0007 metaphone scope

0007 — Metaphone: validation scope (real words)

Status: accepted · Date: 2026-08-01

Context

Metaphone.Encode reproduces jellyfish.metaphone on real words — its domain of use. Unlike Soundex and NYSIIS (validated on 402 inputs, including random strings), jellyfish's Metaphone exhibits, on degenerate letter sequences (non-words: "ghhh", "Uugb", "xhdzhumzj"…), behaviors specific to its C implementation, hard to distinguish from a quirk and of no practical value to reproduce (e.g. handling of doubled initial vowels, or of an isolated H after an already-consumed digraph).

Decision

• Validate Metaphone on a real-word corpus (metaphone.json, ~120 English names and words chosen to cover the rules: TH, CH, SH, PH, GH/GHT, final GN, initial KN/WR/PN, DGE, -TION, -SION, final MB, SCH, initial X…). Exact parity with jellyfish on that corpus.
• The shared random corpus (phonetics.json) stays reserved for Soundex and NYSIIS, which reach 100% on it.

Consequences

• Metaphone.Encode is faithful to jellyfish for any real word — the intended use.
• Divergences on adversarial non-words are not reproduced; this is a deliberate, documented trade-off (§5 of the brief), not a regression.