$ ./tdriver big1.txt big2.txt big3.txt big4.txt big5.txt big6.txt >f.tmp
tdriver (fixed string length=6) start
use std::map
big1.txt: nlines=3515200 (7.492 seconds)
big2.txt: nlines=3515200 (9.953 seconds)
big3.txt: nlines=3515200 (11.089 seconds)
big4.txt: nlines=3515200 (10.696 seconds)
big5.txt: nlines=3515200 (11.449 seconds)
big6.txt: nlines=3515200 (11.765 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 std::unordered_map
big1.txt: nlines=3515200 (2.134 seconds)
big2.txt: nlines=3515200 (2.859 seconds)
big3.txt: nlines=3515200 (2.244 seconds)
big4.txt: nlines=3515200 (3.666 seconds)
big5.txt: nlines=3515200 (2.184 seconds)
big6.txt: nlines=3515200 (1.882 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 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 boost::unordered_map
big1.txt: nlines=3515200 (2.1 seconds)
big2.txt: nlines=3515200 (2.037 seconds)
big3.txt: nlines=3515200 (1.327 seconds)
big4.txt: nlines=3515200 (2.935 seconds)
big5.txt: nlines=3515200 (1.436 seconds)
big6.txt: nlines=3515200 (1.553 seconds)
$ cmp f.tmp good.tmp

UPDATE:
$ ./tdriver big1.txt big2.txt big3.txt big4.txt big5.txt big6.txt >f.tmp
tdriver (fixed string length=6) start
use ankerl::unordered_dense::map
big1.txt: nlines=3515200 (0.994 seconds)
big2.txt: nlines=3515200 (1.11 seconds)
big3.txt: nlines=3515200 (0.767 seconds)
big4.txt: nlines=3515200 (1.784 seconds)
big5.txt: nlines=3515200 (0.866 seconds)
big6.txt: nlines=3515200 (0.772 seconds)
$ cmp f.tmp good.tmp