/**
 * Provider failover on retriable upstream errors (Story 3.3 — FR18 / NFR-R1).
 *
 * Story 3.5 BYOK: when user BYOK is active, call with skipSystemFallback: true.
 */

import { APICallError } from 'ai'
import { QuotaExceededError } from '@/lib/entitlements'
import { getProviderInstance, type ProviderType } from './factory'
import {
  resolveAiRoute,
  VALID_PROVIDERS,
  type AiFeatureLane,
  type AiGatewayProvider,
  type ResolvedAiRoute,
} from './router'
import type { AIProvider } from './types'

export const FALLBACK_BUDGET_MS = 1500

const VALID_PROVIDER_LIST = [...VALID_PROVIDERS].join(', ')

const LANE_FALLBACK_KEYS: Record<
  AiFeatureLane,
  { provider: string; model: string; ollamaUrl?: string }
> = {
  chat: {
    provider: 'AI_PROVIDER_CHAT_FALLBACK',
    model: 'AI_MODEL_CHAT_FALLBACK',
    ollamaUrl: 'OLLAMA_BASE_URL_CHAT',
  },
  tags: {
    provider: 'AI_PROVIDER_TAGS_FALLBACK',
    model: 'AI_MODEL_TAGS_FALLBACK',
    ollamaUrl: 'OLLAMA_BASE_URL_TAGS',
  },
  embedding: {
    provider: 'AI_PROVIDER_EMBEDDING_FALLBACK',
    model: 'AI_MODEL_EMBEDDING_FALLBACK',
    ollamaUrl: 'OLLAMA_BASE_URL_EMBEDDING',
  },
}

function pick(config: Record<string, string>, key: string): string | undefined {
  const v = config[key]
  if (v != null && v !== '') return v
  const e = process.env[key]
  return e != null && e !== '' ? e : undefined
}

function cfgOnly(config: Record<string, string>, key: string): string | undefined {
  const v = config[key]
  return v != null && v !== '' ? v : undefined
}

function extractProviderErrorStatusDepth(err: unknown, depth: number): number | undefined {
  if (depth > 5) return undefined
  if (err instanceof QuotaExceededError) return 402
  if (APICallError.isInstance(err)) {
    return err.statusCode ?? undefined
  }
  if (err && typeof err === 'object') {
    const o = err as Record<string, unknown>
    if (typeof o.statusCode === 'number') return o.statusCode
    if (typeof o.status === 'number') return o.status
    if (o.cause) return extractProviderErrorStatusDepth(o.cause, depth + 1)
  }
  return undefined
}

export function extractProviderErrorStatus(err: unknown): number | undefined {
  return extractProviderErrorStatusDepth(err, 0)
}

/** True for HTTP 429 and 5xx provider failures; false for quota and other 4xx. */
export function isRetriableProviderError(err: unknown): boolean {
  if (err instanceof QuotaExceededError) return false
  if (err && typeof err === 'object') {
    const code = (err as { code?: string }).code
    if (code === 'QUOTA_EXCEEDED') return false
  }
  const status = extractProviderErrorStatus(err)
  if (status === undefined) return false
  if (status === 429) return true
  if (status >= 500 && status < 600) return true
  return false
}

/**
 * Resolve secondary route from *_FALLBACK keys only (primary keys untouched).
 */
export function resolveAiFallbackRoute(
  lane: AiFeatureLane,
  config: Record<string, string>
): ResolvedAiRoute | null {
  const keys = LANE_FALLBACK_KEYS[lane]
  const providerRaw = pick(config, keys.provider)?.trim()
  if (!providerRaw) return null

  const providerType = providerRaw.toLowerCase()

  if (!VALID_PROVIDERS.has(providerType)) {
    throw new Error(
      `Unknown fallback provider '${providerRaw}'. Valid options: ${VALID_PROVIDER_LIST}`
    )
  }

  if (lane === 'embedding' && (providerType === 'anthropic' || providerType === 'anthropic_custom')) {
    throw new Error(
      'AI_PROVIDER_EMBEDDING_FALLBACK cannot use "anthropic" or "anthropic_custom": no embeddings on this gateway.'
    )
  }

  const primary = resolveAiRoute(lane, config)

  if (providerType === primary.providerType) return null

  const modelName =
    pick(config, keys.model) ??
    (lane === 'chat'
      ? pick(config, 'AI_MODEL_CHAT')
      : lane === 'tags'
        ? pick(config, 'AI_MODEL_TAGS')
        : pick(config, 'AI_MODEL_EMBEDDING')) ??
    primary.modelName

  const embeddingModelName =
    pick(config, 'AI_MODEL_EMBEDDING_FALLBACK') ??
    pick(config, 'AI_MODEL_EMBEDDING') ??
    primary.embeddingModelName

  const ollamaBaseUrl =
    cfgOnly(config, keys.ollamaUrl!) ||
    cfgOnly(config, 'OLLAMA_BASE_URL')

  return {
    lane,
    providerType: providerType as AiGatewayProvider,
    modelName,
    embeddingModelName,
    ollamaBaseUrl,
    meta: {},
  }
}

function getProviderForRoute(config: Record<string, string>, route: ResolvedAiRoute): AIProvider {
  return getProviderInstance(
    route.providerType as ProviderType,
    config,
    route.modelName,
    route.embeddingModelName,
    route.ollamaBaseUrl
  )
}

function getPrimaryProvider(lane: AiFeatureLane, config: Record<string, string>): {
  provider: AIProvider
  route: ResolvedAiRoute
} {
  const route = resolveAiRoute(lane, config)
  return { route, provider: getProviderForRoute(config, route) }
}

function getSecondaryProvider(lane: AiFeatureLane, config: Record<string, string>): {
  provider: AIProvider
  route: ResolvedAiRoute
} | null {
  try {
    const fbRoute = resolveAiFallbackRoute(lane, config)
    if (!fbRoute) return null
    return { route: fbRoute, provider: getProviderForRoute(config, fbRoute) }
  } catch {
    return null
  }
}

function logFallbackSuccess(meta: {
  lane: AiFeatureLane
  primaryProvider: string
  secondaryProvider: string
  primaryStatus?: number
  fallbackMs: number
}): void {
  if (process.env.NODE_ENV !== 'production' || process.env.MEMENTO_AI_ROUTE_DEBUG === '1') {
    console.debug('[ai-fallback]', JSON.stringify(meta))
    if (meta.fallbackMs > FALLBACK_BUDGET_MS) {
      console.warn(
        `[ai-fallback] NFR-R1 budget exceeded: ${meta.fallbackMs.toFixed(1)}ms > ${FALLBACK_BUDGET_MS}ms`
      )
    }
  }
}

export interface WithAiProviderFallbackOptions {
  /** Story 3.5: skip system secondary when user BYOK is active */
  skipSystemFallback?: boolean
}

/**
 * Run an AI operation on the primary provider; on retriable failure, try secondary once.
 */
export async function withAiProviderFallback<T>(
  lane: AiFeatureLane,
  config: Record<string, string>,
  run: (provider: AIProvider) => Promise<T>,
  options?: WithAiProviderFallbackOptions
): Promise<T> {
  if (options?.skipSystemFallback) {
    const primary = getPrimaryProvider(lane, config)
    return run(primary.provider)
  }

  const primary = getPrimaryProvider(lane, config)
  try {
    return await run(primary.provider)
  } catch (err) {
    if (!isRetriableProviderError(err)) throw err

    const fallbackStart = performance.now()
    const secondary = getSecondaryProvider(lane, config)
    if (!secondary) throw err

    const primaryStatus = extractProviderErrorStatus(err)
    try {
      const result = await run(secondary.provider)
      logFallbackSuccess({
        lane,
        primaryProvider: primary.route.providerType,
        secondaryProvider: secondary.route.providerType,
        primaryStatus,
        fallbackMs: performance.now() - fallbackStart,
      })
      return result
    } catch (secondaryErr) {
      console.error(
        `[ai-fallback] secondary also failed for lane '${lane}':`,
        secondaryErr instanceof Error ? secondaryErr.message : secondaryErr
      )
      throw secondaryErr
    }
  }
}