galaxy-game/backend/internal/diplomail/translator/libretranslate.go

package translator

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"strings"
	"time"
)

// LibreTranslateEngine is the engine identifier persisted in
// `diplomail_translations.translator` for cache rows produced by the
// LibreTranslate client.
const LibreTranslateEngine = "libretranslate"

// LibreTranslateConfig configures the HTTP client. URL is the base
// of the deployed instance (without `/translate`). Timeout bounds a
// single HTTP request; the worker layers retry / backoff on top.
type LibreTranslateConfig struct {
	URL     string
	Timeout time.Duration
}

// ErrUnsupportedLanguagePair classifies a LibreTranslate 400 response
// that indicates the engine cannot translate between the requested
// source / target codes. The worker treats this as terminal: no
// further retries, deliver the original.
var ErrUnsupportedLanguagePair = errors.New("translator: language pair not supported by libretranslate")

// NewLibreTranslate constructs a Translator that posts to
// `<URL>/translate`. Returns an error when URL is empty so wiring
// catches "translator misconfigured" at startup rather than at
// first-translation-attempt.
func NewLibreTranslate(cfg LibreTranslateConfig) (Translator, error) {
	url := strings.TrimRight(strings.TrimSpace(cfg.URL), "/")
	if url == "" {
		return nil, errors.New("translator: libretranslate URL must be set")
	}
	timeout := cfg.Timeout
	if timeout <= 0 {
		timeout = 10 * time.Second
	}
	return &libreTranslate{
		endpoint: url + "/translate",
		client:   &http.Client{Timeout: timeout},
	}, nil
}

type libreTranslate struct {
	endpoint string
	client   *http.Client
}

// requestBody is the LibreTranslate POST /translate input shape.
// `q` is sent as a two-element array so the engine returns one
// translation per element in the same call (subject + body).
type requestBody struct {
	Q      []string `json:"q"`
	Source string   `json:"source"`
	Target string   `json:"target"`
	Format string   `json:"format"`
}

// responseBody is the LibreTranslate output shape when `q` is an
// array. The single-string-q variant is a different shape; we never
// emit a single-q request so the client always sees the array form.
type responseBody struct {
	TranslatedText []string `json:"translatedText"`
	Error          string   `json:"error,omitempty"`
}

// Translate posts subject + body to LibreTranslate, normalising the
// language codes and classifying the response. The 400 / unsupported-
// pair path is signalled by `ErrUnsupportedLanguagePair`. All other
// HTTP errors (timeout, 5xx, network failure) come back as wrapped
// errors so the worker can backoff and retry.
func (l *libreTranslate) Translate(ctx context.Context, srcLang, dstLang, subject, body string) (Result, error) {
	src := normaliseLanguageCode(srcLang)
	dst := normaliseLanguageCode(dstLang)
	if src == "" || dst == "" {
		return Result{}, fmt.Errorf("translator: missing source or target language (src=%q dst=%q)", srcLang, dstLang)
	}
	if src == dst {
		return Result{Subject: subject, Body: body, Engine: NoopEngine}, nil
	}

	reqBody, err := json.Marshal(requestBody{
		Q:      []string{subject, body},
		Source: src,
		Target: dst,
		Format: "text",
	})
	if err != nil {
		return Result{}, fmt.Errorf("translator: marshal request: %w", err)
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, l.endpoint, bytes.NewReader(reqBody))
	if err != nil {
		return Result{}, fmt.Errorf("translator: build request: %w", err)
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Accept", "application/json")

	resp, err := l.client.Do(req)
	if err != nil {
		return Result{}, fmt.Errorf("translator: do request: %w", err)
	}
	defer func() { _ = resp.Body.Close() }()

	raw, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return Result{}, fmt.Errorf("translator: read response: %w", err)
	}
	if resp.StatusCode == http.StatusBadRequest {
		return Result{}, fmt.Errorf("%w: %s", ErrUnsupportedLanguagePair, strings.TrimSpace(string(raw)))
	}
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return Result{}, fmt.Errorf("translator: libretranslate http %d: %s", resp.StatusCode, strings.TrimSpace(string(raw)))
	}

	var out responseBody
	if err := json.Unmarshal(raw, &out); err != nil {
		return Result{}, fmt.Errorf("translator: unmarshal response: %w", err)
	}
	if out.Error != "" {
		return Result{}, fmt.Errorf("translator: libretranslate error: %s", out.Error)
	}
	if len(out.TranslatedText) != 2 {
		return Result{}, fmt.Errorf("translator: libretranslate returned %d strings, want 2", len(out.TranslatedText))
	}
	return Result{
		Subject: out.TranslatedText[0],
		Body:    out.TranslatedText[1],
		Engine:  LibreTranslateEngine,
	}, nil
}

// normaliseLanguageCode collapses a BCP 47 tag to the ISO 639-1 base
// that LibreTranslate expects (`en-US` → `en`, `EN` → `en`). The
// helper is mirrored on the diplomail service side; both sides need
// to use the same normalisation so cache keys line up.
func normaliseLanguageCode(tag string) string {
	tag = strings.TrimSpace(tag)
	if tag == "" {
		return ""
	}
	if i := strings.IndexAny(tag, "-_"); i > 0 {
		tag = tag[:i]
	}
	return strings.ToLower(tag)
}