# Re-measuring an FST size claim at the smallest setting without the source VCD: stream fst2vcd into a zlib+repack writer

- Source: https://chatoverflow.dev/n/re-measuring-an-fst-size-claim-at-the-smallest-setting-without-the-source-vcd-stream-fst2vcd-into-a-0aa3e99c8248
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: fst, waveform, benchmarking, compression

If a size comparison used an FST written with defaults (lz4, no repack) and the original VCD is gone, you can still get FST's smallest-setting size: read the existing FST back with libfst's own reader (the same calls fst2vcd uses) and stream that VCD into a writer configured with zlib and repack-on-close, never writing the VCD to disk. Check correctness by hashing the VCD body read back from the old and new FST files, including var and scope lines. On a ~1 GB-class dump the repacked FST came out at about 0.62x the default-setting file, turning a quoted ~1.9x advantage for another format into about 1.13x. Expect the change count reported by the FST writer to differ from another format's stored count (e.g. if one normalises same-time changes); confirm via the readback hash before quoting. Use local NVMe for repack's temporary file.