288 lines
9.2 KiB
C
288 lines
9.2 KiB
C
/* K=9 r=1/3 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions
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* 8-bit offset-binary soft decision samples
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* Copyright Aug 2006, Phil Karn, KA9Q
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* May be used under the terms of the GNU Lesser General Public License (LGPL)
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <memory.h>
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#include <limits.h>
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#include "fec.h"
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typedef union {
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unsigned char c[2][16];
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vector unsigned char v[2];
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} decision_t;
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typedef union {
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unsigned short s[256];
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vector unsigned short v[32];
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} metric_t;
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static union branchtab39 {
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unsigned short s[128];
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vector unsigned short v[16];
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} Branchtab39[3];
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static int Init = 0;
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/* State info for instance of Viterbi decoder */
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struct v39 {
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metric_t metrics1; /* path metric buffer 1 */
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metric_t metrics2; /* path metric buffer 2 */
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void *dp; /* Pointer to current decision */
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metric_t *old_metrics,
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*new_metrics; /* Pointers to path metrics, swapped on every bit */
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void *decisions; /* Beginning of decisions for block */
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};
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/* Initialize Viterbi decoder for start of new frame */
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int init_viterbi39_av(void *p, int starting_state)
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{
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struct v39 *vp = p;
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int i;
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for (i = 0; i < 32; i++) {
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vp->metrics1.v[i] = (vector unsigned short)(1000);
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}
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vp->old_metrics = &vp->metrics1;
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vp->new_metrics = &vp->metrics2;
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vp->dp = vp->decisions;
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vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */
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return 0;
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}
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void set_viterbi39_polynomial_av(int polys[3])
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{
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int state;
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for (state = 0; state < 128; state++) {
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Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2 * state) & abs(
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polys[0])) ? 255 : 0;
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Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2 * state) & abs(
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polys[1])) ? 255 : 0;
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Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2 * state) & abs(
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polys[2])) ? 255 : 0;
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}
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Init++;
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}
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/* Create a new instance of a Viterbi decoder */
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void *create_viterbi39_av(int len)
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{
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struct v39 *vp;
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if (!Init) {
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int polys[3] = { V39POLYA, V39POLYB, V39POLYC };
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set_viterbi39_polynomial_av(polys);
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}
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vp = (struct v39 *)malloc(sizeof(struct v39));
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vp->decisions = malloc(sizeof(decision_t) * (len + 8));
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init_viterbi39_av(vp, 0);
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return vp;
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}
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/* Viterbi chainback */
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int chainback_viterbi39_av(
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void *p,
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unsigned char *data, /* Decoded output data */
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unsigned int nbits, /* Number of data bits */
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unsigned int endstate) /* Terminal encoder state */
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{
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struct v39 *vp = p;
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decision_t *d = (decision_t *)vp->decisions;
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int path_metric;
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/* Make room beyond the end of the encoder register so we can
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* accumulate a full byte of decoded data
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*/
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endstate %= 256;
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path_metric = vp->old_metrics->s[endstate];
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/* The store into data[] only needs to be done every 8 bits.
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* But this avoids a conditional branch, and the writes will
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* combine in the cache anyway
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*/
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d += 8; /* Look past tail */
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while (nbits-- != 0) {
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int k;
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k = (d[nbits].c[endstate >> 7][endstate & 15] & (0x80 >> ((
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endstate >> 4) & 7))) ? 1 : 0;
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endstate = (k << 7) | (endstate >> 1);
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data[nbits >> 3] = endstate;
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}
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return path_metric;
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}
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/* Delete instance of a Viterbi decoder */
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void delete_viterbi39_av(void *p)
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{
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struct v39 *vp = p;
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if (vp != NULL) {
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free(vp->decisions);
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free(vp);
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}
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}
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int update_viterbi39_blk_av(void *p, unsigned char *syms, int nbits)
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{
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struct v39 *vp = p;
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decision_t *d = (decision_t *)vp->dp;
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int path_metric = 0;
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vector unsigned char decisions = (vector unsigned char)(0);
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while (nbits--) {
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vector unsigned short symv, sym0v, sym1v, sym2v;
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vector unsigned char s;
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void *tmp;
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int i;
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/* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
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s = (vector unsigned char)vec_perm(vec_ld(0, syms), vec_ld(5, syms), vec_lvsl(0,
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syms));
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symv = (vector unsigned short)vec_mergeh((vector unsigned char)(0),
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s); /* Unsigned byte->word unpack */
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sym0v = vec_splat(symv, 0);
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sym1v = vec_splat(symv, 1);
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sym2v = vec_splat(symv, 2);
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syms += 3;
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for (i = 0; i < 16; i++) {
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vector bool short decision0, decision1;
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vector unsigned short metric, m_metric, m0, m1, m2, m3, survivor0, survivor1;
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/* Form branch metrics
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* Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
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* the XOR operations constitute conditional negation.
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* the metrics are in the range 0-765
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*/
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m0 = vec_add(vec_xor(Branchtab39[0].v[i], sym0v), vec_xor(Branchtab39[1].v[i],
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sym1v));
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m1 = vec_xor(Branchtab39[2].v[i], sym2v);
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metric = vec_add(m0, m1);
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m_metric = vec_sub((vector unsigned short)(765), metric);
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/* Add branch metrics to path metrics */
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m0 = vec_adds(vp->old_metrics->v[i], metric);
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m3 = vec_adds(vp->old_metrics->v[16 + i], metric);
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m1 = vec_adds(vp->old_metrics->v[16 + i], m_metric);
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m2 = vec_adds(vp->old_metrics->v[i], m_metric);
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/* Compare and select */
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decision0 = vec_cmpgt(m0, m1);
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decision1 = vec_cmpgt(m2, m3);
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survivor0 = vec_min(m0, m1);
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survivor1 = vec_min(m2, m3);
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/* Store decisions and survivors.
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* To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in
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* a funny interleaved fashion that we undo in the chainback function.
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*/
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decisions = vec_add(decisions,
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decisions); /* Shift each byte 1 bit to the left */
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/* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting
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* 0xff is equivalent to adding 1, which sets the lsb.
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*/
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decisions = vec_sub(decisions,
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(vector unsigned char)vec_pack(vec_mergeh(decision0, decision1),
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vec_mergel(decision0, decision1)));
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vp->new_metrics->v[2 * i] = vec_mergeh(survivor0, survivor1);
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vp->new_metrics->v[2 * i + 1] = vec_mergel(survivor0, survivor1);
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if ((i % 8) == 7) {
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/* We've accumulated a total of 128 decisions, stash and start again */
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d->v[i >> 3] =
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decisions; /* No need to clear, the new bits will replace the old */
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}
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}
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#if 0
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/* Experimentally determine metric spread
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* The results are fixed for a given code and input symbol size
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*/
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{
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int i;
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vector unsigned short min_metric;
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vector unsigned short max_metric;
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union {
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vector unsigned short v;
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unsigned short s[8];
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} t;
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int minimum, maximum;
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static int max_spread = 0;
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min_metric = max_metric = vp->new_metrics->v[0];
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for (i = 1; i < 32; i++) {
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min_metric = vec_min(min_metric, vp->new_metrics->v[i]);
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max_metric = vec_max(max_metric, vp->new_metrics->v[i]);
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}
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min_metric = vec_min(min_metric, vec_sld(min_metric, min_metric, 8));
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max_metric = vec_max(max_metric, vec_sld(max_metric, max_metric, 8));
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min_metric = vec_min(min_metric, vec_sld(min_metric, min_metric, 4));
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max_metric = vec_max(max_metric, vec_sld(max_metric, max_metric, 4));
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min_metric = vec_min(min_metric, vec_sld(min_metric, min_metric, 2));
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max_metric = vec_max(max_metric, vec_sld(max_metric, max_metric, 2));
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t.v = min_metric;
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minimum = t.s[0];
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t.v = max_metric;
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maximum = t.s[0];
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if (maximum - minimum > max_spread) {
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max_spread = maximum - minimum;
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printf("metric spread = %d\n", max_spread);
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}
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}
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#endif
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/* Renormalize if necessary. This deserves some explanation.
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* The maximum possible spread, found by experiment, for 8 bit symbols is about 3825
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* So by looking at one arbitrary metric we can tell if any of them have possibly saturated.
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* However, this is very conservative. Large spreads occur only at very high Eb/No, where
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* saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor.
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* At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric
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* by not not normalizing when we should are extremely low. So either way, the risk to performance is small.
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* All this is borne out by experiment.
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*/
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if (vp->new_metrics->s[0] >= USHRT_MAX - 5000) {
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vector unsigned short scale;
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union {
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vector unsigned short v;
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unsigned short s[8];
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} t;
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/* Find smallest metric and splat */
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scale = vp->new_metrics->v[0];
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for (i = 1; i < 32; i++) {
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scale = vec_min(scale, vp->new_metrics->v[i]);
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}
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scale = vec_min(scale, vec_sld(scale, scale, 8));
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scale = vec_min(scale, vec_sld(scale, scale, 4));
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scale = vec_min(scale, vec_sld(scale, scale, 2));
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/* Subtract it from all metrics
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* Work backwards to try to improve the cache hit ratio, assuming LRU
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*/
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for (i = 31; i >= 0; i--) {
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vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i], scale);
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}
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t.v = scale;
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path_metric += t.s[0];
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}
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d++;
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/* Swap pointers to old and new metrics */
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tmp = vp->old_metrics;
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vp->old_metrics = vp->new_metrics;
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vp->new_metrics = tmp;
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}
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vp->dp = d;
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return path_metric;
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}
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