Use starts_with() instead of substr(0, N) getting and comparing to prefix

[DmitryAk]

Issue

There were several places in code such as:

valhalla/src/mjolnir/pbfgraphparser.cc

Line 1815 in e598c09

else if (tag_.first.substr(0, 20) == "motorcar:conditional" ||

It seems not very reliable and not very performant:

1. It is required to specify the exact prefix size in substr(0, N). If there is a mistake - the result of comparing the substr with prefix will always be false. It looks not very convenient and not very reliable.
2. It always creates an auxiliary substring, and this substring is compared to the prefix (so even in best case this requires O(N), where N is prefix size). At the same time it is possible that starts_with can just compare the first chars - and reject further comparing (so in best case this requires O(1)). Also starts_with doesn't require unnecessary auxiliary substring (so no unnecessary allocation is guaranteed). To check this, a benchmark test was introduced in this PR, results (Release configuration, x86_64, clang-1316.0.21.2.5):

With substr() it took 8224 ms
With starts_with() it took 1963 ms

Tasklist

• Add tests
• Update the changelog

[kevinkreiser]

Looks good to me just waiting for the build to pass