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reduce code size, add MINIMP3_NONSTANDARD_BUT_LOGICAL, more robust L3_ldexp_q2

--- a/README.md

+++ b/README.md

@@ -69,6 +69,7 @@

 ```

 //#define MINIMP3_ONLY_MP3

 //#define MINIMP3_ONLY_SIMD

+//#define MINIMP3_NONSTANDARD_BUT_LOGICAL

 #define MINIMP3_IMPLEMENTATION

 #include "minimp3.h"

 ...

@@ -80,6 +81,7 @@

 You can ``#include`` ``minimp3.h`` in as many files as you like.

 Also you can use ``MINIMP3_ONLY_MP3`` define to strip MP1/MP2 decoding code.

 MINIMP3_ONLY_SIMD define controls generic (non SSE/NEON) code generation (always enabled on x64/arm64 targets).

+MINIMP3_NONSTANDARD_BUT_LOGICAL define saves some code bytes, and enforces non-stadnard but logical behaviour of mono-stereo transition (rare case).

 Then. we decode the input stream frame-by-frame:

--- a/minimp3.h

+++ b/minimp3.h

@@ -247,11 +247,8 @@

     uint32_t next, cache = 0, s = bs->pos & 7;

     int shl = n + s;

     const uint8_t *p = bs->buf + (bs->pos >> 3);

-    if (bs->pos + n > bs->limit)

-    {

-        bs->pos = bs->limit;

+    if ((bs->pos += n) > bs->limit)

         return 0;

-    }

     next = *p++ & (255 >> s);

     while ((shl -= 8) > 0)

     {

@@ -258,7 +255,6 @@

         cache |= next << shl;

         next = *p++;

     }

-    bs->pos += n;

     return cache | (next >> -shl);

 }

@@ -315,7 +311,7 @@

 }

 #ifndef MINIMP3_ONLY_MP3

-static const L12_subband_alloc_t * L12_subband_alloc_table(const uint8_t *hdr, L12_scale_info *sci)

+static const L12_subband_alloc_t *L12_subband_alloc_table(const uint8_t *hdr, L12_scale_info *sci)

 {

     const L12_subband_alloc_t *alloc;

     int mode = HDR_GET_STEREO_MODE(hdr);

@@ -643,10 +639,16 @@

     scf[0] = scf[1] = scf[2] = 0;

 }

-static float L3_ldexp_q2(int e)

+static float L3_ldexp_q2(float y, int exp_q2)

 {

     static const float g_expfrac[4] = { 9.31322575e-10f,7.83145814e-10f,6.58544508e-10f,5.53767716e-10f };

-    return g_expfrac[e & 3]*(1 << 30 >> (e >> 2));

+    int e;

+    do

+    {

+        e = MINIMP3_MIN(30*4, exp_q2);

+        y *= g_expfrac[e & 3]*(1 << 30 >> (e >> 2));

+    } while ((exp_q2 -= e) > 0);

+    return y;

 }

 static void L3_decode_scalefactors(const uint8_t *hdr, uint8_t *ist_pos, bs_t *bs, const L3_gr_info_t *gr, float *scf, int ch)

@@ -694,8 +696,7 @@

             iscf[gr->n_long_sfb + i + 1] += gr->subblock_gain[1] << sh;

             iscf[gr->n_long_sfb + i + 2] += gr->subblock_gain[2] << sh;

         }

-    }

-    else if (gr->preflag)

+    } else if (gr->preflag)

     {

         static const uint8_t g_preamp[10] = { 1,1,1,1,2,2,3,3,3,2 };

         for (i = 0; i < 10; i++)

@@ -705,21 +706,10 @@

     }

     gain_exp = gr->global_gain + BITS_DEQUANTIZER_OUT*4 - 210 - (HDR_IS_MS_STEREO(hdr) ? 2 : 0);

-    gain = 1 << (MAX_SCFI/4);

-

-    while (gain_exp < MAX_SCFI)

-    {

-        int dexp = MINIMP3_MIN(30*4, MAX_SCFI - gain_exp);

-        gain *= L3_ldexp_q2(dexp);

-        gain_exp += dexp;

-    }

-

+    gain = L3_ldexp_q2(1 << (MAX_SCFI/4),  MAX_SCFI - gain_exp);

     for (i = 0; i < (int)(gr->n_long_sfb + gr->n_short_sfb); i++)

     {

-        if (((iscf[i] << scf_shift) >> 2) >= 31)

-            scf[i] = gain*L3_ldexp_q2((iscf[i] << scf_shift) - 30*4) * /*L3_ldexp_q2(30*4)*/ 9.313226e-10f;

-        else

-            scf[i] = gain*L3_ldexp_q2(iscf[i] << scf_shift);

+        scf[i] = L3_ldexp_q2(gain, iscf[i] << scf_shift);

     }

 }

@@ -927,7 +917,7 @@

             } else

             {

                 kl = 1;

-                kr = L3_ldexp_q2((ipos + 1) >> 1 << mpeg2_sh);

+                kr = L3_ldexp_q2(1, (ipos + 1) >> 1 << mpeg2_sh);

                 if (ipos & 1)

                 {

                     kl = kr;

@@ -1572,7 +1562,7 @@

     {

         mp3d_synth(grbuf + i, pcm + 32*nch*i, nch, lins + i*64);

     }

-

+#ifndef MINIMP3_NONSTANDARD_BUT_LOGICAL

     if (nch == 1)

     {

         for (i = 0; i < 15*64; i += 2)

@@ -1580,6 +1570,7 @@

             qmf_state[i] = lins[nbands*64 + i];

         }

     } else

+#endif

     {

         memcpy(qmf_state, lins + nbands*64, sizeof(float)*15*64);

     }

--- a/minimp3_test.c

+++ b/minimp3_test.c

@@ -1,5 +1,6 @@

 /*#define MINIMP3_ONLY_MP3*/

 /*#define MINIMP3_ONLY_SIMD*/

+/*#define MINIMP3_NONSTANDARD_BUT_LOGICAL*/

 #define MINIMP3_IMPLEMENTATION

 #include "minimp3.h"

 #include <stdio.h>

binary files a/vectors/l2-nonstandard-test32-size.pcm b/vectors/l2-nonstandard-test32-size.pcm differ