// ---------------------------------------------------------------------------
// ui / fmt.ts — EXIT wave 2 (W2-6, contract Y2).
//
// THE formatters. Y1's rule 3 is the reason this file exists: *"the server
// NEVER sends a pre-formatted string ⇒ one formatter change fixes every page."*
// A number arrives as a number with an `fmt` code beside it, and this module is
// the only place that decides how it reads.
//
// ⚠⚠ TWO `fmt` VOCABULARIES EXIST AND THEY ARE NOT INTERCHANGEABLE (Y1 rule 1).
// This file owns the **Y1 fmt enum** — `money · num · pct · int · date`, CLOSED —
// used by `kpis`, `table` and `validation` blocks. A `chart` block's `fields[]`
// instead carries the **viz `FieldType`** vocabulary (`currency · int · pct ·
// date · text · status`), because those objects are fed straight into
// `chartData(spec, rows, fieldByKey)`. `fmt:"money"` inside a chart field
// typechecks nowhere near here and renders as raw text — it is the bug that
// looks fine. Keep the two apart: `Fmt` below is never assigned to a viz field.
//
// The rules are the SHIPPED ones, mirrored from `platform/ui/primitives.py`
// so a number reads identically in the Streamlit page and in the shell during
// the strangle. Where this file deliberately differs, it says so.
// ---------------------------------------------------------------------------

/** Y1's closed format enum. A column with NO `fmt` renders verbatim (rule 2) —
 *  that is how a text column is expressed without opening the enum. */
export type Fmt = "money" | "num" | "pct" | "int" | "date";

/** What every formatter returns for a value it has nothing to say about.
 *  An em dash, never "0" and never "" — a missing number and a zero are
 *  different facts and a KPI card must not conflate them. */
export const BLANK = "—";

/** Above this, a percentage stops being readable and becomes a multiple.
 *  `+14,975%` reads as noise; `+151×` reads as a fact (Y1 rule 3). */
export const PCT_AS_MULTIPLE_AT = 999;

function toNumber(v: unknown): number | null {
  if (v == null || v === "") return null;
  const n = typeof v === "number" ? v : Number(v);
  return Number.isFinite(n) ? n : null;
}

/**
 * The house number rule, from `ui/primitives.py:num` verbatim: comma-grouped,
 * NO decimals — except a non-whole single-digit value (|v| < 10), which keeps
 * one. So big numbers read clean (1,204,882), small ratios keep their meaning
 * (7.7), and whole numbers never carry a trailing `.0` ($0, not $0.0).
 */
export function num(v: unknown, dp?: number): string {
  const n = toNumber(v);
  if (n == null) return BLANK;
  const places = dp ?? (Math.abs(n) < 10 && !Number.isInteger(n) ? 1 : 0);
  return n.toLocaleString(undefined, {
    minimumFractionDigits: places,
    maximumFractionDigits: places,
  });
}

/** Money. The sign goes OUTSIDE the currency mark (`-$1,204`, not `$-1,204`) —
 *  `primitives.py:money`'s shape, and the one that reads as a loss at a glance. */
export function money(v: unknown, dp?: number): string {
  const n = toNumber(v);
  if (n == null) return BLANK;
  return n < 0 ? `-$${num(Math.abs(n), dp)}` : `$${num(n, dp)}`;
}

/** Whole counts. Always 0 dp — an order count with a decimal is a bug wearing
 *  a format. */
export function int(v: unknown): string {
  const n = toNumber(v);
  if (n == null) return BLANK;
  return Math.round(n).toLocaleString();
}

export interface PctOptions {
  /** Render a leading `+` on non-negative values (deltas do; shares do not). */
  signed?: boolean;
  dp?: number;
}

/**
 * Percentages, and the ONE place the big-ratio rule lives (Y1 rule 3).
 *
 * A percentage over ~1000% is arithmetically fine and rhetorically useless: it
 * means the prior-year base was tiny, and `+14,975%` invites the reader to
 * treat a rounding artefact as a result. Past `PCT_AS_MULTIPLE_AT` the value is
 * restated as the MULTIPLE it actually is — `1 + v/100`, zero decimals, exactly
 * `app.py:2851`'s formula. The caller supplies the comparison word (Y1 sends
 * `delta_label: "vs LY"`), so this returns `+151×` and never invents a suffix.
 *
 * ⚠ The value is a PERCENTAGE ALREADY — 12.4 means 12.4%. It is never
 * multiplied by 100 here. Semantic percentages elsewhere in this codebase are
 * 0–1 while transform `*_pct` values are 0–100 ([[analyst-chart-library]]), and
 * a blanket rescale would be silently wrong for half of them.
 */
export function pct(v: unknown, opts: PctOptions = {}): string {
  const n = toNumber(v);
  if (n == null) return BLANK;
  if (n >= PCT_AS_MULTIPLE_AT) {
    const mult = (1 + n / 100).toLocaleString(undefined, {
      minimumFractionDigits: 0,
      maximumFractionDigits: 0,
    });
    return `+${mult}×`;
  }
  const sign = opts.signed && n >= 0 ? "+" : "";
  return `${sign}${num(n, opts.dp)}%`;
}

const MONTHS = ["Jan", "Feb", "Mar", "Apr", "May", "Jun",
                "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];

/**
 * Dates, from what the server actually sends: an ISO date, an ISO datetime, or
 * a bare `YYYY-MM` month key.
 *
 * ⚠ Every branch is computed in UTC ON PURPOSE. `new Date("2026-05-01")` parses
 * as UTC midnight, so `toLocaleDateString()` in any negative-offset timezone
 * renders it as 30 April — a report row dated the last day of the previous
 * month. The grid's cell formatter (`customer-grid/display.ts:dateTimeText`)
 * still has that behaviour and documents it as byte-compatibility with a
 * pre-wave-5 path; a formatter written today should not inherit it, so this one
 * does not. That is a deliberate difference, not a drift.
 *
 * An unparseable value renders VERBATIM rather than "Invalid Date" — the raw
 * string is at least true.
 */
export function date(v: unknown): string {
  if (v == null || v === "") return BLANK;
  const s = String(v);
  const month = /^(\d{4})-(\d{2})$/.exec(s);
  if (month) {
    const m = Number(month[2]);
    if (m >= 1 && m <= 12) return `${MONTHS[m - 1]} ${month[1].slice(2)}`;
  }
  const iso = s.includes(" ") ? s.replace(" ", "T") : s;
  const withZone =
    /T\d{2}:\d{2}/.test(iso) && !/(?:[zZ]|[+-]\d{2}:?\d{2})$/.test(iso) ? `${iso}Z` : iso;
  const d = new Date(withZone);
  if (Number.isNaN(d.getTime())) return s;
  return d.toLocaleDateString(undefined, {
    year: "numeric",
    month: "short",
    day: "numeric",
    timeZone: "UTC",
  });
}

/** Y2's named surface. The contract fixes these five names. */
export const fmt = { money, num, pct, int, date } as const;

/**
 * Render one value under an OPTIONAL format code — the dispatcher every block
 * renderer uses.
 *
 * ⛔ `code` undefined means RENDER VERBATIM (Y1 rule 2), which is how a text
 * column exists without opening the closed enum. It does NOT mean "guess": a
 * formatter picked from the value's runtime type would format an account number
 * as `4,412` and a postcode as `01,234`.
 *
 * An UNKNOWN code also renders verbatim rather than throwing — same reasoning
 * as Y1 rule 7's unknown-block rule: the server may learn a format before this
 * client does, and one new code must not blank a whole page.
 */
export function formatValue(v: unknown, code?: string, opts?: PctOptions): string {
  if (code === undefined || code === null || code === "") {
    return v == null || v === "" ? BLANK : String(v);
  }
  switch (code) {
    case "money": return money(v);
    case "num":   return num(v);
    case "pct":   return pct(v, opts);
    case "int":   return int(v);
    case "date":  return date(v);
    default:      return v == null || v === "" ? BLANK : String(v);
  }
}