/**
 * v1.11.4 — per-night sleep aggregation (iOS #1, HIGH).
 *
 * `SLEEP_DURATION` is stored in MINUTES, one row per sleep STAGE per
 * night (IN_BED / AWAKE / ASLEEP / REM / CORE / DEEP) since v1.4.23. The
 * value is NOT cumulative — a single stage row carries only that stage's
 * minutes. Every "last night's sleep" surface therefore has to SUM the
 * stage rows belonging to one night before it shows a headline number;
 * reading the single most-recent row gives one stage, not the night.
 *
 * This helper is the one shared place that turns the raw per-stage rows
 * into per-night totals. The headline "time asleep" is light(CORE) +
 * deep + REM; IN_BED and AWAKE are excluded from the asleep total,
 * matching the AASM convention.
 *
 * Granular-over-bare (v1.11.5)
 * ----------------------------
 * Apple Health writes BOTH an unspecified `ASLEEP` AGGREGATE row AND the
 * granular `CORE` / `DEEP` / `REM` breakdown for the SAME sleep period. The
 * granular rows partition the same time the aggregate covers, so counting
 * bare-ASLEEP together with the granular rows ~doubles the night. We therefore
 * prefer the granular partition: when ANY granular stage (CORE / DEEP / REM)
 * is present for a session we drop the redundant bare `ASLEEP` aggregate (and
 * any stage-less row) EVERYWHERE — the asleep total, the per-stage `stages`
 * breakdown, and the hypnogram `segments` list — so none of those views holds
 * the granular set plus its ~equal bare twin. The bare `ASLEEP` aggregate (or
 * a stage-less legacy / manual row) is the session's only asleep signal when
 * no granular stage exists. WHOOP never emits a bare `ASLEEP` aggregate (its
 * stage map writes CORE / DEEP / REM / AWAKE / IN_BED only — see
 * `whoop/client.ts`), so this gate is Apple-Health-only in practice.
 *
 * Night grouping
 * --------------
 * A "night" is one SLEEP SESSION, not a calendar day. `measuredAt` is the
 * stage segment's END instant (`apple-health-mapping.ts` sets
 * `takenAt = endDate`), and a device writes one row per stage segment, so a
 * normal overnight sleep (asleep before local midnight) spreads its segments
 * across BOTH sides of midnight. Keying each segment by its own calendar day
 * would split one physical night into two partial nights and undercount the
 * "last night" headline.
 *
 * Instead we cluster the segments into sessions by time gap: sort by
 * `measuredAt`, and start a new session whenever the gap from the previous
 * segment exceeds `SESSION_GAP_MS` (3 h). Stage segments inside one sleep
 * session are contiguous (minutes to ~2 h apart), so a > 3 h gap reliably
 * marks the boundary between a daytime nap and the overnight session, or
 * between two genuine sessions in one day — they stay separable. The gap is
 * measured on the absolute UTC instant, so it is immune to DST shifts.
 *
 * Each session is keyed by the LOCAL WAKE DAY — `userDayKey` of the session's
 * LAST segment (its end instant) in the user's timezone. This matches the
 * Apple Health convention (a sleep session is attributed to the date you wake
 * up) and keeps the keying tz/DST-correct because it runs on the real instant.
 *
 * Naps vs the main night
 * ----------------------
 * `reconstructSleepNights` MERGES every session that lands on the same wake
 * day under one key so the headline "time asleep" for a day is the day's
 * total. `reconstructSleepSessions` keeps each session SEPARATE, and
 * `pickMainNightAndNaps` applies the convention the UI needs: the MAIN night
 * is the session with the most asleep minutes (normally the overnight block),
 * and every other session on the same wake day is a NAP, surfaced separately
 * and never folded into the main night's headline.
 *
 * Source de-dup
 * -------------
 * A user paired to more than one sleep source (e.g. WHOOP + Apple Health)
 * gets per-stage rows for the SAME night from each source. Summing them all
 * would ~double the night total. Before summing a session we collapse it to
 * ONE source using the user's `sleep` source-priority ladder
 * (`DEFAULT_SOURCE_PRIORITY.sleep` = WHOOP > APPLE_HEALTH > WITHINGS): pick the
 * highest-ranked source present in the session, falling back to the source
 * with the most asleep minutes (then source name) when none of the session's
 * sources sits on the ladder. Only that source's segments are summed — sources
 * are never blended within a night. This mirrors the per-(type, day) collapse
 * the rest of the v1.11.x numeric surfaces apply.
 *
 * Unit
 * ----
 * Totals are returned in MINUTES — the canonical `SLEEP_DURATION` unit
 * (`getUnitForType` → "minutes") — so the existing web consumers that
 * already treat the summary as minutes (e.g. the insights overview's
 * `formatHoursMinutes`) keep working unchanged; they now average per-
 * night totals instead of per-stage rows, which is strictly more
 * correct. Callers that want hours convert at the edge (`/ 60`).
 */
import type { MeasurementSource, SleepStage } from "@/generated/prisma/client";
import { userDayKey } from "@/lib/tz/format";
import { summarize, type DataSummary } from "@/lib/analytics/trends";
import {
  getSourceLadder,
  parseSourcePriority,
} from "@/lib/validations/source-priority";

/**
 * The GRANULAR asleep stages (iOS 16+ `asleepCore` / `asleepDeep` /
 * `asleepREM`). A bare `ASLEEP` row is the iOS 15- `asleepUnspecified`
 * AGGREGATE — Apple Health writes BOTH the unspecified aggregate AND the
 * granular breakdown for the SAME period, so keeping bare-ASLEEP alongside the
 * granular rows ~doubles the night. The granular rows partition the same time
 * the aggregate covers, so when any granular stage is present for a session we
 * keep ONLY the granular stages and drop the bare aggregate; the bare row is
 * the asleep signal only when no granular stage exists. WHOOP never writes a
 * bare `ASLEEP` aggregate (see `whoop/client.ts` `SLEEP_STAGE_MAP`).
 */
const GRANULAR_ASLEEP_STAGES: ReadonlySet<SleepStage> = new Set<SleepStage>([
  "REM",
  "CORE",
  "DEEP",
]);

/**
 * True when any row in the set carries a GRANULAR asleep stage
 * (CORE / DEEP / REM). When this holds, the bare `ASLEEP` aggregate (and any
 * stage-less row) is the redundant unspecified twin of the granular partition
 * and must be dropped from EVERY downstream view — the asleep total, the
 * per-stage `stages` map, and the hypnogram `segments` list — so none of them
 * double-counts the same period.
 */
function sawGranularStage(rows: readonly SleepStageRow[]): boolean {
  for (const r of rows) {
    const stage = r.sleepStage;
    if (stage != null && GRANULAR_ASLEEP_STAGES.has(stage)) return true;
  }
  return false;
}

/**
 * A stage row is a redundant bare-asleep aggregate (must be dropped) when the
 * session/night ALSO carries a granular CORE/DEEP/REM partition. Only the bare
 * `ASLEEP` aggregate and stage-less (`null`) rows are redundant — IN_BED /
 * AWAKE and the granular stages themselves are always kept.
 */
function isRedundantBareAsleep(
  stage: SleepStage | null,
  sawGranular: boolean,
): boolean {
  return sawGranular && (stage === "ASLEEP" || stage == null);
}

/**
 * Sum a set of stage rows into time-asleep minutes WITHOUT double-counting a
 * bare `ASLEEP` aggregate against the granular CORE/DEEP/REM rows it overlaps.
 * IN_BED + AWAKE are never asleep; a stage-less (`null`) row is the night's
 * total only when no granular stage is present.
 */
function asleepMinutesOf(rows: readonly SleepStageRow[]): number {
  let granular = 0;
  let fallback = 0;
  let sawGranular = false;
  for (const r of rows) {
    const minutes = Number.isFinite(r.value) ? r.value : 0;
    const stage = r.sleepStage;
    if (stage != null && GRANULAR_ASLEEP_STAGES.has(stage)) {
      granular += minutes;
      sawGranular = true;
    } else if (stage === "ASLEEP" || stage == null) {
      // Bare unspecified aggregate (`ASLEEP`) or a stage-less legacy/manual
      // row — only used when no granular stage carries the night.
      fallback += minutes;
    }
    // IN_BED / AWAKE never count toward asleep.
  }
  return sawGranular ? granular : fallback;
}

/**
 * Gap (ms) between consecutive stage segments that starts a new sleep
 * session. Stage segments within one night are contiguous (minutes to a
 * couple of hours apart); a > 3 h gap separates a nap from the overnight
 * block, or two genuine sessions in one day.
 */
const SESSION_GAP_MS = 3 * 60 * 60 * 1000;

export interface SleepStageRow {
  value: number;
  measuredAt: Date;
  sleepStage: SleepStage | null;
  /**
   * Ingest source of the stage row. Used to collapse a multi-source night
   * (e.g. WHOOP + Apple Health) to one canonical source before summing.
   * Optional so legacy callers and fixtures that don't carry a source still
   * type-check (a missing source is treated as a single anonymous source —
   * the de-dup is a no-op for single-source nights).
   */
  source?: MeasurementSource | null;
}

/** One reconstructed night: the asleep total + the per-stage breakdown. */
export interface SleepNight {
  /** Wake-day key (YYYY-MM-DD) in the user's timezone. */
  night: string;
  /** Latest stage instant in the night — the night's representative timestamp. */
  measuredAt: Date;
  /** Time asleep in minutes = CORE + DEEP + REM, or bare ASLEEP when granular stages are absent. */
  asleepMinutes: number;
  /** In-bed minutes when an IN_BED row exists, else null. */
  inBedMinutes: number | null;
  /** Awake-in-bed minutes when an AWAKE row exists, else null. */
  awakeMinutes: number | null;
  /** Per-stage minutes for the night (only stages the device reported). */
  stages: Partial<Record<SleepStage, number>>;
}

/** Sentinel for rows that carry no source — collapses to one bucket. */
const NO_SOURCE = "__none__";

/**
 * Pick the one canonical source for a session. The user's `sleep`
 * source-priority ladder wins (lowest ladder index = highest priority).
 * Sources absent from the ladder fall back to "most asleep minutes", then
 * a stable source-name tiebreak. Single-source (or source-less) sessions
 * resolve to that one bucket with no work.
 */
function pickSessionSource(
  rows: SleepStageRow[],
  sleepLadder: readonly MeasurementSource[],
): string {
  // Group rows by source, then sum each source's asleep minutes with the
  // granular-over-bare rule so a source that writes BOTH a bare ASLEEP
  // aggregate and the granular breakdown is not over-weighted in the tiebreak.
  const rowsBySource = new Map<string, SleepStageRow[]>();
  for (const r of rows) {
    const src = r.source ?? NO_SOURCE;
    const list = rowsBySource.get(src) ?? [];
    list.push(r);
    rowsBySource.set(src, list);
  }
  const asleepBySource = new Map<string, number>();
  for (const [src, srcRows] of rowsBySource) {
    const minutes = asleepMinutesOf(srcRows);
    if (minutes > 0) asleepBySource.set(src, minutes);
  }
  // No asleep minutes anywhere (IN_BED / AWAKE only) — fall back to the set
  // of all present sources so the session still resolves to a single bucket.
  const sources =
    asleepBySource.size > 0
      ? [...asleepBySource.keys()]
      : [...new Set(rows.map((r) => r.source ?? NO_SOURCE))];
  if (sources.length <= 1) return sources[0] ?? NO_SOURCE;

  const rankOf = (src: string): number => {
    const i = sleepLadder.indexOf(src as MeasurementSource);
    return i === -1 ? Number.MAX_SAFE_INTEGER : i;
  };
  return sources.sort((a, b) => {
    const ra = rankOf(a);
    const rb = rankOf(b);
    if (ra !== rb) return ra - rb;
    // Neither (or both) on the ladder — most asleep minutes wins, then name.
    const ma = asleepBySource.get(a) ?? 0;
    const mb = asleepBySource.get(b) ?? 0;
    if (ma !== mb) return mb - ma;
    return a < b ? -1 : 1;
  })[0];
}

/**
 * Group raw per-stage rows into per-night totals. Stages are clustered into
 * sleep SESSIONS by time gap (a > `SESSION_GAP_MS` gap starts a new session),
 * each session keyed by the LOCAL WAKE DAY of its last segment, then collapsed
 * to ONE source via the user's `sleep` priority ladder before summing.
 *
 * Pure — the caller does the bounded DB read. `priorityJson` is the user's
 * persisted `sourcePriorityJson` (or null for the defaults). Nights are
 * returned sorted ascending by key so the last element is the most recent
 * night.
 */
export function reconstructSleepNights(
  rows: SleepStageRow[],
  tz: string,
  priorityJson: unknown = null,
): SleepNight[] {
  if (rows.length === 0) return [];
  const sleepLadder = getSourceLadder(parseSourcePriority(priorityJson), "sleep");

  // Cluster into sessions by the gap between a segment's START and the latest
  // END seen so far in the current session. `measuredAt` is the segment END;
  // the segment START is `end − value minutes`. A new session begins only when
  // the next segment STARTS more than SESSION_GAP_MS after the running end —
  // so a single long stage block (a 4 h CORE) does NOT split a night, and two
  // sources' interleaved/overlapping rows for the same night stay together
  // (their gap is ≤ 0). Gaps are absolute-time, so the clustering is DST-immune.
  // Sort by START so contiguous segments compare end-to-start in order.
  const startOf = (r: SleepStageRow): number =>
    r.measuredAt.getTime() -
    (Number.isFinite(r.value) ? r.value : 0) * 60_000;
  const sorted = [...rows].sort((a, b) => startOf(a) - startOf(b));
  const sessions: SleepStageRow[][] = [];
  let current: SleepStageRow[] = [];
  let sessionEnd = Number.NEGATIVE_INFINITY;
  for (const r of sorted) {
    const start = startOf(r);
    const end = r.measuredAt.getTime();
    if (current.length > 0 && start - sessionEnd > SESSION_GAP_MS) {
      sessions.push(current);
      current = [];
      sessionEnd = Number.NEGATIVE_INFINITY;
    }
    current.push(r);
    if (end > sessionEnd) sessionEnd = end;
  }
  if (current.length > 0) sessions.push(current);

  // Two sessions can land on the same wake day (e.g. a genuine overnight plus
  // a same-day nap); collect each session's canonical-source rows under one
  // wake-day key. The rows are grouped PER SESSION (not flattened) so the
  // granular-over-bare gate and the asleep total are decided per session — a
  // bare-only nap keeps its minutes even when the overnight session is
  // granular (the v1.11.5 merged-night nap under-count fix).
  const byNight = new Map<string, SleepStageRow[][]>();
  for (const session of sessions) {
    const canonical = pickSessionSource(session, sleepLadder);
    const kept = session.filter((r) => (r.source ?? NO_SOURCE) === canonical);
    const pool = kept.length > 0 ? kept : session;
    // The wake day is the LATEST segment END in the session (Apple Health
    // attributes a session to the morning you wake up).
    const wakeInstant = pool.reduce(
      (max, r) => (r.measuredAt.getTime() > max.getTime() ? r.measuredAt : max),
      pool[0].measuredAt,
    );
    const key = userDayKey(wakeInstant, tz);
    const list = byNight.get(key) ?? [];
    list.push(pool);
    byNight.set(key, list);
  }

  const nights: SleepNight[] = [];
  for (const [night, nightSessions] of byNight) {
    let inBed = 0;
    let awake = 0;
    let sawInBed = false;
    let sawAwake = false;
    let asleep = 0;
    // `nightSessions` and each session pool are non-empty by construction: a
    // key is only added with a `pool` that has ≥ 1 row (the source-filter falls
    // back to the full session when the canonical filter empties it), so
    // `nightSessions[0][0]` is safe. Unlike `reconstructSleepSessions`, this
    // function never applies the granular-over-bare filter to the indexed pool,
    // so it cannot empty the array here. Keep that invariant on future edits.
    let latest = nightSessions[0][0].measuredAt;
    const stages: Partial<Record<SleepStage, number>> = {};
    for (const sessionRows of nightSessions) {
      // v1.11.5 — decide the granular-over-bare gate PER SESSION. A granular
      // overnight does not strip a bare-only nap's minutes, and the per-stage
      // `stages` map drops the redundant bare ASLEEP aggregate only for the
      // session that also carries the granular partition.
      const sawGranular = sawGranularStage(sessionRows);
      // Asleep total: sum per session so a granular overnight + a bare-only
      // nap both contribute (the merged-night nap under-count fix).
      asleep += asleepMinutesOf(sessionRows);
      for (const r of sessionRows) {
        const minutes = Number.isFinite(r.value) ? r.value : 0;
        if (r.measuredAt.getTime() > latest.getTime()) latest = r.measuredAt;
        const stage = r.sleepStage;
        // Drop the redundant bare ASLEEP aggregate (and stage-less rows) from
        // the per-stage breakdown when this session has the granular partition,
        // so `stages` is never the granular set PLUS its ~equal bare twin.
        if (isRedundantBareAsleep(stage, sawGranular)) continue;
        if (stage) {
          stages[stage] = (stages[stage] ?? 0) + minutes;
        }
        if (stage === "IN_BED") {
          inBed += minutes;
          sawInBed = true;
        } else if (stage === "AWAKE") {
          awake += minutes;
          sawAwake = true;
        }
      }
    }
    nights.push({
      night,
      measuredAt: latest,
      asleepMinutes: asleep,
      inBedMinutes: sawInBed ? inBed : null,
      awakeMinutes: sawAwake ? awake : null,
      stages,
    });
  }
  return nights.sort((a, b) => (a.night < b.night ? -1 : 1));
}

/**
 * One reconstructed sleep SEGMENT — a single stage row resolved to its
 * absolute start / end span. `start = measuredAt − value·60_000` because
 * `measuredAt` is the segment END instant (`apple-health-mapping.ts` sets
 * `takenAt = endDate`). Used by the hypnogram view to lay each stage onto
 * a clock-time lane.
 */
export interface SleepSegment {
  stage: SleepStage | null;
  /** Absolute segment start instant (end − duration). */
  start: Date;
  /** Absolute segment end instant (= the stored `measuredAt`). */
  end: Date;
  /** Duration in minutes (= the stored `value`). */
  minutes: number;
}

/**
 * One reconstructed sleep SESSION — a contiguous block of segments (no
 * gap > `SESSION_GAP_MS`) collapsed to ONE canonical source. This is the
 * unit the hypnogram renders and the nap convention separates (main night
 * vs nap). Distinct from `SleepNight`, which merges same-wake-day sessions
 * under one key for the headline number.
 */
export interface SleepSession {
  /** Wake-day key (YYYY-MM-DD) of the session's last segment, in user tz. */
  night: string;
  /** The canonical ingest source whose segments were kept (null = none). */
  source: MeasurementSource | null;
  /** Session start instant — earliest segment start. */
  start: Date;
  /** Session end instant — latest segment end (the wake instant). */
  end: Date;
  /** Time asleep in minutes (granular-over-bare rule). */
  asleepMinutes: number;
  /** In-bed minutes when an IN_BED segment exists, else null. */
  inBedMinutes: number | null;
  /** Awake-in-bed minutes when an AWAKE segment exists, else null. */
  awakeMinutes: number | null;
  /** Per-stage minutes for the session (only stages the device reported). */
  stages: Partial<Record<SleepStage, number>>;
  /**
   * Count of AWAKE segments between the first and last asleep segment —
   * the mid-sleep awakenings (a leading / trailing AWAKE before sleep
   * onset or after final wake is not an awakening). Computed from the
   * canonical source's segments only.
   */
  awakenings: number;
  /** The canonical source's segments, sorted by start, for the hypnogram. */
  segments: SleepSegment[];
}

/** Resolve a stage row to its absolute span (start = end − duration). */
function segmentOf(r: SleepStageRow): SleepSegment {
  const minutes = Number.isFinite(r.value) ? r.value : 0;
  const end = r.measuredAt;
  const start = new Date(end.getTime() - minutes * 60_000);
  return { stage: r.sleepStage, start, end, minutes };
}

/**
 * Count mid-sleep awakenings from a session's segments. An AWAKE segment
 * counts only when it sits BETWEEN the first and last asleep segment — a
 * leading AWAKE before sleep onset or a trailing AWAKE after the final
 * wake is settling-in / lie-in, not a fragmentation event. IN_BED is
 * ignored entirely.
 */
function countAwakenings(segments: readonly SleepSegment[]): number {
  const sorted = [...segments].sort(
    (a, b) => a.start.getTime() - b.start.getTime(),
  );
  const isAsleep = (s: SleepSegment): boolean =>
    s.stage != null &&
    (GRANULAR_ASLEEP_STAGES.has(s.stage) || s.stage === "ASLEEP");
  let firstAsleep = -1;
  let lastAsleep = -1;
  for (let i = 0; i < sorted.length; i++) {
    if (isAsleep(sorted[i])) {
      if (firstAsleep === -1) firstAsleep = i;
      lastAsleep = i;
    }
  }
  if (firstAsleep === -1 || lastAsleep <= firstAsleep) return 0;
  let count = 0;
  for (let i = firstAsleep + 1; i < lastAsleep; i++) {
    if (sorted[i].stage === "AWAKE") count += 1;
  }
  return count;
}

/**
 * Reconstruct per-SESSION sleep blocks from raw per-stage rows. Unlike
 * `reconstructSleepNights` (which merges same-wake-day sessions under one
 * key for the single headline number), this returns EACH session
 * separately so a caller can render the night's hypnogram and surface a
 * daytime nap as its own block. Every session is collapsed to ONE
 * canonical source via the user's `sleep` priority ladder, so two sources'
 * timelines never overlay.
 *
 * Pure — the caller does the bounded DB read. Sessions are returned sorted
 * ascending by start, so the last element is the most recent session.
 */
export function reconstructSleepSessions(
  rows: SleepStageRow[],
  tz: string,
  priorityJson: unknown = null,
): SleepSession[] {
  if (rows.length === 0) return [];
  const sleepLadder = getSourceLadder(parseSourcePriority(priorityJson), "sleep");

  const startOf = (r: SleepStageRow): number =>
    r.measuredAt.getTime() -
    (Number.isFinite(r.value) ? r.value : 0) * 60_000;
  const sorted = [...rows].sort((a, b) => startOf(a) - startOf(b));
  const rawSessions: SleepStageRow[][] = [];
  let current: SleepStageRow[] = [];
  let sessionEnd = Number.NEGATIVE_INFINITY;
  for (const r of sorted) {
    const start = startOf(r);
    const end = r.measuredAt.getTime();
    if (current.length > 0 && start - sessionEnd > SESSION_GAP_MS) {
      rawSessions.push(current);
      current = [];
      sessionEnd = Number.NEGATIVE_INFINITY;
    }
    current.push(r);
    if (end > sessionEnd) sessionEnd = end;
  }
  if (current.length > 0) rawSessions.push(current);

  const sessions: SleepSession[] = [];
  for (const session of rawSessions) {
    const canonical = pickSessionSource(session, sleepLadder);
    const kept = session.filter((r) => (r.source ?? NO_SOURCE) === canonical);
    const pool = kept.length > 0 ? kept : session;
    // v1.11.5 — drop the redundant bare ASLEEP aggregate (and stage-less
    // rows) from the hypnogram segments AND the per-stage breakdown when the
    // session also carries the granular CORE/DEEP/REM partition, so the
    // hypnogram never draws a double-height ASLEEP lane on top of the
    // granular stages and the `stages` map is the granular set OR the bare
    // aggregate, never both.
    const sawGranular = sawGranularStage(pool);
    const segments = pool
      .filter((r) => !isRedundantBareAsleep(r.sleepStage, sawGranular))
      .map(segmentOf)
      .sort((a, b) => a.start.getTime() - b.start.getTime());

    // The granular-over-bare filter can empty a non-empty pool (a session whose
    // only rows are the redundant bare ASLEEP aggregate / stage-less twins of a
    // granular partition that did not survive into this pool). A session with no
    // renderable/scorable segment contributes nothing — skip it before indexing
    // `segments[0]`. This keeps `reconstructSleepSessions` total: it must NEVER
    // throw on a shape the dedup empties, so `GET /api/sleep/night` (and every
    // other consumer) returns a valid empty night, never a 500.
    if (segments.length === 0) continue;

    let inBed = 0;
    let awake = 0;
    let sawInBed = false;
    let sawAwake = false;
    const stages: Partial<Record<SleepStage, number>> = {};
    let earliest = segments[0].start;
    let latest = segments[0].end;
    for (const seg of segments) {
      if (seg.start.getTime() < earliest.getTime()) earliest = seg.start;
      if (seg.end.getTime() > latest.getTime()) latest = seg.end;
      const stage = seg.stage;
      if (stage) stages[stage] = (stages[stage] ?? 0) + seg.minutes;
      if (stage === "IN_BED") {
        inBed += seg.minutes;
        sawInBed = true;
      } else if (stage === "AWAKE") {
        awake += seg.minutes;
        sawAwake = true;
      }
    }
    const asleep = asleepMinutesOf(pool);
    sessions.push({
      night: userDayKey(latest, tz),
      source:
        canonical === NO_SOURCE ? null : (canonical as MeasurementSource),
      start: earliest,
      end: latest,
      asleepMinutes: asleep,
      inBedMinutes: sawInBed ? inBed : null,
      awakeMinutes: sawAwake ? awake : null,
      stages,
      awakenings: countAwakenings(segments),
      segments,
    });
  }
  return sessions.sort((a, b) => a.start.getTime() - b.start.getTime());
}

/**
 * Apply the NAP convention to a wake-day's sessions: the MAIN night is the
 * session with the most asleep minutes (normally the overnight block); every
 * other session on the same wake day is a nap, surfaced separately and never
 * folded into the main night's headline.
 *
 * Returns the main session plus the naps (sorted by start). When the input
 * holds sessions across multiple wake days, only the sessions matching the
 * main session's wake day are considered naps — call once per wake day, or
 * pass a single wake day's sessions.
 */
export function pickMainNightAndNaps(sessions: readonly SleepSession[]): {
  main: SleepSession | null;
  naps: SleepSession[];
} {
  const scorable = sessions.filter((s) => s.asleepMinutes > 0);
  if (scorable.length === 0) return { main: null, naps: [] };
  // Main = most asleep minutes; tie-break on the later end (overnight wins).
  const main = [...scorable].sort((a, b) => {
    if (b.asleepMinutes !== a.asleepMinutes)
      return b.asleepMinutes - a.asleepMinutes;
    return b.end.getTime() - a.end.getTime();
  })[0];
  const naps = scorable
    .filter((s) => s !== main && s.night === main.night)
    .sort((a, b) => a.start.getTime() - b.start.getTime());
  return { main, naps };
}

export interface SleepNightSummary {
  /**
   * A `DataSummary` whose every value is a per-night TIME-ASLEEP total
   * (minutes). Drop-in replacement for the per-stage `summaries.
   * SLEEP_DURATION` the slim slice produced. Null when no night has any
   * asleep minutes in the supplied rows.
   */
  summary: DataSummary;
  /** The most recent night (for the dashboard/series headline), or null. */
  latestNight: SleepNight | null;
}

/**
 * Reconstruct nights from raw rows and summarise the per-night asleep
 * totals. Nights with zero asleep minutes are dropped (an IN_BED-only or
 * AWAKE-only fragment is not a scorable night). The returned `summary`
 * uses one DataPoint per night so `summarize()`'s windowed avg7 / avg30
 * become "average nightly sleep over the trailing N days".
 */
export function summarizeSleepNights(
  rows: SleepStageRow[],
  tz: string,
  priorityJson: unknown = null,
): SleepNightSummary {
  const nights = reconstructSleepNights(rows, tz, priorityJson).filter(
    (n) => n.asleepMinutes > 0,
  );
  const dataPoints = nights.map((n) => ({
    date: n.measuredAt,
    value: n.asleepMinutes,
  }));
  return {
    summary: summarize(dataPoints),
    latestNight: nights.length > 0 ? nights[nights.length - 1] : null,
  };
}