commit 15ad38f4fde9cd8366afd8e538df4f73b53696e2
author Wan-Teh Chang <wtc@google.com> Thu Aug 16 10:31:53 2018 -0700
committer Wan-Teh Chang <wtc@google.com> Fri Sep 07 18:17:17 2018 +0000
tree 4896458f242a78ed4c1ec7977af003f64305830f
parent 4137f7daaa1c3947bba45681a4ff32142ca3097d

Pass n to get_msb in the primitive_quniform funcs.

The functions this CL modifies were added in the following two CLs:
https://aomedia-review.googlesource.com/c/aom/+/8789
https://aomedia-review.googlesource.com/c/aom/+/20820

These functions pass n - 1 to get_msb(). I believe these functions
implement the ns(n) and NS(n) descriptors of the AV1 spec. ns(n) and
NS(n) pass n, not n - 1, to FloorLog2(). (FloorLog2() is the same as
libaom's get_msb() function.)

A careful analysis of the code shows passing n instead of n - 1 to
get_msb() results in the same outcome for these functions. Passing n
matches the AV1 spec and is slightly more efficient when n is a power
of 2.

Change-Id: Ibeff837059e9c9de2b4ab9cb857a2ea60b1c4767

aom_dsp/binary_codes_reader.c

diff --git a/aom_dsp/binary_codes_reader.c b/aom_dsp/binary_codes_reader.c
index 23e6121..0108801 100644
--- a/aom_dsp/binary_codes_reader.c
+++ b/aom_dsp/binary_codes_reader.c
@@ -36,7 +36,7 @@
 uint16_t aom_read_primitive_quniform_(aom_reader *r,
                                       uint16_t n ACCT_STR_PARAM) {
   if (n <= 1) return 0;
-  const int l = get_msb(n - 1) + 1;
+  const int l = get_msb(n) + 1;
   const int m = (1 << l) - n;
   const int v = aom_read_literal(r, l - 1, ACCT_STR_NAME);
   return v < m ? v : (v << 1) - m + aom_read_bit(r, ACCT_STR_NAME);
@@ -45,7 +45,7 @@
 static uint16_t aom_rb_read_primitive_quniform(struct aom_read_bit_buffer *rb,
                                                uint16_t n) {
   if (n <= 1) return 0;
-  const int l = get_msb(n - 1) + 1;
+  const int l = get_msb(n) + 1;
   const int m = (1 << l) - n;
   const int v = aom_rb_read_literal(rb, l - 1);
   return v < m ? v : (v << 1) - m + aom_rb_read_bit(rb);

aom_dsp/binary_codes_writer.c

diff --git a/aom_dsp/binary_codes_writer.c b/aom_dsp/binary_codes_writer.c
index 4e53b15..ee7a9f5 100644
--- a/aom_dsp/binary_codes_writer.c
+++ b/aom_dsp/binary_codes_writer.c
@@ -59,7 +59,7 @@
 // Encodes a value v in [0, n-1] quasi-uniformly
 void aom_write_primitive_quniform(aom_writer *w, uint16_t n, uint16_t v) {
   if (n <= 1) return;
-  const int l = get_msb(n - 1) + 1;
+  const int l = get_msb(n) + 1;
   const int m = (1 << l) - n;
   if (v < m) {
     aom_write_literal(w, v, l - 1);
@@ -72,7 +72,7 @@
 static void aom_wb_write_primitive_quniform(struct aom_write_bit_buffer *wb,
                                             uint16_t n, uint16_t v) {
   if (n <= 1) return;
-  const int l = get_msb(n - 1) + 1;
+  const int l = get_msb(n) + 1;
   const int m = (1 << l) - n;
   if (v < m) {
     aom_wb_write_literal(wb, v, l - 1);
@@ -84,7 +84,7 @@
 
 int aom_count_primitive_quniform(uint16_t n, uint16_t v) {
   if (n <= 1) return 0;
-  const int l = get_msb(n - 1) + 1;
+  const int l = get_msb(n) + 1;
   const int m = (1 << l) - n;
   return v < m ? l - 1 : l;
 }