Use std::map for faster lookup

[kevinbackhouse]

OSS-Fuzz reported a timeout. It's not serious problem - just a moderately large file that's a bit slow to process. But I noticed that it's spending a lot of time in this line of code:

exiv2/src/tiffimage_int.cpp

Line 1811 in ee5dae4

const TiffGroupStruct* ts = find(tiffGroupStruct_, TiffGroupStruct::Key(extendedTag, group));

It's doing a linear search of a 358-element array, which is a bit slow. This PR replaces the array with a std::map, which improves the running time on this particular file by approximately 10%.

poc:

time exiv2 poc.jpg

[codecov]

Codecov Report

Merging #2299 (7498fa6) into main (1d0530f) will increase coverage by 0.00%.
The diff coverage is 84.44%.

@@           Coverage Diff           @@
##             main    #2299   +/-   ##
=======================================
  Coverage   63.53%   63.54%           
=======================================
  Files         118      118           
  Lines       19595    19597    +2     
  Branches     9576     9579    +3     
=======================================
+ Hits        12450    12452    +2     
  Misses       5079     5079           
  Partials     2066     2066           

Impacted Files	Coverage Δ
src/tiffimage_int.cpp	78.63% <83.72%> (+0.29%)	⬆️
src/tiffimage_int.hpp	86.66% <100.00%> (-0.84%)	⬇️

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[neheb]

On a somewhat related note, I wonder if it was a mistake to replace function pointers with std::function...

[neheb]

Can this whole struct be replaced with

using TiffImgTagStruct = std::pair<uint16_t, IfdId>;

?

[kevinbackhouse]

Yes, I agree. And this looks inefficient too:

exiv2/src/tiffimage_int.cpp

Line 2118 in ee5dae4

return find(tiffImageTags, TiffImgTagStruct::Key(tag, group));

I think those changes should go in a separate PR though.

[neheb]

From what I can tell, this find function is a wrapper over std::find.

[neheb]

 Error:  Failed to execute: D:\a\exiv2\exiv2\build\bin\iotest s0 s1 s2 http://127.0.0.1:12760/table.jpg
Error:  The asserted return code is [0], but got 1

[kevinbackhouse]

I think I need to investigate why one of the tests failed.