$ ./tdriver big1.txt big2.txt big3.txt big4.txt big5.txt big6.txt >f.tmp
tdriver (fixed string length=6) start
use phmap::parallel_flat_hash_map
big1.txt: nlines=3515200 (0.597 seconds)
big2.txt: nlines=3515200 (1.08 seconds)
big3.txt: nlines=3515200 (1.604 seconds)
big4.txt: nlines=3515200 (0.795 seconds)
big5.txt: nlines=3515200 (2.317 seconds)
big6.txt: nlines=3515200 (0.894 seconds)
$ cmp f.tmp good.tmp

$ ./tdriver big1.txt big2.txt big3.txt big4.txt big5.txt big6.txt >f.tmp
tdriver (fixed string length=6) start
use absl::flat_hash_map
big1.txt: nlines=3515200 (0.568 seconds)
big2.txt: nlines=3515200 (0.626 seconds)
big3.txt: nlines=3515200 (1.067 seconds)
big4.txt: nlines=3515200 (0.767 seconds)
big5.txt: nlines=3515200 (1.057 seconds)
big6.txt: nlines=3515200 (0.820 seconds)
$ cmp f.tmp good.tmp