Low: Event index order non-deterministic (map iteration)

[metis-sprock]

Description

The footer's event index is built by iterating over a Go map, which has intentionally randomized iteration order. This means the event index section of the footer will have different ordering between writes of the same data, making tape files non-reproducible.

Evidence

File: pkg/codec/tape.go:114-121

// Writing the footer event index:
for eventType, offsets := range w.eventIndex {
    // map iteration order is randomized in Go
    // footer bytes differ between identical writes
}

Go's map iteration order is randomized by design (since Go 1.12). Two identical tape files written from the same data will produce different footer bytes.

Impact

Non-reproducible output. Byte-for-byte comparison of tape files fails even with identical input. This affects:

• Checksumming / integrity verification
• Diffing tape files
• Test determinism

Suggested Fix

Sort the event types before writing:

types := make([]string, 0, len(w.eventIndex))
for t := range w.eventIndex {
    types = append(types, t)
}
sort.Strings(types)
for _, t := range types {
    offsets := w.eventIndex[t]
    // write in deterministic order
}

🤖 Filed by Metis