shithub: dav1d

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Simplify rate calculation in update_cdf

Use the fact that the count is at most 32.
Also, apply the same in msac_decode_bool_adapt.

--- a/src/msac.c

+++ b/src/msac.c

@@ -143,14 +143,14 @@

 static void update_cdf(uint16_t *const cdf, const unsigned val,

                        const unsigned n_symbols)

 {

-    const int rate = 4 + (cdf[n_symbols] > 15) + (cdf[n_symbols] > 31) +

-                     (n_symbols > 3);

+    const unsigned count = cdf[n_symbols];

+    const int rate = ((count >> 4) | 4) + (n_symbols > 3);

     unsigned i;

-    for (i = 0; i < val; ++i)

+    for (i = 0; i < val; i++)

         cdf[i] += (32768 - cdf[i]) >> rate;

-    for (i = val; i < n_symbols - 1; i++)

+    for (; i < n_symbols - 1; i++)

         cdf[i] -= cdf[i] >> rate;

-    cdf[n_symbols] += (cdf[n_symbols] < 32);

+    cdf[n_symbols] = count + (count < 32);

 }

 unsigned msac_decode_symbol_adapt(MsacContext *const c,

@@ -165,13 +165,14 @@

     const unsigned bit = msac_decode_bool(c, *cdf >> EC_PROB_SHIFT);

     // update_cdf() specialized for boolean CDFs

-    const int rate = 4 + (cdf[1] > 15) + (cdf[1] > 31);

+    const unsigned count = cdf[1];

+    const int rate = (count >> 4) | 4;

     if (bit) {

         cdf[0] += (32768 - cdf[0]) >> rate;

     } else {

         cdf[0] -= cdf[0] >> rate;

     }

-    cdf[1] += (cdf[1] < 32);

+    cdf[1] = count + (count < 32);

     return bit;

 }