This is makeindex, version 2.15 [TeX Live 2020] (kpathsea + Thai support).
Scanning input file output/book.idx....done (345 entries accepted, 0 rejected).
Sorting entries......done (3076 comparisons).
Generating output file output/book.ind....done (401 lines written, 0 warnings).
Output written in output/book.ind.
Transcript written in output/book.ilg.