import { ref, computed, type ComputedRef } from 'vue'

export interface BannerFallbackOptions {
  /** Primary image URL candidates, tried in order */
  primaryUrls: () => (string | undefined | null)[]
  /** Async fetch to try when all primary URLs fail */
  apiFetch: () => Promise<{ posterUrl: string | null; backdropUrl: string | null }>
  /** Title for gradient generation */
  title: () => string
  /** Optional seed override for gradient hue */
  gradientSeed?: () => string
}

export interface BannerFallbackReturn {
  bannerSrc: ComputedRef<string | null>
  fallbackGradient: ComputedRef<string>
  onBannerError: () => void
}

export function useBannerFallback(options: BannerFallbackOptions): BannerFallbackReturn {
  const primaryIndex = ref(0)
  const stage = ref<'primary' | 'api' | 'done'>('primary')
  const apiUrl = ref<string | null>(null)
  let apiFetching = false

  const bannerSrc = computed<string | null>(() => {
    if (stage.value === 'done') return null

    if (stage.value === 'primary') {
      const urls = options.primaryUrls()
      // Find first non-null URL starting from primaryIndex
      for (let i = primaryIndex.value; i < urls.length; i++) {
        if (urls[i]) return urls[i]!
      }
      // No primary URLs available — skip to API immediately
      return null
    }

    if (stage.value === 'api' && apiUrl.value) return apiUrl.value

    return null
  })

  const fallbackGradient = computed(() => {
    const seed = options.gradientSeed ? options.gradientSeed() : options.title()
    const hue = [...seed].reduce((acc, c) => acc + c.charCodeAt(0), 0) % 360
    return `linear-gradient(135deg, hsl(${hue}, 25%, 12%) 0%, hsl(${(hue + 40) % 360}, 20%, 8%) 100%)`
  })

  async function onBannerError() {
    if (stage.value === 'primary') {
      const urls = options.primaryUrls()
      // Advance to next primary URL
      let nextIdx = primaryIndex.value + 1
      while (nextIdx < urls.length && !urls[nextIdx]) nextIdx++

      if (nextIdx < urls.length) {
        primaryIndex.value = nextIdx
        return
      }

      // All primaries exhausted — try API
      if (!apiFetching) {
        apiFetching = true
        try {
          const result = await options.apiFetch()
          const url = result.backdropUrl ?? result.posterUrl
          if (url) {
            apiUrl.value = url
            stage.value = 'api'
            return
          }
        } catch { /* ignore */ }
      }
      stage.value = 'done'
      return
    }

    if (stage.value === 'api') {
      stage.value = 'done'
    }
  }

  // If no primary URLs at all, trigger API fetch on first render
  const urls = options.primaryUrls()
  const hasAnyPrimary = urls.some(u => !!u)
  if (!hasAnyPrimary && !apiFetching) {
    apiFetching = true
    options.apiFetch().then(result => {
      const url = result.backdropUrl ?? result.posterUrl
      if (url) {
        apiUrl.value = url
        stage.value = 'api'
      } else {
        stage.value = 'done'
      }
    }).catch(() => {
      stage.value = 'done'
    })
  }

  return { bannerSrc, fallbackGradient, onBannerError }
}