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Correct SILK encoder gain limit

Ensure that the SILK encoder's log gain is 63, not 64, when encoding
a maximum-value delta gain index of 40.  This matches the decoder
and RFC 6716 4.2.7.4, and prevents an assertion failure in the rare
case that the gain is later independently coded.

--- a/silk/gain_quant.c

+++ b/silk/gain_quant.c

@@ -76,6 +76,7 @@

             /* Accumulate deltas */

             if( ind[ k ] > double_step_size_threshold ) {

                 *prev_ind += silk_LSHIFT( ind[ k ], 1 ) - double_step_size_threshold;

+                *prev_ind = silk_min_int( *prev_ind, N_LEVELS_QGAIN - 1 );

             } else {

                 *prev_ind += ind[ k ];

             }

--- a/tests/opus_encode_regressions.c

+++ b/tests/opus_encode_regressions.c

@@ -983,6 +983,42 @@

     return 0;

 }

+static int silk_gain_assert(void)

+{

+    OpusEncoder *enc;

+    int err;

+    int data_len;

+    unsigned char data[1000];

+    static const short pcm1[160] = { 0 };

+    static const short pcm2[960] =

+    {

+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        32767, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

+        0, 0, 0, 0, 32767

+    };

+

+    enc = opus_encoder_create(8000, 1, OPUS_APPLICATION_AUDIO, &err);

+    opus_encoder_ctl(enc, OPUS_SET_COMPLEXITY(3));

+    opus_encoder_ctl(enc, OPUS_SET_MAX_BANDWIDTH(OPUS_BANDWIDTH_NARROWBAND));

+    opus_encoder_ctl(enc, OPUS_SET_BITRATE(6000));

+    data_len = opus_encode(enc, pcm1, 160, data, 1000);

+    assert(data_len > 0);

+

+    opus_encoder_ctl(enc, OPUS_SET_VBR(0));

+    opus_encoder_ctl(enc, OPUS_SET_COMPLEXITY(0));

+    opus_encoder_ctl(enc, OPUS_SET_MAX_BANDWIDTH(OPUS_BANDWIDTH_MEDIUMBAND));

+    opus_encoder_ctl(enc, OPUS_SET_BITRATE(2867));

+    data_len = opus_encode(enc, pcm2, 960, data, 1000);

+    assert(data_len > 0);

+

+    opus_encoder_destroy(enc);

+    return 0;

+}

+

 void regression_test(void)

 {

    fprintf(stderr, "Running simple tests for bugs that have been fixed previously\n");

@@ -992,4 +1028,5 @@

    surround_analysis_uninit();

    ec_enc_shrink_assert();

    ec_enc_shrink_assert2();

+   silk_gain_assert();

 }