/**
 * Shared window-confidence helper (v1.17 W1b).
 *
 * Several insight tiles narrate a metric over a fixed trailing window —
 * the BD-Zielbereich tile reports the BP in-target share over the last
 * 90 days, and future tiles (glucose TIR / GMI, sleep efficiency, …)
 * will do the same over their own windows. A static "· 90 T" label is
 * dishonest for a user who has only a few weeks of history or only a
 * handful of readings: the headline reads like a stable quarter-long
 * average when it is really a thin sample.
 *
 * This helper turns a window length + the readings observed inside it
 * into two truthful signals every windowed tile can reuse:
 *
 *   1. `effectiveSpanDays` — the real calendar span of the data, capped
 *      at the window. A user with 23 days of history sees "· 23 T"; once
 *      ~90 days of history exist the span saturates at the window and the
 *      label reads "· 90 T". `null` when the window holds no readings.
 *
 *   2. `sufficient` — `false` when fewer than `MIN_READINGS_FOR_CONFIDENCE`
 *      readings fall in the window. The caller renders a "collecting data"
 *      placeholder instead of a confident percentage, and the Health-Score
 *      pillar treats the metric as absent rather than grading a thin sample.
 *
 * Pure & deterministic so the unit suite can pin both outputs.
 */

/**
 * Minimum readings inside the window before a percentage is trustworthy.
 * Below this the tile narrates "collecting data" and the score pillar is
 * suppressed. Five is the same floor the trend cards use before they draw
 * a first slope — a sample small enough to be noise.
 */
export const MIN_READINGS_FOR_CONFIDENCE = 5;

const DAY_MS = 24 * 60 * 60 * 1000;

/**
 * `true` once a window holds enough readings to narrate a confident
 * percentage. Shared by the server (Health-Score pillar gate) and the
 * client (tile placeholder) so both apply the identical floor.
 */
export function isWindowSufficient(readingCount: number): boolean {
  return readingCount >= MIN_READINGS_FOR_CONFIDENCE;
}

export interface WindowConfidence {
  /**
   * Real calendar span of the in-window data, in days, capped at
   * `windowDays`. `null` when the window holds no readings (caller renders
   * the "—" / placeholder rather than "· 0 T").
   */
  effectiveSpanDays: number | null;
  /** `true` once at least `MIN_READINGS_FOR_CONFIDENCE` readings exist. */
  sufficient: boolean;
}

/**
 * v1.17 W1b — derive the BD-Zielbereich tile's confidence inputs (90-day
 * pair count + effective label span) from a BP envelope's 90-day window.
 * Shared by the dashboard snapshot and the `/api/analytics` thick slice so
 * the two surfaces can never compute the tile's gate or span differently.
 */
export function deriveBpWindow90(
  last90: { pairs: number } | null,
  last90EarliestAt: Date | null,
  now: Date,
): { count: number; spanDays: number | null } {
  const count = last90?.pairs ?? 0;
  const spanDays = computeWindowConfidence({
    windowDays: 90,
    readingCount: count,
    earliestReadingAt: last90EarliestAt,
    now,
  }).effectiveSpanDays;
  return { count, spanDays };
}

/**
 * Compute the effective label span + sufficiency flag for a windowed
 * metric.
 *
 * @param windowDays         the configured trailing window (e.g. 90).
 * @param readingCount       number of readings counted inside the window.
 * @param earliestReadingAt  timestamp of the oldest in-window reading, or
 *                           `null` when the window is empty. The span is
 *                           `now - earliest`, ceil-rounded to whole days,
 *                           and never exceeds `windowDays`. At least 1 day
 *                           so a same-day-only sample still reads "· 1 T".
 * @param now                clock anchor (injectable for deterministic tests).
 */
export function computeWindowConfidence(input: {
  windowDays: number;
  readingCount: number;
  earliestReadingAt: Date | null;
  now?: Date;
}): WindowConfidence {
  const { windowDays, readingCount, earliestReadingAt } = input;
  const now = input.now ?? new Date();

  const sufficient = isWindowSufficient(readingCount);

  if (readingCount <= 0 || earliestReadingAt === null) {
    return { effectiveSpanDays: null, sufficient };
  }

  const rawSpanMs = now.getTime() - earliestReadingAt.getTime();
  // A reading logged moments ago still spans one calendar day of history.
  const rawSpanDays = Math.max(1, Math.ceil(rawSpanMs / DAY_MS));
  const effectiveSpanDays = Math.min(rawSpanDays, windowDays);

  return { effectiveSpanDays, sufficient };
}