package llm

// Which language a pass corrects, and which language it explains in.
//
// Until Phase 28 there was no question to answer: every prompt was written
// around English prose explained in English, and the pair language reached her
// only when she asked for it (Ask Petal, the explanation translator). That is
// the right default for a writer practising English and the wrong one for a
// document she wrote in her own language, where Petal would read Portuguese,
// say nothing about it, and file a mechanics note about the one English
// sentence at the end.
//
// The two fields are two different decisions reading two different pieces of
// state, and collapsing them would be the bug:
//
//   - Correct follows the DOCUMENT. Portuguese prose gets Portuguese
//     corrections; that is the whole point.
//   - Explain follows the WRITER — the half of her pair she is *not* learning
//     (users.direction), because an explanation is teaching, and teaching lands
//     in the language she reads most easily.
//
// Today those two coincide for every account that exists: `learnerPairs` is
// {"zh"}, so fr, es and pt-PT writers are all `learning_en` and their
// non-learned half *is* the pair language. That equality is a fact about
// today's roster, not about the design — the same shape of assumption that had
// to be unpicked from `pair_lang` in migration 0016. Keep them apart.
type Target struct {
	// Correct is the language the writing is in, and so the language both
	// `original` and `replacement` must be written in.
	Correct Lang
	// Explain is the language each explanation is written in.
	Explain Lang
	// Pair is the writer's pair language regardless of what this document is
	// written in. The collocation coach's parenthetical gloss is addressed to
	// her rather than to the document, so it reads this and not Correct.
	Pair Lang
}

// English as the prompts name it. Not in `langs`: that map answers "which
// language is the writer's half of the pair", and English is the constant on
// the other side of every pair Petal supports.
var English = Lang{Code: "en", Name: "English", Why: "why"}

// EnglishTarget is the pre-Phase-28 behaviour, made explicit: an English
// document, corrected and explained in English, for a writer whose pair
// language is `pair`. Every existing user is on this path and the prompt it
// produces is byte-identical to the one that shipped before this phase.
func EnglishTarget(pair Lang) Target {
	return Target{Correct: English, Explain: English, Pair: pair}
}

// Flipped reports whether this document is in the pair language rather than in
// English — i.e. whether the pass is reading her own language.
func (t Target) Flipped() bool { return t.Correct.Code != English.Code }