// srcset strings of a featured image from the Journal API's
// featured_images[].variants (D-85, D-71): one for the WebP <source> and
// one for the <img> in the original format, the original itself added
// when it is wider than every variant, plus the size the <img> reserves.
// Pure, no Vue or Nuxt imports, so `node --test` drives it.

/** One size of a featured image (featured_images[].variants). */
export interface JournalImageVariant {
  width: number
  height: number
  /** The thumbnail in the original's format. */
  url: string
  /** Its WebP copy. */
  webp_url: string
  /** The original's size. */
  source_width?: number
  source_height?: number
}

/** What <picture> renders from the variants. */
export interface JournalImageSrcsets {
  /** srcset of the type="image/webp" <source>. */
  webp: string
  /** srcset of the <img>, in the original format. */
  original: string
  /** The size the <img> reserves: the original's, else the largest variant's. */
  width: number
  height: number
}

/** A URL safe inside a srcset candidate (no spaces or commas). */
function candidateURL(url: string): string {
  return url.replace(/ /g, '%20').replace(/,/g, '%2C')
}

function validVariant(v: unknown): v is JournalImageVariant {
  if (!v || typeof v !== 'object') return false
  const o = v as Record<string, unknown>
  return (
    Number.isInteger(o.width) && (o.width as number) > 0 &&
    Number.isInteger(o.height) && (o.height as number) > 0 &&
    typeof o.url === 'string' && o.url !== '' &&
    typeof o.webp_url === 'string' && o.webp_url !== ''
  )
}

/**
 * The srcsets of an image from its variants, narrowest first and each
 * width once. originalUrl and originalWidth (the variants' source_width
 * when not given) add the original to the <img> srcset when it is wider
 * than every variant. No usable variant answers null: render the original
 * as before.
 */
export function imageSrcsets(
  variants: readonly unknown[] | null | undefined,
  originalUrl?: string | null,
  originalWidth?: number | null,
): JournalImageSrcsets | null {
  if (!Array.isArray(variants)) return null
  const byWidth = new Map<number, JournalImageVariant>()
  for (const v of variants) {
    if (validVariant(v) && !byWidth.has(v.width)) byWidth.set(v.width, v)
  }
  const list = [...byWidth.values()].sort((a, b) => a.width - b.width)
  const largest = list[list.length - 1]
  if (!largest) return null
  const webp = list.map((v) => `${candidateURL(v.webp_url)} ${v.width}w`)
  const original = list.map((v) => `${candidateURL(v.url)} ${v.width}w`)
  const sourceWidth = originalWidth ?? largest.source_width ?? null
  const sourceHeight = largest.source_height ?? null
  if (originalUrl && sourceWidth && Number.isInteger(sourceWidth) && sourceWidth > largest.width) {
    original.push(`${candidateURL(originalUrl)} ${sourceWidth}w`)
  }
  const hasSource = !!sourceWidth && !!sourceHeight && Number.isInteger(sourceWidth) && Number.isInteger(sourceHeight)
  return {
    webp: webp.join(', '),
    original: original.join(', '),
    width: hasSource ? (sourceWidth as number) : largest.width,
    height: hasSource ? (sourceHeight as number) : largest.height,
  }
}