package weixin_ilink

import (
	"context"
	"crypto/aes"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"net/http"
	"strings"
)

// ErrKeyTooShort is returned when the inbound derivation cannot yield an AES
// key of the required block size.
var ErrKeyTooShort = errors.New("weixin_ilink: derived key shorter than a block")

// deriveInboundKey derives the AES-128 key for an inbound media payload. Per
// design/08 §3.3 the key is extracted from the bot_token: base64-decode the
// token and take the trailing 16 bytes. This is marked ⚠️ in the reference, so
// DownloadMediaWithKey lets callers supply an explicit key when the platform
// carries one per item instead.
func deriveInboundKey(botToken string) ([]byte, error) {
	token := strings.TrimSpace(botToken)
	if token == "" {
		return nil, ErrInvalidConfig
	}
	b, err := base64.StdEncoding.DecodeString(token)
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: decode bot_token for key: %w", err)
	}
	if len(b) < aes.BlockSize {
		return nil, ErrKeyTooShort
	}
	return b[len(b)-aes.BlockSize:], nil
}

// DownloadMedia downloads an inbound CDN reference and returns the decrypted
// plaintext using the key derived from the configured bot_token. It fails with
// ErrMediaTooLarge when the ciphertext exceeds MaxMediaSize.
func (a *Adapter) DownloadMedia(ctx context.Context, cdnURL string) ([]byte, error) {
	key, err := deriveInboundKey(a.cfg.BotToken)
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: derive inbound key: %w", err)
	}
	return a.downloadWithKey(ctx, cdnURL, key)
}

// DownloadMediaWithKey is DownloadMedia with an explicit per-item AES key,
// used when the protocol delivers the key alongside the media reference.
func (a *Adapter) DownloadMediaWithKey(ctx context.Context, cdnURL string, key []byte) ([]byte, error) {
	if len(key) != aes.BlockSize {
		return nil, fmt.Errorf("weixin_ilink: key must be %d bytes: %w", aes.BlockSize, ErrKeyTooShort)
	}
	return a.downloadWithKey(ctx, cdnURL, key)
}

// downloadWithKey fetches the CDN bytes and removes the AES-ECB layer.
func (a *Adapter) downloadWithKey(ctx context.Context, cdnURL string, key []byte) ([]byte, error) {
	ciphertext, err := a.fetchBody(ctx, a.cdnURL(cdnURL))
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: fetch media: %w", err)
	}
	plaintext, err := aesECBDecrypt(key, ciphertext)
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: decrypt media: %w", err)
	}
	return plaintext, nil
}

// fetchBody GETs rawURL and returns the body, refusing anything over
// MaxMediaSize so a single huge file cannot exhaust memory.
func (a *Adapter) fetchBody(ctx context.Context, rawURL string) ([]byte, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, rawURL, nil)
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: build media request: %w", err)
	}
	a.authorizationHeader(req)
	resp, err := a.httpClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: download media: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return nil, fmt.Errorf("weixin_ilink: download media: status %d", resp.StatusCode)
	}
	limit := a.cfg.MaxMediaSize + 1
	data, err := io.ReadAll(io.LimitReader(resp.Body, limit))
	if err != nil {
		return nil, fmt.Errorf("weixin_ilink: read media: %w", err)
	}
	if int64(len(data)) > a.cfg.MaxMediaSize {
		return nil, ErrMediaTooLarge
	}
	return data, nil
}