Convert gtfs_transfer_table to C++

[mpadge]

Coz's it's all way too slow, and very easily C++-able.Ref #14

[mpadge]

Before translation to C++

library (gtfsrouter)
packageVersion ("gtfsrouter")
#> [1] '0.0.5.1'
gtfs <- extract_gtfs ("./feeds/vbb.zip")
#> ▶ Unzipping GTFS archive
#> ✔ Unzipped GTFS archive
#> ▶ Extracting GTFS feed✔ Extracted GTFS feed 
#> ▶ Converting stop times to seconds✔ Converted stop times to seconds 
#> ▶ Converting transfer times to seconds✔ Converted transfer times to seconds
system.time (
    tr <- gtfs_transfer_table (gtfs)
)
#> ▶ Finding neighbouring services for each stop
#> Loading required namespace: pbapply
#> ✔ Found neighbouring services for each stop
#> ▶ Expanding to include in-place transfers
#> ✔ Expanded to include in-place transfers
#>    user  system elapsed 
#> 109.298   1.418 110.357

^{Created on 2021-06-28 by the reprex package (v2.0.0.9000)}

After translation:

library (gtfsrouter)
packageVersion ("gtfsrouter")
#> [1] '0.0.5.4'
gtfs <- extract_gtfs ("./feeds/vbb.zip")
#> ▶ Unzipping GTFS archive
#> ✔ Unzipped GTFS archive
#> ▶ Extracting GTFS feed✔ Extracted GTFS feed 
#> ▶ Converting stop times to seconds✔ Converted stop times to seconds 
#> ▶ Converting transfer times to seconds✔ Converted transfer times to seconds
system.time (
    tr <- gtfs_transfer_table (gtfs)
)
#> ▶ Finding neighbouring services for each stop✔ Found neighbouring services for each stop
#>    user  system elapsed 
#>   9.252   0.831   9.461

dim(tr)
#> [1] 14170   4

^{Created on 2021-06-28 by the reprex package (v2.0.0.9000)}

More than 10 times faster for this relatively huge transfer table with 14,170 transfers. Now finally entirely efficient enough to be generally useful.

[mpadge]

Re-opening because current code just makes new transfer table, but does not append to original table if present, as in this code from @AlexandraKapp. Next commit will insert lines to append new values to old.