This is makeindex, version 2.15 [TeX Live 2019] (kpathsea + Thai support).

Scanning input file main.idx....done (21 entries accepted, 0 rejected).
Sorting entries....done (100 comparisons).
Generating output file main.ind....done (85 lines written, 0 warnings).
Output written in main.ind.
Transcript written in main.ilg.