package weixin_ilink

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

// ErrKeyTooShort 在入站密钥推导无法给出所需分组长度的 AES 密钥时返回。
var ErrKeyTooShort = errors.New("weixin_ilink: derived key shorter than a block")

// deriveInboundKey 为入站媒体负载推导 AES-128 密钥。按 design/08 §3.3，密钥从
// bot_token 中提取：先 base64 解码 token，再取末尾 16 字节。参考实现把这一步标记为
// ⚠️，因此 DownloadMediaWithKey 允许调用方在平台按每条媒体下发密钥时显式传入密钥。
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 下载一条入站 CDN 引用，并用从已配置 bot_token 推导出的密钥返回
// 明文。密文超过 MaxMediaSize 时以 ErrMediaTooLarge 失败。
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 是 DownloadMedia 的显式按条 AES 密钥版本，用于协议把密钥
// 与媒体引用一并下发的场景。
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 拉取 CDN 字节并剥掉 AES-ECB 层。
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 对 rawURL 发 GET 并返回响应体，超过 MaxMediaSize 的一律拒绝，避免单个
// 超大文件耗尽内存。
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
}