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Making the IMDCT work on interleaved data

Saves a copy in the decoder

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -440,42 +440,38 @@

    const int C = CHANNELS(_C);

    const int N = mode->shortMdctSize<<LM;

    const int overlap = OVERLAP(mode);

+   VARDECL(opus_val32, x);

+   SAVE_STACK;

+

+   ALLOC(x, N+overlap, opus_val32);

    c=0; do {

       int j;

-         VARDECL(opus_val32, x);

-         VARDECL(opus_val32, tmp);

-         int b;

-         int N2 = N;

-         int B = 1;

-         SAVE_STACK;

+      int b;

+      int N2 = N;

+      int B = 1;

-         ALLOC(x, N+overlap, opus_val32);

-         ALLOC(tmp, N, opus_val32);

+      if (shortBlocks)

+      {

+         N2 = mode->shortMdctSize;

+         B = shortBlocks;

+      }

+      /* Prevents problems from the imdct doing the overlap-add */

+      CELT_MEMSET(x, 0, overlap);

-         if (shortBlocks)

-         {

-            N2 = mode->shortMdctSize;

-            B = shortBlocks;

-         }

-         /* Prevents problems from the imdct doing the overlap-add */

-         CELT_MEMSET(x, 0, overlap);

+      for (b=0;b<B;b++)

+      {

+         /* IMDCT on the interleaved the sub-frames */

+         clt_mdct_backward(&mode->mdct, &X[b+c*N2*B], x+N2*b, mode->window, overlap, shortBlocks ? mode->maxLM : mode->maxLM-LM, B);

+      }

-         for (b=0;b<B;b++)

-         {

-            /* De-interleaving the sub-frames */

-            for (j=0;j<N2;j++)

-               tmp[j] = X[(j*B+b)+c*N2*B];

-            clt_mdct_backward(&mode->mdct, tmp, x+N2*b, mode->window, overlap, shortBlocks ? mode->maxLM : mode->maxLM-LM);

-         }

-

-         for (j=0;j<overlap;j++)

-            out_mem[c][j] = x[j] + overlap_mem[c][j];

-         for (;j<N;j++)

-            out_mem[c][j] = x[j];

-         for (j=0;j<overlap;j++)

-            overlap_mem[c][j] = x[N+j];

-         RESTORE_STACK;

+      for (j=0;j<overlap;j++)

+         out_mem[c][j] = x[j] + overlap_mem[c][j];

+      for (;j<N;j++)

+         out_mem[c][j] = x[j];

+      for (j=0;j<overlap;j++)

+         overlap_mem[c][j] = x[N+j];

    } while (++c<C);

+   RESTORE_STACK;

 }

 static void deemphasis(celt_sig *in[], opus_val16 *pcm, int N, int _C, int downsample, const opus_val16 *coef, celt_sig *mem)

--- a/libcelt/mdct.c

+++ b/libcelt/mdct.c

@@ -202,7 +202,8 @@

    RESTORE_STACK;

 }

-void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar * restrict out, const opus_val16 * restrict window, int overlap, int shift)

+void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar * restrict out,

+      const opus_val16 * restrict window, int overlap, int shift, int stride)

 {

    int i;

    int N, N2, N4;

@@ -227,7 +228,7 @@

    {

       /* Temp pointers to make it really clear to the compiler what we're doing */

       const kiss_fft_scalar * restrict xp1 = in;

-      const kiss_fft_scalar * restrict xp2 = in+N2-1;

+      const kiss_fft_scalar * restrict xp2 = in+stride*(N2-1);

       kiss_fft_scalar * restrict yp = f2;

       const kiss_twiddle_scalar *t = &l->trig[0];

       for(i=0;i<N4;i++)

@@ -238,8 +239,8 @@

          /* works because the cos is nearly one */

          *yp++ = yr - S_MUL(yi,sine);

          *yp++ = yi + S_MUL(yr,sine);

-         xp1+=2;

-         xp2-=2;

+         xp1+=2*stride;

+         xp2-=2*stride;

       }

    }

--- a/libcelt/mdct.h

+++ b/libcelt/mdct.h

@@ -60,6 +60,7 @@

 /** Compute a backward MDCT (no scaling) and performs weighted overlap-add

     (scales implicitly by 1/2) */

-void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar *out, const opus_val16 * restrict window, int overlap, int shift);

+void clt_mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar *out,

+      const opus_val16 * restrict window, int overlap, int shift, int stride);

 #endif

--- a/libcelt/tests/mdct-test.c

+++ b/libcelt/tests/mdct-test.c

@@ -122,7 +122,7 @@

     {

        for (k=0;k<nfft;++k)

           out[k] = 0;

-       clt_mdct_backward(&cfg,in,out, window, nfft/2, 0);

+       clt_mdct_backward(&cfg,in,out, window, nfft/2, 0, 1);

        check_inv(in,out,nfft,isinverse);

     } else {

        clt_mdct_forward(&cfg,in,out,window, nfft/2, 0);