Merge pull request #903 from openzim/fix_locate

Correctly locate part range in which read data.

src/file_compound.h

@@ -87,20 +87,16 @@ class LIBZIM_PRIVATE_API FileCompound : private std::map<Range, FilePart*, less_
  }
 
  PartRange locate(offset_t offset, zsize_t size) const {
-#if ! defined(__APPLE__)
- return equal_range(Range(offset, offset+size));
-#else
- // Workaround for https://github.com/openzim/libzim/issues/398
- // Under MacOS the implementation of std::map::equal_range() makes
- // assumptions about the properties of the key comparison function and
- // abuses the std::map requirement that it must contain unique keys. As
- // a result, when a map m is queried with an element k that is
- // equivalent to more than one keys present in m,
- // m.equal_range(k).first may be different from m.lower_bound(k) (the
- // latter one returning the correct result).
- const Range queryRange(offset, offset+size);
- return {lower_bound(queryRange), upper_bound(queryRange)};
-#endif // ! defined(__APPLE__)
+ const Range queryRange(offset, offset+size);
+ // equal_range expects comparator to satisfy the `Compare` requirement.
+ // (ie `comp(a, b) == !comp(b, a)`) which is not the case for `less_range`
+ // If not satisfy, this is UB.
+ // Even if UB, stdlib's equal_range behaves "correctly".
+ // But libc++ (used in Apple, Android, ..) is not.
+ // In all case, we are triggering a UB and it is to us to not call equal_range.
+ // So let's use lower_bound and upper_bound which doesn't need such requirement.
+ // See https://stackoverflow.com/questions/67042750/should-setequal-range-return-pair-setlower-bound-setupper-bound
+ return {lower_bound(queryRange), upper_bound(queryRange)};
  }
 
  private: // functions