package i18n

import (
	"context"
	"strconv"
	"strings"
)

// Default is the fallback UI/API locale when Accept-Language is missing or unsupported.
const Default = "en"

// Supported UI/API locales for public error/validation copy.
// Keep aligned with apps/web/src/lib/i18n/locales.ts (UI_LOCALES).
var Supported = []string{"en", "es", "fr", "de", "it", "pt", "nl", "pl", "ja"}

var supportedSet map[string]struct{}

func init() {
	supportedSet = make(map[string]struct{}, len(Supported))
	for _, code := range Supported {
		supportedSet[code] = struct{}{}
	}
}

type ctxKey struct{}

// WithLocale stores a resolved locale on ctx.
func WithLocale(ctx context.Context, locale string) context.Context {
	return context.WithValue(ctx, ctxKey{}, Normalize(locale))
}

// FromContext returns the locale stored by middleware, or Default.
func FromContext(ctx context.Context) string {
	if ctx == nil {
		return Default
	}
	if v, ok := ctx.Value(ctxKey{}).(string); ok && v != "" {
		return v
	}
	return Default
}

// Normalize lowercases/trims and maps to a supported primary language tag, or Default.
func Normalize(raw string) string {
	code := strings.ToLower(strings.TrimSpace(raw))
	if code == "" || code == "*" {
		return Default
	}
	if i := strings.IndexByte(code, '-'); i > 0 {
		code = code[:i]
	}
	if i := strings.IndexByte(code, '_'); i > 0 {
		code = code[:i]
	}
	if _, ok := supportedSet[code]; ok {
		return code
	}
	return Default
}

// IsSupported reports whether the primary language tag is in Supported.
func IsSupported(raw string) bool {
	code := strings.ToLower(strings.TrimSpace(raw))
	if code == "" {
		return false
	}
	if i := strings.IndexByte(code, '-'); i > 0 {
		code = code[:i]
	}
	if i := strings.IndexByte(code, '_'); i > 0 {
		code = code[:i]
	}
	_, ok := supportedSet[code]
	return ok
}

// Resolve picks the best supported locale from an Accept-Language header value.
// Quality values are respected; unsupported tags are skipped; empty → Default.
func Resolve(acceptLanguage string) string {
	header := strings.TrimSpace(acceptLanguage)
	if header == "" {
		return Default
	}
	bestTag := ""
	bestQ := -1.0
	for _, part := range strings.Split(header, ",") {
		part = strings.TrimSpace(part)
		if part == "" {
			continue
		}
		tag := part
		q := 1.0
		if i := strings.IndexByte(part, ';'); i >= 0 {
			tag = strings.TrimSpace(part[:i])
			for _, p := range strings.Split(part[i+1:], ";") {
				p = strings.TrimSpace(p)
				if len(p) >= 2 && (p[0] == 'q' || p[0] == 'Q') && p[1] == '=' {
					if parsed, err := strconv.ParseFloat(strings.TrimSpace(p[2:]), 64); err == nil {
						q = parsed
					}
				}
			}
		}
		primary := strings.ToLower(strings.TrimSpace(tag))
		if primary == "*" {
			if q > bestQ {
				bestQ = q
				bestTag = Default
			}
			continue
		}
		if !IsSupported(primary) {
			continue
		}
		norm := Normalize(primary)
		if q > bestQ {
			bestQ = q
			bestTag = norm
		}
	}
	if bestTag == "" {
		return Default
	}
	return bestTag
}