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trim: only warn about positions beyond end of file

If the user specifies positions after the input audio length, warn but
don't fail; they might intend "at most ..." semantics.

It's especially important for cases like the man page example
  sox infile.wav outfile.wav trim 0 30 : newfile : restart
that would otherwise fail if the input length is not exactly a
multiple of 30 seconds.

--- a/src/trim.c

+++ b/src/trim.c

@@ -92,11 +92,12 @@

       case a_start: res = s; break;

       case a_latest: res = last_seen + s; break;

       case a_end:

-        if (s > in_length) {

+        if (s <= in_length)

+          res = in_length - s;

+        else {

           lsx_fail("Position %u is before start of audio.", i+1);

           return SOX_EOF;

         }

-        res = in_length - s;

         break;

     }

     last_seen = p->pos[i].sample = res;

@@ -112,16 +113,11 @@

     }

     last_seen = p->pos[i].sample;

   }

-  if (p->num_pos && in_length != SOX_UNKNOWN_LEN &&

-      p->pos[0].sample > in_length) {

-    lsx_fail("Start position after end of audio.");

-    return SOX_EOF;

-  }

-  if (p->num_pos && in_length != SOX_UNKNOWN_LEN &&

-      p->pos[p->num_pos-1].sample > in_length) {

-    lsx_fail("End position after end of audio.");

-    return SOX_EOF;

-  }

+  if (p->num_pos && in_length != SOX_UNKNOWN_LEN)

+    if (p->pos[0].sample > in_length ||

+        p->pos[p->num_pos-1].sample > in_length)

+      lsx_warn("%s position after expected end of audio.",

+          p->pos[0].sample > in_length? "Start" : "End");

   if (p->num_pos == 1 && !p->pos[0].sample)

     return SOX_EFF_NULL;

@@ -134,10 +130,10 @@

     effp->out_signal.length = 0;

     for (i = 0; i+1 < p->num_pos ; i += 2)

       effp->out_signal.length +=

-        p->pos[i+1].sample - p->pos[i].sample;

+        min(p->pos[i+1].sample, in_length) - min(p->pos[i].sample, in_length);

     if (open_end)

       effp->out_signal.length +=

-        in_length - p->pos[p->num_pos-1].sample;

+        in_length - min(p->pos[p->num_pos-1].sample, in_length);

     effp->out_signal.length *= effp->in_signal.channels;

   }