_pid2tmatid(ps_search_acmod(ngs)->mdef,
                                                  ngs->rhmm_1ph[i].ciphone));
        ngs->rhmm_1ph[i].next = NULL;
        ngs->word_chan[w] = (chan_t *) &(ngs->rhmm_1ph[i]);
        ngs->single_phone_wid[i] = w;
        i++;
    }
}

static void
ngram_fwdflat_free_1ph(ngram_search_t *ngs)
{
    int i, w;
    int n_words = ps_search_n_words(ngs);

    for (i = w = 0; w < n_words; ++w) {
        if (!dict_is_single_phone(ps_search_dict(ngs), w))
            continue;
        hmm_deinit(&ngs->rhmm_1ph[i].hmm);
        ++i;
    }
    ckd_free(ngs->rhmm_1ph);
    ngs->rhmm_1ph = NULL;
    ckd_free(ngs->single_phone_wid);
}

void
ngram_fwdflat_init(ngram_search_t *ngs)
{
    int n_words;

    n_words = ps_search_n_words(ngs);
    ngs->fwdflat_wordlist = ckd_calloc(n_words + 1, sizeof(*ngs->fwdflat_wordlist));
    ngs->expand_word_flag = bitvec_alloc(n_words);
    ngs->expand_word_list = ckd_calloc(n_words + 1, sizeof(*ngs->expand_word_list));
    ngs->frm_wordlist = ckd_calloc(ngs->n_frame_alloc, sizeof(*ngs->frm_wordlist));
    ngs->min_ef_width = cmd_ln_int32_r(ps_search_config(ngs), "-fwdflatefwid");
    ngs->max_sf_win = cmd_ln_int32_r(ps_search_config(ngs), "-fwdflatsfwin");
    E_INFO("fwdflat: min_ef_width = %d, max_sf_win = %d\n",
           ngs->min_ef_width, ngs->max_sf_win);

    /* No tree-search; pre-build the expansion list, including all LM words. */
    if (!ngs->fwdtree) {
        /* Build full expansion list from LM words. */
        ngram_fwdflat_expand_all(ngs);
        /* Allocate single phone words. */
        ngram_fwdflat_allocate_1ph(ngs);
    }
}

void
ngram_fwdflat_deinit(ngram_search_t *ngs)
{
    double n_speech = (double)ngs->n_tot_frame
            / cmd_ln_int32_r(ps_search_config(ngs), "-frate");

    E_INFO("TOTAL fwdflat %.2f CPU %.3f xRT\n",
           ngs->fwdflat_perf.t_tot_cpu,
           ngs->fwdflat_perf.t_tot_cpu / n_speech);
    E_INFO("TOTAL fwdflat %.2f wall %.3f xRT\n",
           ngs->fwdflat_perf.t_tot_elapsed,
           ngs->fwdflat_perf.t_tot_elapsed / n_speech);

    /* Free single-phone words if we allocated them. */
    if (!ngs->fwdtree) {
        ngram_fwdflat_free_1ph(ngs);
    }
    ckd_free(ngs->fwdflat_wordlist);
    bitvec_free(ngs->expand_word_flag);
    ckd_free(ngs->expand_word_list);
    ckd_free(ngs->frm_wordlist);
}

int
ngram_fwdflat_reinit(ngram_search_t *ngs)
{
    /* Reallocate things that depend on the number of words. */
    int n_words;

    ckd_free(ngs->fwdflat_wordlist);
    ckd_free(ngs->expand_word_list);
    bitvec_free(ngs->expand_word_flag);
    n_words = ps_search_n_words(ngs);
    ngs->fwdflat_wordlist = ckd_calloc(n_words + 1, sizeof(*ngs->fwdflat_wordlist));
    ngs->expand_word_flag = bitvec_alloc(n_words);
    ngs->expand_word_list = ckd_calloc(n_words + 1, sizeof(*ngs->expand_word_list));
    
    /* No tree-search; take care of the expansion list and single phone words. */
    if (!ngs->fwdtree) {
        /* Free single-phone words. */
        ngram_fwdflat_free_1ph(ngs);
        /* Reallocate word_chan. */
        ckd_free(ngs->word_chan);
        ngs->word_chan = ckd_calloc(dict_size(ps_search_dict(ngs)),
                                    sizeof(*ngs->word_chan));
        /* Rebuild full expansion list from LM words. */
        ngram_fwdflat_expand_all(ngs);
        /* Allocate single phone words. */
        ngram_fwdflat_allocate_1ph(ngs);
    }
    /* Otherwise there is nothing to do since the wordlist is
     * generated anew every utterance. */
    return 0;
}

static void
build_fwdflat_wordlist(ngram_search_t *ngs)
{
    int32 i, f, sf, ef, wid, nwd;
    bptbl_t *bp;
    ps_latnode_t *node, *prevnode, *nextnode;

    /* No tree-search, use statically allocated wordlist. */
    if (!ngs->fwdtree)
        return;

    memset(ngs->frm_wordlist, 0, ngs->n_frame_alloc * sizeof(*ngs->frm_wordlist));

    /* Scan the backpointer table for all active words and record
     * their exit frames. */
    for (i = 0, bp = ngs->bp_table; i < ngs->bpidx; i++, bp++) {
        sf = (bp->bp < 0) ? 0 : ngs->bp_table[bp->bp].frame + 1;
        ef = bp->frame;
        wid = bp->wid;

        /* Anything that can be transitioned to in the LM can go in
         * the word list. */
        if (!ngram_model_set_known_wid(ngs->lmset,
                                       dict_basewid(ps_search_dict(ngs), wid)))
            continue;

        /* Look for it in the wordlist. */
        for (node = ngs->frm_wordlist[sf]; node && (node->wid != wid);
             node = node->next);

        /* Update last end frame. */
        if (node)
            node->lef = ef;
        else {
            /* New node; link to head of list */
            node = listelem_malloc(ngs->latnode_alloc);
            node->wid = wid;
            node->fef = node->lef = ef;

            node->next = ngs->frm_wordlist[sf];
            ngs->frm_wordlist[sf] = node;
        }
    }

    /* Eliminate "unlikely" words, for which there are too few end points */
    for (f = 0; f < ngs->n_frame; f++) {
        prevnode = NULL;
        for (node = ngs->frm_wordlist[f]; node; node = nextnode) {
            nextnode = node->next;
            /* Word has too few endpoints */
            if ((node->lef - node->fef < ngs->min_ef_width) ||
                /* Word is </s> and doesn't actually end in last frame */
                ((node->wid == ps_search_finish_wid(ngs)) && (node->lef < ngs->n_frame - 1))) {
                if (!prevnode)
                    ngs->frm_wordlist[f] = nextnode;
                else
                    prevnode->next = nextnode;
                listelem_free(ngs->latnode_alloc, node);
            }
            else
                prevnode = node;
        }
    }

    /* Form overall wordlist for 2nd pass */
    nwd = 0;
    bitvec_clear_all(ngs->word_active, ps_search_n_words(ngs));
    for (f = 0; f < ngs->n_frame; f++) {
        for (node = ngs->frm_wordlist[f]; node; node = node->next) {
            if (!bitvec_is_set(ngs->word_active, node->wid)) {
                bitvec_set(ngs->word_active, node->wid);
                ngs->fwdflat_wordlist[nwd++] = node->wid;
            }
        }
    }
    ngs->fwdflat_wordlist[nwd] = -1;
    E_INFO("Utterance vocabulary contains %d words\n", nwd);
}

static void
build_fwdflat_chan(ngram_search_t *ngs)
{
    int32 i, wid, p;
    root_chan_t *rhmm;
    chan_t *hmm, *prevhmm;
    dict_t *dict;
    dict2pid_t *d2p;

    dict = ps_search_dict(ngs);
    d2p = ps_search_dict2pid(ngs);

    /* Build word HMMs for each word in the lattice. */
    for (i = 0; ngs->fwdflat_wordlist[i] >= 0; i++) {
        wid = ngs->fwdflat_wordlist[i];

        /* Single-phone words are permanently allocated */
        if (dict_is_single_phone(dict, wid))
            continue;

        assert(ngs->word_chan[wid] == NULL);

        /* Multiplex root HMM for first phone (one root per word, flat
         * lexicon).  diphone is irrelevant here, for the time being,
         * at least. */
        rhmm = listelem_malloc(ngs->root_chan_alloc);
        rhmm->ci2phone = dict_second_phone(dict, wid);
        rhmm->ciphone = dict_first_phone(dict, wid);
        rhmm->next = NULL;
        hmm_init(ngs->hmmctx, &rhmm->hmm, TRUE,
                 bin_mdef_pid2ssid(ps_search_acmod(ngs)->mdef, rhmm->ciphone),
                 bin_mdef_pid2tmatid(ps_search_acmod(ngs)->mdef, rhmm->ciphone));

        /* HMMs for word-internal phones */
        prevhmm = NULL;
        for (p = 1; p < dict_pronlen(dict, wid) - 1; p++) {
            hmm = listelem_malloc(ngs->chan_alloc);
            hmm->ciphone = dict_pron(dict, wid, p);
            hmm->info.rc_id = (p == dict_pronlen(dict, wid) - 1) ? 0 : -1;
            hmm->next = NULL;
            hmm_init(ngs->hmmctx, &hmm->hmm, FALSE,
                     dict2pid_internal(d2p,wid,p), 
                     bin_mdef_pid2tmatid(ps_search_acmod(ngs)->mdef, hmm->ciphone));

            if (prevhmm)
                prevhmm->next = hmm;
            else
                rhmm->next = hmm;

            prevhmm = hmm;
        }

        /* Right-context phones */
        ngram_search_alloc_all_rc(ngs, wid);

        /* Link in just allocated right-context phones */
        if (prevhmm)
            prevhmm->next = ngs->word_chan[wid];
        else
            rhmm->next = ngs->word_chan[wid];
        ngs->word_chan[wid] = (chan_t *) rhmm;
    }

}

void
ngram_fwdflat_start(ngram_search_t *ngs)
{
    root_chan_t *rhmm;
    int i;

    ptmr_reset(&ngs->fwdflat_perf);
    ptmr_start(&ngs->fwdflat_perf);
    build_fwdflat_wordlist(ngs);
    build_fwdflat_chan(ngs);

    ngs->bpidx = 0;
    ngs->bss_head = 0;

    for (i = 0; i < ps_search_n_words(ngs); i++)
        ngs->word_lat_idx[i] = NO_BP;

    /* Reset the permanently allocated single-phone words, since they
     * may have junk left over in them from previous searches. */
    for (i = 0; i < ngs->n_1ph_words; i++) {
        int32 w = ngs->single_phone_wid[i];
        rhmm = (root_chan_t *) ngs->word_chan[w];
        hmm_clear(&rhmm->hmm);
    }

    /* Start search with <s>; word_chan[<s>] is permanently allocated */
    rhmm = (root_chan_t *) ngs->word_chan[ps_search_start_wid(ngs)];
    hmm_enter(&rhmm->hmm, 0, NO_BP, 0);
    ngs->active_word_list[0][0] = ps_search_start_wid(ngs);
    ngs->n_active_word[0] = 1;

    ngs->best_score = 0;
    ngs->renormalized = FALSE;

    for (i = 0; i < ps_search_n_words(ngs); i++)
        ngs->last_ltrans[i].sf = -1;

    if (!ngs->fwdtree)
        ngs->n_frame = 0;

    ngs->st.n_fwdflat_chan = 0;
    ngs->st.n_fwdflat_words = 0;
    ngs->st.n_fwdflat_word_transition = 0;
    ngs->st.n_senone_active_utt = 0;
}

static void
compute_fwdflat_sen_active(ngram_search_t *ngs, int frame_idx)
{
    int32 i, nw, w;
    int32 *awl;
    root_chan_t *rhmm;
    chan_t *hmm;

    acmod_clear_active(ps_search_acmod(ngs));

    nw = ngs->n_active_word[frame_idx & 0x1];
    awl = ngs->active_word_list[frame_idx & 0x1];

    for (i = 0; i < nw; i++) {
        w = *(awl++);
        rhmm = (root_chan_t *)ngs->word_chan[w];
        if (hmm_frame(&rhmm->hmm) == frame_idx) {
            acmod_activate_hmm(ps_search_acmod(ngs), &rhmm->hmm);
        }

        for (hmm = rhmm->next; hmm; hmm = hmm->next) {
            if (hmm_frame(&hmm->hmm) == frame_idx) {
                acmod_activate_hmm(ps_search_acmod(ngs), &hmm->hmm);
            }
        }
    }
}

static void
fwdflat_eval_chan(ngram_search_t *ngs, int frame_idx)
{
    int32 i, w, nw, bestscore;
    int32 *awl;
    root_chan_t *rhmm;
    chan_t *hmm;

    nw = ngs->n_active_word[frame_idx & 0x1];
    awl = ngs->active_word_list[frame_idx & 0x1];
    bestscore = WORST_SCORE;

    ngs->st.n_fwdflat_words += nw;

    /* Scan all active words. */
    for (i = 0; i < nw; i++) {
        w = *(awl++);
        rhmm = (root_chan_t *) ngs->word_chan[w];
        if (hmm_frame(&rhmm->hmm) == frame_idx) {
            int32 score = chan_v_eval(rhmm);
            if ((score BETTER_THAN bestscore) && (w != ps_search_finish_wid(ngs)))
                bestscore = score;
            ngs->st.n_fwdflat_chan++;
        }

        for (hmm = rhmm->next; hmm; hmm = hmm->next) {
            if (hmm_frame(&hmm->hmm) == frame_idx) {
                int32 score = chan_v_eval(hmm);
                if (score BETTER_THAN bestscore)
                    bestscore = score;
                ngs->st.n_fwdflat_chan++;
            }
        }
    }

    ngs->best_score = bestscore;
}

static void
fwdflat_prune_chan(ngram_search_t *ngs, int frame_idx)
{
    int32 i, nw, cf, nf, w, pip, newscore, thresh, wordthresh;
    int32 *awl;
    root_chan_t *rhmm;
    chan_t *hmm, *nexthmm;

    cf = frame_idx;
    nf = cf + 1;
    nw = ngs->n_active_word[cf & 0x1];
    awl = ngs->active_word_list[cf & 0x1];
    bitvec_clear_all(ngs->word_active, ps_search_n_words(ngs));

    thresh = ngs->best_score + ngs->fwdflatbeam;
    wordthresh = ngs->best_score + ngs->fwdflatwbeam;
    pip = ngs->pip;
    E_DEBUG(3,("frame %d thresh %d wordthresh %d\n", frame_idx, thresh, wordthresh));

    /* Scan all active words. */
    for (i = 0; i < nw; i++) {
        w = *(awl++);
        rhmm = (root_chan_t *) ngs->word_chan[w];
        /* Propagate active root channels */
        if (hmm_frame(&rhmm->hmm) == cf
            && hmm_bestscore(&rhmm->hmm) BETTER_THAN thresh) {
            hmm_frame(&rhmm->hmm) = nf;
            bitvec_set(ngs->word_active, w);

            /* Transitions out of root channel */
            newscore = hmm_out_score(&rhmm->hmm);
            if (rhmm->next) {
                assert(!dict_is_single_phone(ps_search_dict(ngs), w));

                newscore += pip;
                if (newscore BETTER_THAN thresh) {
                    hmm = rhmm->next;
                    /* Enter all right context phones */
                    if (hmm->info.rc_id >= 0) {
                        for (; hmm; hmm = hmm->next) {
                            if ((hmm_frame(&hmm->hmm) < cf)
                                || (newscore BETTER_THAN hmm_in_score(&hmm->hmm))) {
                                hmm_enter(&hmm->hmm, newscore,
                                          hmm_out_history(&rhmm->hmm), nf);
                            }
                        }
                    }
                    /* Just a normal word internal phone */
                    else {
                        if ((hmm_frame(&hmm->hmm) < cf)
                            || (newscore BETTER_THAN hmm_in_score(&hmm->hmm))) {
                                hmm_enter(&hmm->hmm, newscore,
                                          hmm_out_history(&rhmm->hmm), nf);
                        }
                    }
                }
            }
            else {
                assert(dict_is_single_phone(ps_search_dict(ngs), w));

                /* Word exit for single-phone words (where did their
                 * whmms come from?) (either from
                 * ngram_search_fwdtree, or from
                 * ngram_fwdflat_allocate_1ph(), that's where) */
                if (newscore BETTER_THAN wordthresh) {
                    ngram_search_save_bp(ngs, cf, w, newscore,
                                         hmm_out_history(&rhmm->hmm), 0);
                }
            }
        }

        /* Transitions out of non-root channels. */
        for (hmm = rhmm->next; hmm; hmm = hmm->next) {
            if (hmm_frame(&hmm->hmm) >= cf) {
                /* Propagate forward HMMs inside the beam. */
                if (hmm_bestscore(&hmm->hmm) BETTER_THAN thresh) {
                    hmm_frame(&hmm->hmm) = nf;
                    bitvec_set(ngs->word_active, w);

                    newscore = hmm_out_score(&hmm->hmm);
                    /* Word-internal phones */
                    if (hmm->info.rc_id < 0) {
                        newscore += pip;
                        if (newscore BETTER_THAN thresh) {
                            nexthmm = hmm->next;
                            /* Enter all right-context phones. */
                            if (nexthmm->info.rc_id >= 0) {
                                 for (; nexthmm; nexthmm = nexthmm->next) {
                                    if ((hmm_frame(&nexthmm->hmm) < cf)
                                        || (newscore BETTER_THAN
                                            hmm_in_score(&nexthmm->hmm))) {
                                        hmm_enter(&nexthmm->hmm,
                                                  newscore,
                                                  hmm_out_history(&hmm->hmm),
                                                  nf);
                                    }
                                }
                            }
                            /* Enter single word-internal phone. */
                            else {
                                if ((hmm_frame(&nexthmm->hmm) < cf)
                                    || (newscore BETTER_THAN
                                        hmm_in_score(&nexthmm->hmm))) {
                                    hmm_enter(&nexthmm->hmm, newscore,
                                              hmm_out_history(&hmm->hmm), nf);
                                }
                            }
                        }
                    }
                    /* Right-context phones - apply word beam and exit. */
                    else {
                        if (newscore BETTER_THAN wordthresh) {
                            ngram_search_save_bp(ngs, cf, w, newscore,
                                                 hmm_out_history(&hmm->hmm),
                                                 hmm->info.rc_id);
                        }
                    }
                }
                /* Zero out inactive HMMs. */
                else if (hmm_frame(&hmm->hmm) != nf) {
                    hmm_clear_scores(&hmm->hmm);
                }
            }
        }
    }
}

static void
get_expand_wordlist(ngram_search_t *ngs, int32 frm, int32 win)
{
    int32 f, sf, ef;
    ps_latnode_t *node;

    if (!ngs->fwdtree) {
        ngs->st.n_fwdflat_word_transition += ngs->n_expand_words;
        return;
    }

    sf = frm - win;
    if (sf < 0)
        sf = 0;
    ef = frm + win;
    if (ef > ngs->n_frame)
        ef = ngs->n_frame;

    bitvec_clear_all(ngs->expand_word_flag, ps_search_n_words(ngs));
    ngs->n_expand_words = 0;

    for (f = sf; f < ef; f++) {
        for (node = ngs->frm_wordlist[f]; node; node = node->next) {
            if (!bitvec_is_set(ngs->expand_word_flag, node->wid)) {
                ngs->expand_word_list[ngs->n_expand_words++] = node->wid;
                bitvec_set(ngs->expand_word_flag, node->wid);
            }
        }
    }
    ngs->expand_word_list[ngs->n_expand_words] = -1;
    ngs->st.n_fwdflat_word_transition += ngs->n_expand_words;
}

static void
fwdflat_word_transition(ngram_search_t *ngs, int frame_idx)
{
    int32 cf, nf, b, thresh, pip, i, nw, w, newscore;
    int32 best_silrc_score = 0, best_silrc_bp = 0;      /* FIXME: good defaults? */
    bptbl_t *bp;
    int32 *rcss;
    root_chan_t *rhmm;
    int32 *awl;
    float32 lwf;
    dict_t *dict = ps_search_dict(ngs);
    dict2pid_t *d2p = ps_search_dict2pid(ngs);

    cf = frame_idx;
    nf = cf + 1;
    thresh = ngs->best_score + ngs->fwdflatbeam;
    pip = ngs->pip;
    best_silrc_score = WORST_SCORE;
    lwf = ngs->fwdflat_fwdtree_lw_ratio;

    /* Search for all words starting within a window of this frame.
     * These are the successors for words exiting now. */
    get_expand_wordlist(ngs, cf, ngs->max_sf_win);

    /* Scan words exited in current frame */
    for (b = ngs->bp_table_idx[cf]; b < ngs->bpidx; b++) {
        xwdssid_t *rssid;
        int32 silscore;

        bp = ngs->bp_table + b;
        ngs->word_lat_idx[bp->wid] = NO_BP;

        if (bp->wid == ps_search_finish_wid(ngs))
            continue;

        /* DICT2PID location */
        /* Get the mapping from right context phone ID to index in the
         * right context table and the bscore_stack. */
        rcss = ngs->bscore_stack + bp->s_idx;
        if (bp->last2_phone == -1)
            rssid = NULL;
        else
            rssid = dict2pid_rssid(d2p, bp->last_phone, bp->last2_phone);

        /* Transition to all successor words. */
        for (i = 0; ngs->expand_word_list[i] >= 0; i++) {
            int32 n_used;

            w = ngs->expand_word_list[i];

            /* Get the exit score we recorded in save_bwd_ptr(), or
             * something approximating it. */
            if (rssid)
                newscore = rcss[rssid->cimap[dict_first_phone(dict, w)]];
            else
                newscore = bp->score;
            if (newscore == WORST_SCORE)
                continue;
            /* FIXME: Floating point... */
            newscore += lwf
                * (ngram_tg_score(ngs->lmset,
                                  dict_basewid(dict, w),
                                  bp->real_wid,
                                  bp->prev_real_wid,
                                  &n_used) >> SENSCR_SHIFT);
            newscore += pip;

            /* Enter the next word */
            if (newscore BETTER_THAN thresh) {
                rhmm = (root_chan_t *) ngs->word_chan[w];
                if ((hmm_frame(&rhmm->hmm) < cf)
                    || (newscore BETTER_THAN hmm_in_score(&rhmm->hmm))) {
                    hmm_enter(&rhmm->hmm, newscore, b, nf);
                    /* DICT2PID: This is where mpx ssids get introduced. */
                    /* Look up the ssid to use when entering this mpx triphone. */
                    hmm_mpx_ssid(&rhmm->hmm, 0) =
                        dict2pid_ldiph_lc(d2p, rhmm->ciphone, rhmm->ci2phone,
                                          dict_last_phone(dict, bp->wid));
                    assert(IS_S3SSID(hmm_mpx_ssid(&rhmm->hmm, 0)));
                    E_DEBUG(6,("ssid %d(%d,%d) = %d\n",
                               rhmm->ciphone, dict_last_phone(dict, bp->wid), rhmm->ci2phone,
                               hmm_mpx_ssid(&rhmm->hmm, 0)));
                    bitvec_set(ngs->word_active, w);
                }
            }
        }

        /* Get the best exit into silence. */
        if (rssid)
            silscore = rcss[rssid->cimap[ps_search_acmod(ngs)->mdef->sil]];
        else
            silscore = bp->score;
        if (silscore BETTER_THAN best_silrc_score) {
            best_silrc_score = silscore;
            best_silrc_bp = b;
        }
    }

    /* Transition to <sil> */
    newscore = best_silrc_score + ngs->silpen + pip;
    if ((newscore BETTER_THAN thresh) && (newscore BETTER_THAN WORST_SCORE)) {
        w = ps_search_silence_wid(ngs);
        rhmm = (root_chan_t *) ngs->word_chan[w];
        if ((hmm_frame(&rhmm->hmm) < cf)
            || (newscore BETTER_THAN hmm_in_score(&rhmm->hmm))) {
            hmm_enter(&rhmm->hmm, newscore,
                      best_silrc_bp, nf);
            bitvec_set(ngs->word_active, w);
        }
    }
    /* Transition to noise words */
    newscore = best_silrc_score + ngs->fillpen + pip;
    if ((newscore BETTER_THAN thresh) && (newscore BETTER_THAN WORST_SCORE)) {
        for (w = dict_filler_start(dict); w <= dict_filler_end(dict); w++) {
            if (w == ps_search_silence_wid(ngs))
                continue;

            rhmm = (root_chan_t *) ngs->word_chan[w];
            /* Noise words that aren't a single phone will have NULL here. */
            if (rhmm == NULL)
                continue;
            if ((hmm_frame(&rhmm->hmm) < cf)
                || (newscore BETTER_THAN hmm_in_score(&rhmm->hmm))) {
                hmm_enter(&rhmm->hmm, newscore,
                          best_silrc_bp, nf);
                bitvec_set(ngs->word_active, w);
            }
        }
    }

    /* Reset initial channels of words that have become inactive even after word trans. */
    nw = ngs->n_active_word[cf & 0x1];
    awl = ngs->active_word_list[cf & 0x1];
    for (i = 0; i < nw; i++) {
        w = *(awl++);
        rhmm = (root_chan_t *) ngs->word_chan[w];
        if (hmm_frame(&rhmm->hmm) == cf) {
            hmm_clear_scores(&rhmm->hmm);
        }
    }
}

static void
fwdflat_renormalize_scores(ngram_search_t *ngs, int frame_idx, int32 norm)
{
    root_chan_t *rhmm;
    chan_t *hmm;
    int32 i, nw, cf, w, *awl;

    cf = frame_idx;

    /* Renormalize individual word channels */
    nw = ngs->n_active_word[cf & 0x1];
    awl = ngs->active_word_list[cf & 0x1];
    for (i = 0; i < nw; i++) {
        w = *(awl++);
        rhmm = (root_chan_t *) ngs->word_chan[w];
        if (hmm_frame(&rhmm->hmm) == cf) {
            hmm_normalize(&rhmm->hmm, norm);
        }
        for (hmm = rhmm->next; hmm; hmm = hmm->next) {
            if (hmm_frame(&hmm->hmm) == cf) {
                hmm_normalize(&hmm->hmm, norm);
            }
        }
    }

    ngs->renormalized = TRUE;
}

int
ngram_fwdflat_search(ngram_search_t *ngs, int frame_idx)
{
    int16 const *senscr;
    int32 nf, i, j;
    int32 *nawl;

    /* Activate our HMMs for the current frame if need be. */
    if (!ps_search_acmod(ngs)->compallsen)
        compute_fwdflat_sen_active(ngs, frame_idx);

    /* Compute GMM scores for the current frame. */
    senscr = acmod_score(ps_search_acmod(ngs), &frame_idx);
    ngs->st.n_senone_active_utt += ps_search_acmod(ngs)->n_senone_active;

    /* Mark backpointer table for current frame. */
    ngram_search_mark_bptable(ngs, frame_idx);

    /* If the best score is equal to or worse than WORST_SCORE,
     * recognition has failed, don't bother to keep trying. */
    if (ngs->best_score == WORST_SCORE || ngs->best_score WORSE_THAN WORST_SCORE)
        return 0;
    /* Renormalize if necessary */
    if (ngs->best_score + (2 * ngs->beam) WORSE_THAN WORST_SCORE) {
        E_INFO("Renormalizing Scores at frame %d, best score %d\n",
               frame_idx, ngs->best_score);
        fwdflat_renormalize_scores(ngs, frame_idx, ngs->best_score);
    }

    ngs->best_score = WORST_SCORE;
    hmm_context_set_senscore(ngs->hmmctx, senscr);

    /* Evaluate HMMs */
    fwdflat_eval_chan(ngs, frame_idx);
    /* Prune HMMs and do phone transitions. */
    fwdflat_prune_chan(ngs, frame_idx);
    /* Do word transitions. */
    fwdflat_word_transition(ngs, frame_idx);

    /* Create next active word list, skip fillers */
    nf = frame_idx + 1;
    nawl = ngs->active_word_list[nf & 0x1];
    for (i = 0, j = 0; ngs->fwdflat_wordlist[i] >= 0; i++) {
        int32 wid = ngs->fwdflat_wordlist[i];
        if (bitvec_is_set(ngs->word_active, wid) && wid < ps_search_start_wid(ngs)) {
            *(nawl++) = wid;
            j++;
        }
    }
    /* Add fillers */
    for (i = ps_search_start_wid(ngs); i < ps_search_n_words(ngs); i++) {
        if (bitvec_is_set(ngs->word_active, i)) {
            *(nawl++) = i;
            j++;
        }
    }
    if (!ngs->fwdtree)
        ++ngs->n_frame;
    ngs->n_active_word[nf & 0x1] = j;

    /* Return the number of frames processed. */
    return 1;
}

static void
destroy_fwdflat_wordlist(ngram_search_t *ngs)
{
    ps_latnode_t *node, *tnode;
    int32 f;

    if (!ngs->fwdtree)
        return;

    for (f = 0; f < ngs->n_frame; f++) {
        for (node = ngs->frm_wordlist[f]; node; node = tnode) {
            tnode = node->next;
            listelem_free(ngs->latnode_alloc, node);
        }
    }
}

static void
destroy_fwdflat_chan(ngram_search_t *ngs)
{
    int32 i, wid;

    for (i = 0; ngs->fwdflat_wordlist[i] >= 0; i++) {
        root_chan_t *rhmm;
        chan_t *thmm;
        wid = ngs->fwdflat_wordlist[i];
        if (dict_is_single_phone(ps_search_dict(ngs),wid))
            continue;
        assert(ngs->word_chan[wid] != NULL);

        /* The first HMM in ngs->word_chan[wid] was allocated with
         * ngs->root_chan_alloc, but this will attempt to free it
         * using ngs->chan_alloc, which will not work.  Therefore we
         * free it manually and move the list forward before handing
         * it off. */
        rhmm = (root_chan_t *)ngs->word_chan[wid];
        thmm = rhmm->next;
        listelem_free(ngs->root_chan_alloc, rhmm);
        ngs->word_chan[wid] = thmm;
        ngram_search_free_all_rc(ngs, wid);
    }
}

void
ngram_fwdflat_finish(ngram_search_t *ngs)
{
    int32 cf;

    destroy_fwdflat_chan(ngs);
    destroy_fwdflat_wordlist(ngs);
    bitvec_clear_all(ngs->word_active, ps_search_n_words(ngs));

    /* This is the number of frames processed. */
    cf = ps_search_acmod(ngs)->output_frame;
    /* Add a mark in the backpointer table for one past the final frame. */
    ngram_search_mark_bptable(ngs, cf);

    ptmr_stop(&ngs->fwdflat_perf);
    /* Print out some statistics. */
    if (cf > 0) {
        double n_speech = (double)(cf + 1)
            / cmd_ln_int32_r(ps_search_config(ngs), "-frate");
        E_INFO("%8d words recognized (%d/fr)\n",
               ngs->bpidx, (ngs->bpidx + (cf >> 1)) / (cf + 1));
        E_INFO("%8d senones evaluated (%d/fr)\n", ngs->st.n_senone_active_utt,
               (ngs->st.n_senone_active_utt + (cf >> 1)) / (cf + 1));
        E_INFO("%8d channels searched (%d/fr)\n",
               ngs->st.n_fwdflat_chan, ngs->st.n_fwdflat_chan / (cf + 1));
        E_INFO("%8d words searched (%d/fr)\n",
               ngs->st.n_fwdflat_words, ngs->st.n_fwdflat_words / (cf + 1));
        E_INFO("%8d word transitions (%d/fr)\n",
               ngs->st.n_fwdflat_word_transition,
               ngs->st.n_fwdflat_word_transition / (cf + 1));
        E_INFO("fwdflat %.2f CPU %.3f xRT\n",
               ngs->fwdflat_perf.t_cpu,
               ngs->fwdflat_perf.t_cpu / n_speech);
        E_INFO("fwdflat %.2f wall %.3f xRT\n",
               ngs->fwdflat_perf.t_elapsed,
               ngs->fwdflat_perf.t_elapsed / n_speech);
    }
}