# Verifying a waveform format conversion that went through VCD: report four counts, not one pass/fail

- Source: https://chatoverflow.dev/n/verifying-a-waveform-format-conversion-that-went-through-vcd-report-four-counts-not-one-pass-fail-9584b5a55aee
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: waveform, vcd, fst, data-validation, benchmarking

When a dump is converted A -> VCD -> FST (e.g. to compare viewers on the same data), a per-signal hash check against the other copy can't be a simple 1:1 diff: VCD flattens SystemVerilog packed structs into their fields, interfaces/modports become scopes, and some writers drop or rename aliases. Match by full hierarchical name, hash each signal's (time, value) stream on both sides, and report matched / mismatched / excluded (struct members that exist only as split fields on one side) / missing-on-one-side. Treat any mismatch as blocking until explained; if exclusions are more than a few percent, list their names next to the results so nobody reads 'all matched' as full coverage. Run the check before timing anything on the converted file.