lpc: add ff_lpc_calc_ref_coefs_f() function

This commit adds a function to get the reflection coefficients on
floating point samples. It's functionally identical to
ff_lpc_calc_ref_coefs() except it works on float samples and will
return the global prediction gain. The Welch window implementation
which is more optimized works only on int32_t samples so a slower
generic expression was used.
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

libavcodec/lpc.c

@@ -167,6 +167,26 @@ int ff_lpc_calc_ref_coefs(LPCContext *s,
     return order;
 }
 
+double ff_lpc_calc_ref_coefs_f(LPCContext *s, const float *samples, int len,
+                               int order, double *ref)
+{
+    int i;
+    double signal = 0.0f, avg_err = 0.0f;
+    double autoc[MAX_LPC_ORDER+1] = {0}, error[MAX_LPC_ORDER] = {0};
+    const double c = (len - 1)/2.0f;
+
+    /* Welch window */
+    for (i = 0; i < len; i++)
+        s->windowed_samples[i] = 1.0f - ((samples[i]-c)/c)*((samples[i]-c)/c);
+
+    s->lpc_compute_autocorr(s->windowed_samples, len, order, autoc);
+    signal = autoc[0];
+    compute_ref_coefs(autoc, order, ref, error);
+    for (i = 0; i < order; i++)
+        avg_err = (avg_err + error[i])/2.0f;
+    return signal/avg_err;
+}
+
 /**
  * Calculate LPC coefficients for multiple orders
  *

libavcodec/lpc.h

@@ -100,6 +100,8 @@ int ff_lpc_calc_coefs(LPCContext *s,
 int ff_lpc_calc_ref_coefs(LPCContext *s,
                           const int32_t *samples, int order, double *ref);
 
+double ff_lpc_calc_ref_coefs_f(LPCContext *s, const float *samples, int len,
+                               int order, double *ref);
 
 /**
  * Initialize LPCContext.