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Improve default output length calculation for effects

As audio length within the effects chain is given in terms of (non-wide)
samples, it changes for effects that modify the number of channels or
the sample rate even if the actual time for the audio remains the same.

Account for this when calculating a default output length for effects
that have SOX_EFF_CHAN and/or SOX_EFF RATE set, but not SOX_EFF_LENGTH.

--- a/src/effects.c

+++ b/src/effects.c

@@ -147,10 +147,7 @@

         in->precision : SOX_SAMPLE_PRECISION;

   if (!(effp->handler.flags & SOX_EFF_GAIN))

     effp->out_signal.mult = in->mult;

-  if (!(effp->handler.flags & SOX_EFF_LENGTH))

-    effp->out_signal.length = in->length;

-

   effp->flows =

     (effp->handler.flags & SOX_EFF_MCHAN)? 1 : effp->in_signal.channels;

   effp->clips = 0;

@@ -171,6 +168,16 @@

   }

   if (in->mult)

     lsx_debug("mult=%g", *in->mult);

+

+  if (!(effp->handler.flags & SOX_EFF_LENGTH)) {

+    effp->out_signal.length = in->length;

+    if (effp->handler.flags & SOX_EFF_CHAN)

+      effp->out_signal.length =

+        effp->out_signal.length / in->channels * effp->out_signal.channels;

+    if (effp->handler.flags & SOX_EFF_RATE)

+      effp->out_signal.length =

+        effp->out_signal.length / in->rate * effp->out_signal.rate + .5;

+  }

   *in = effp->out_signal;