/**
 * 资产预签名上传状态机（纯函数 reducer）
 *
 * 三段链路 init（POST /assets 预签名）→ PUT（staging 直传）→ complete
 * （核验生效并返回签名 URL）。任何一步失败进入 error（携带阶段与原因，
 * 可重新开始）；complete 幂等，成功态保留签名 URL 与过期时间用于预览。
 * 抽成纯 reducer 以便单测与 UI 解耦。
 */

export type AssetUploadStage =
  'idle' | 'initializing' | 'uploading' | 'completing' | 'done' | 'error'

export interface AssetUploadState {
  stage: AssetUploadStage
  /** 失败发生的阶段（error 态定位重试入口） */
  failedStage?: 'init' | 'put' | 'complete'
  /** 错误消息（已本地化/服务端文案） */
  error?: string
  /** complete 返回的签名预览 URL */
  previewUrl?: string
  /** 签名 URL 过期时间（ISO） */
  previewExpiresAt?: string
}

export type AssetUploadEvent =
  | { type: 'START' }
  | { type: 'INIT_OK' }
  | { type: 'PUT_OK' }
  | { type: 'COMPLETE_OK'; url: string; expiresAt: string }
  | { type: 'FAIL'; stage: 'init' | 'put' | 'complete'; message?: string }
  | { type: 'RESET' }

export const INITIAL_ASSET_UPLOAD_STATE: AssetUploadState = { stage: 'idle' }

/**
 * 状态转移：非法转移（如 idle 收到 PUT_OK）保持原状——UI 侧的迟到响应
 * 不得污染已重置的新会话。
 */
export function nextAssetUploadState(
  state: AssetUploadState,
  event: AssetUploadEvent
): AssetUploadState {
  switch (event.type) {
    case 'START':
      if (state.stage === 'idle' || state.stage === 'done' || state.stage === 'error') {
        return { stage: 'initializing' }
      }
      return state
    case 'INIT_OK':
      if (state.stage === 'initializing') return { stage: 'uploading' }
      return state
    case 'PUT_OK':
      if (state.stage === 'uploading') return { stage: 'completing' }
      return state
    case 'COMPLETE_OK':
      if (state.stage === 'completing') {
        return {
          stage: 'done',
          previewUrl: event.url,
          previewExpiresAt: event.expiresAt
        }
      }
      return state
    case 'FAIL':
      if (
        state.stage === 'initializing' ||
        state.stage === 'uploading' ||
        state.stage === 'completing'
      ) {
        return { stage: 'error', failedStage: event.stage, error: event.message }
      }
      return state
    case 'RESET':
      return INITIAL_ASSET_UPLOAD_STATE
    default:
      return state
  }
}