// Package i18n is the hub's customer-facing message bundle: one flat key → text map per language,
// embedded in the binary, Hungarian first and authoritative.
//
// Design: felhom.eu/documentation/architecture/10-localisation.md, slice 3 (R-558).
//
// THIS BUNDLE IS FOR THE HOUSEHOLD, NOT THE OPERATOR. Everything the operator reads — the hub's own
// pages, its logs, and FormatOperatorEmail — stays exactly as it is, in the language it is already
// in. A key that would change an operator surface does not belong here.
//
// Three rules, the same three the controller's bundle follows:
//
//  1. HUNGARIAN IS THE SOURCE. hu.json holds every key. A key missing from hu is a programming
//     error and Msg says so loudly rather than rendering a blank line into a customer's mail.
//  2. FALLBACK IS HUNGARIAN, AND IT IS COUNTED. A key absent from en.json renders the Hungarian and
//     is reported by MissingKeys, which the gate holds at zero. A household never sees a key name
//     or an empty space where a sentence should be.
//  3. THE BOX'S OWN SENTENCES ARE NOT IN HERE. Roughly a third of the customer mails carry a
//     sentence composed on the box ("the remote target is not set"). The hub cannot translate those
//     and does not try: the box sends the household's version beside the Hungarian one
//     (`message_customer`), and the hub picks. See FormatCustomerEmail.
//
// Unlike the controller's bundle there is no template expansion here: a mail is plain text, so the
// contextual-escaping problem that shaped the controller's design does not exist. Msg is a plain
// lookup and Msgf formats with the Go verbs the Hungarian already uses.
package i18n

import (
	"embed"
	"encoding/json"
	"fmt"
	"io/fs"
	"sort"
	"strings"
	"sync"
)

//go:embed locales/*.json
var localeFS embed.FS

// Default is the language of a household that never chose one — and the only language every key has.
const Default = "hu"

// Supported lists the languages a household may be written to in, Default first.
var Supported = []string{"hu", "en"}

// Bundle holds every language's messages.
type Bundle struct {
	msgs map[string]map[string]string
}

// Normalize maps any input to a supported language; everything unknown is Default.
//
// This is the ONLY gate between a reported language and a rendered mail. A box that reports
// garbage, an old box that reports nothing, and a box that reports "EN " all land on a real
// language rather than on an empty lookup.
func Normalize(lang string) string {
	l := strings.ToLower(strings.TrimSpace(lang))
	for _, s := range Supported {
		if l == s {
			return s
		}
	}
	return Default
}

// IsSupported reports whether lang is exactly one of Supported. Used where an input must be
// REFUSED rather than coerced — the customer-creation form, which should not silently store `hu`
// for an operator who typed something else.
func IsSupported(lang string) bool {
	for _, s := range Supported {
		if lang == s {
			return true
		}
	}
	return false
}

// Load reads the embedded bundles.
func Load() (*Bundle, error) { return loadFrom(localeFS) }

func loadFrom(fsys fs.FS) (*Bundle, error) {
	entries, err := fs.Glob(fsys, "locales/*.json")
	if err != nil {
		return nil, err
	}
	b := &Bundle{msgs: make(map[string]map[string]string, len(entries))}
	for _, e := range entries {
		raw, err := fs.ReadFile(fsys, e)
		if err != nil {
			return nil, fmt.Errorf("reading %s: %w", e, err)
		}
		var m map[string]string
		if err := json.Unmarshal(raw, &m); err != nil {
			return nil, fmt.Errorf("parsing %s: %w", e, err)
		}
		lang := strings.TrimSuffix(strings.TrimPrefix(e, "locales/"), ".json")
		b.msgs[lang] = m
	}
	if _, ok := b.msgs[Default]; !ok {
		return nil, fmt.Errorf("i18n: no %s.json — Hungarian is the source and must exist", Default)
	}
	return b, nil
}

var (
	sharedOnce sync.Once
	shared     *Bundle
	sharedErr  error
)

// Shared returns the process-wide bundle, loaded once.
//
// It panics if the embedded bundle cannot be parsed. That is deliberate and it is a BUILD-TIME
// class of fault, not a runtime one: the JSON is compiled into the binary, so a parse failure means
// every mail this binary will ever send is broken. Failing at the first send, loudly, beats sending
// a thousand mails with blank bodies.
func Shared() *Bundle {
	sharedOnce.Do(func() { shared, sharedErr = Load() })
	if sharedErr != nil {
		panic("i18n: embedded bundle is unloadable: " + sharedErr.Error())
	}
	return shared
}

// Msg returns the message for key in lang, falling back to Hungarian.
//
// A key that exists in NO language returns a visible marker rather than an empty string. An empty
// string in a mail body is invisible — the customer gets a mail with a hole in it and nobody ever
// learns. The marker is ugly on purpose.
func (b *Bundle) Msg(lang, key string) string {
	if b == nil {
		return "!" + key + "!"
	}
	if m, ok := b.msgs[Normalize(lang)]; ok {
		if v, ok := m[key]; ok && v != "" {
			return v
		}
	}
	if m, ok := b.msgs[Default]; ok {
		if v, ok := m[key]; ok && v != "" {
			return v
		}
	}
	return "!" + key + "!"
}

// Msgf formats the message for key in lang with args.
//
// Word order is the reason this exists rather than string concatenation at the call site: English
// reorders what Hungarian does not, and a translation reorders with Go's explicit argument indexes
// (`%[2]s`) inside its own value. The Hungarian value is then free to stay exactly the format
// string it always was, which is what makes the goldens hold.
func (b *Bundle) Msgf(lang, key string, args ...any) string {
	return fmt.Sprintf(b.Msg(lang, key), args...)
}

// Has reports whether lang carries key in its own file (no fallback).
func (b *Bundle) Has(lang, key string) bool {
	if b == nil {
		return false
	}
	m, ok := b.msgs[lang]
	if !ok {
		return false
	}
	v, ok := m[key]
	return ok && v != ""
}

// Keys lists lang's own keys, sorted.
func (b *Bundle) Keys(lang string) []string {
	if b == nil {
		return nil
	}
	m := b.msgs[lang]
	out := make([]string, 0, len(m))
	for k := range m {
		out = append(out, k)
	}
	sort.Strings(out)
	return out
}

// MissingKeys lists the Hungarian keys that lang does not carry, sorted.
//
// The gate holds this at zero for every supported language. It is the difference between "we
// translated it" and "we believe we translated it".
func (b *Bundle) MissingKeys(lang string) []string {
	if b == nil {
		return nil
	}
	var out []string
	for _, k := range b.Keys(Default) {
		if !b.Has(lang, k) {
			out = append(out, k)
		}
	}
	sort.Strings(out)
	return out
}