[FFmpeg-soc] [PATCH]aacsbr: Replace ceil(log() / M_LN2) with a ceil_log2[] lookup table

[Alex Converse]

I don't trust these bitstream reads to floating point math.

From 9b383d6b7a55f8f2187779eac9a17a50ff98592d Mon Sep 17 00:00:00 2001
From: Alex Converse <alex.conve...@gmail.com>
Date: Wed, 18 Nov 2009 09:20:41 -0500
Subject: [PATCH 03/10] Replace ceil(log() / M_LN2) with a ceil_log2[] lookup table.
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---
 aacsbr.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)


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diff --git a/aacsbr.c b/aacsbr.c
index 2b3d5e2..8d91673 100644
--- a/aacsbr.c
+++ b/aacsbr.c
@@ -489,6 +489,10 @@ static int sbr_make_f_derived(AACContext *ac, SpectralBandReplication *sbr)
     return 0;
 }
 
+static int8_t ceil_log2[] = {
+    0, 0, 1, 2, 2, 3, 3,
+};
+
 static int sbr_grid(AACContext *ac, SpectralBandReplication *sbr,
                     GetBitContext *gb, SBRData *ch_data)
 {
@@ -514,7 +518,7 @@ static int sbr_grid(AACContext *ac, SpectralBandReplication *sbr,
         for (i = 0; i < ch_data->bs_num_rel[1]; i++)
             ch_data->bs_rel_bord[1][i] = (get_bits(gb, 2) << 1) + 2;
 
-        ch_data->bs_pointer = get_bits(gb, ceil(logf(ch_data->bs_num_env[1] + 1) / M_LN2));
+        ch_data->bs_pointer = get_bits(gb, ceil_log2[ch_data->bs_num_env[1] + 1]);
 
         for (i = 0; i < ch_data->bs_num_env[1]; i++)
             ch_data->bs_freq_res[ch_data->bs_num_env[1] - 1 - i] = get_bits1(gb);
@@ -527,7 +531,7 @@ static int sbr_grid(AACContext *ac, SpectralBandReplication *sbr,
         for (i = 0; i < ch_data->bs_num_rel[0]; i++)
             ch_data->bs_rel_bord[0][i] = (get_bits(gb, 2) << 1) + 2;
 
-        ch_data->bs_pointer = get_bits(gb, ceil(logf(ch_data->bs_num_env[1] + 1) / M_LN2));
+        ch_data->bs_pointer = get_bits(gb, ceil_log2[ch_data->bs_num_env[1] + 1]);
 
         for (i = 0; i < ch_data->bs_num_env[1]; i++)
             ch_data->bs_freq_res[i] = get_bits1(gb);
@@ -544,7 +548,7 @@ static int sbr_grid(AACContext *ac, SpectralBandReplication *sbr,
         for (i = 0; i < ch_data->bs_num_rel[1]; i++)
             ch_data->bs_rel_bord[1][i] = (get_bits(gb, 2) << 1) + 2;
 
-        ch_data->bs_pointer = get_bits(gb, ceil(logf(ch_data->bs_num_env[1] + 1) / M_LN2));
+        ch_data->bs_pointer = get_bits(gb, ceil_log2[ch_data->bs_num_env[1] + 1]);
 
         for (i = 0; i < ch_data->bs_num_env[1]; i++)
             ch_data->bs_freq_res[i] = get_bits1(gb);

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