/**
 * Remembers where the reader stopped in a chapter. The position belongs to one account and one
 * chapter, and it is tied to the chapter's content version: after the text is replaced, the old
 * offset would point at different words, so it is dropped instead of restored.
 *
 * The value is a scroll ratio rather than a pixel offset, because the same chapter is read at
 * different window widths (desktop and phone) where a pixel offset would land in the wrong place.
 * This is local to the browser: it is not synced across devices.
 */
export const POSITION_KEY_PREFIX = 'lexgo-learner-position:'

export interface ReadingPosition {
  ratio: number
  sha: string
  at: string
}

export function positionKey(userId: number | null, chapterId: number): string | null {
  if (userId === null || !Number.isInteger(chapterId) || chapterId <= 0) return null
  return `${POSITION_KEY_PREFIX}${userId}:${chapterId}`
}

/** Clamps a stored ratio into [0,1]; anything else is treated as "no usable position". */
export function normalizeRatio(value: unknown): number | null {
  if (typeof value !== 'number' || !Number.isFinite(value)) return null
  if (value <= 0) return null
  return Math.min(value, 1)
}

export function readPosition(storage: Storage, userId: number | null, chapterId: number, sha: string): ReadingPosition | null {
  const key = positionKey(userId, chapterId)
  if (key === null) return null
  try {
    const raw = storage.getItem(key)
    if (raw === null) return null
    const parsed = JSON.parse(raw) as Partial<ReadingPosition> | null
    const ratio = normalizeRatio(parsed?.ratio)
    // A different content version means the offset describes words that are no longer there.
    if (ratio === null || !parsed || parsed.sha !== sha) return null
    return { ratio, sha, at: typeof parsed.at === 'string' ? parsed.at : '' }
  } catch {
    return null
  }
}

export function writePosition(storage: Storage, userId: number | null, chapterId: number, sha: string, ratio: number, now = new Date()): void {
  const key = positionKey(userId, chapterId)
  const usable = normalizeRatio(ratio)
  if (key === null || usable === null || !sha) return
  try {
    storage.setItem(key, JSON.stringify({ ratio: usable, sha, at: now.toISOString() } satisfies ReadingPosition))
  } catch {
    // A full or disabled store must not break reading.
  }
}

export function clearPosition(storage: Storage, userId: number | null, chapterId: number): void {
  const key = positionKey(userId, chapterId)
  if (key === null) return
  try {
    storage.removeItem(key)
  } catch {
    // Ignored for the same reason as above.
  }
}

export interface ReadingPositionOptions {
  /** The scrollable document the reader lives in. */
  view: () => Window
  /** The current scrollable height, used to turn a pixel offset into a ratio. */
  scrollHeight: () => number
}

/** Turns the current window scroll offset into the ratio that is stored. */
export function currentRatio(view: Pick<Window, 'scrollY' | 'innerHeight'>, scrollHeight: number): number {
  const scrollable = scrollHeight - view.innerHeight
  if (scrollable <= 0) return 0
  return Math.min(Math.max(view.scrollY / scrollable, 0), 1)
}