package kiwix

import (
	"strings"
	"unicode"

	"github.com/kami/maven/internal/lexicon"
	"github.com/kami/maven/internal/morph"
)

// Topic reduces a question to the thing it is about, because Kiwix ranks by
// keyword overlap and a whole sentence buries the keyword that matters.
//
// This package's own doc says it: "why is the sky blue" finds a TV episode.
// Measured against the Russian ZIM on 2026-08-09, the sentence and the topic
// return different articles for the same question. "кто написал Войну и мир"
// returns "Радуйся, мир (Доктор Кто)"; "Войну и мир" returns the novel first.
// "что такое TCP" returns "Перехват TCP-соединения"; "TCP" returns TCP. The
// English path had a rewriter doing this with a model call. The Russian path
// reads the book verbatim (V-508) and had nothing.
//
// It drops three things off the front and stops: the narrative request, the
// interrogative, and a verb sitting between them and the noun. Everything else
// is kept, because a word this cannot classify is more likely the topic than
// noise. An empty return means the utterance was question words alone, and the
// caller searches the sentence as before.
func Topic(utterance string) string {
	words := strings.Fields(strings.TrimSpace(utterance))
	cut := 0
	for cut < len(words) {
		w := strings.Trim(strings.ToLower(words[cut]), ".,!?…:;\"'«»")
		if w == "" {
			cut++
			continue
		}
		switch {
		case inList(lexicon.NarrativeRequests(), w),
			inList(lexicon.Interrogatives(), w),
			inList(lexicon.FirstPerson(), w),
			// "что ТАКОЕ x", "кто ТАКОЙ x" — the copula that only ever follows
			// an interrogative, and never a topic on its own.
			cut > 0 && isCopula(w),
			// "расскажи ПРО x", "о x". One-letter and two-letter prepositions
			// are not a closed class worth a lexicon set of their own.
			cut > 0 && isLeadingPreposition(w),
			// "кто НАПИСАЛ Войну и мир". A verb here is the question's own
			// verb, not part of the title. Only after something was already
			// dropped, so "написал отчёт" as a topic survives intact.
			cut > 0 && morph.IsVerbForm(w):
			cut++
		default:
			// The question mark is the sentence's, not the title's, and Kiwix
			// carries it into the keyword match.
			topic := strings.TrimRight(strings.Join(words[cut:], " "), " .,!?…:;\"'«»")
			if !hasLetter(topic) {
				return ""
			}
			return topic
		}
	}
	return ""
}

// TitleCandidates is the topic as it might be titled, best first.
//
// A ZIM title is capitalized and the utterance is not: measured on 2026-08-09,
// `/A/фотосинтез` is a 404 and `/A/Фотосинтез` is a 200. The spoken form is
// tried first anyway, because a title that begins lowercase on purpose
// ("iPhone") would not survive capitalizing it. Both are one request each
// against a server on the same box, and a miss is a 404 rather than a wrong
// article.
func TitleCandidates(topic string) []string {
	if topic == "" {
		return nil
	}
	r := []rune(topic)
	up := unicode.ToUpper(r[0])
	if up == r[0] {
		return []string{topic}
	}
	return []string{topic, string(up) + string(r[1:])}
}

func isCopula(w string) bool {
	switch w {
	case "такое", "такой", "такая", "такие", "is", "are", "was", "were":
		return true
	}
	return false
}

func isLeadingPreposition(w string) bool {
	switch w {
	case "про", "о", "об", "обо", "по", "about", "of", "on":
		return true
	}
	return false
}

func inList(list []string, w string) bool {
	for _, x := range list {
		if x == w {
			return true
		}
	}
	return false
}

// hasLetter is the guard against a topic that reduced to punctuation or digits
// alone, which no ZIM title matches.
func hasLetter(s string) bool {
	for _, r := range s {
		if unicode.IsLetter(r) {
			return true
		}
	}
	return false
}