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Avoid rescanning invalid regions during file open.

If there is a large region of invalid data in the middle of the
 file, avoid scanning it repeatedly during chain boundary
 enumeration.
Because of the bisection, doing that would only be O(n*log(n)), not
 quadratic like op_get_last_page(), but still good to avoid.

--- a/src/opusfile.c

+++ b/src/opusfile.c

@@ -1189,6 +1189,7 @@

     /*We guard against garbage separating the last and first pages of two

        links below.*/

     while(_searched<end_searched){

+      opus_int64 boundary;

       opus_int32 next_bias;

       /*If we don't have a better estimate, use simple bisection.*/

       if(bisect==-1)bisect=_searched+(end_searched-_searched>>1);

@@ -1199,7 +1200,14 @@

       else end_gp=-1;

       ret=op_seek_helper(_of,bisect);

       if(OP_UNLIKELY(ret<0))return ret;

-      last=op_get_next_page(_of,&og,_sr[nsr-1].offset);

+      /*If there is a large region of invalid data in the middle of the file,

+         avoid scanning it repeatedly.

+        Because of the bisection, doing that would only be O(n*log(n)), not

+         quadratic like op_get_last_page(), but still good to avoid.*/

+      OP_ASSERT(end_searched<=_sr[nsr-1].search_start);

+      boundary=OP_MIN(_sr[nsr-1].offset,

+       OP_ADV_OFFSET(end_searched,OP_PAGE_SIZE_MAX-1));

+      last=op_get_next_page(_of,&og,boundary);

       if(OP_UNLIKELY(last<OP_FALSE))return (int)last;

       next_bias=0;

       if(last==OP_FALSE)end_searched=bisect;

-- 

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