import { apiClient } from "../../../api-client";

export interface YieldPoint {
  maturity: string;      // "1M", "3M", "6M", "1Y", "2Y", "3Y", "5Y", "7Y", "10Y", "20Y", "30Y"
  maturityYears: number; // 0.083, 0.25, 0.5, 1, 2, 3, 5, 7, 10, 20, 30
  yield: number | null;  // percent, e.g., 4.29
  asOf?: string | null;  // FRED observation date, absent on older servers
  stale?: boolean;
  fetchedAt?: string;
  error?: string;
}

export function isYieldObservationDate(value: unknown): value is string {
  if (typeof value !== "string" || !/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
  const timestamp = Date.parse(value);
  return Number.isFinite(timestamp) && new Date(timestamp).toISOString().slice(0, 10) === value;
}

export function yieldCurveErrors(points: readonly YieldPoint[]): string[] {
  return points.flatMap((point) => point.error ? [`${point.maturity}: ${point.error}`] : []);
}

export const TREASURY_MATURITIES: Array<{ maturity: string; years: number; seriesId: string }> = [
  { maturity: "1M",  years: 1/12,  seriesId: "DGS1MO" },
  { maturity: "3M",  years: 0.25,  seriesId: "DGS3MO" },
  { maturity: "6M",  years: 0.5,   seriesId: "DGS6MO" },
  { maturity: "1Y",  years: 1,     seriesId: "DGS1" },
  { maturity: "2Y",  years: 2,     seriesId: "DGS2" },
  { maturity: "3Y",  years: 3,     seriesId: "DGS3" },
  { maturity: "5Y",  years: 5,     seriesId: "DGS5" },
  { maturity: "7Y",  years: 7,     seriesId: "DGS7" },
  { maturity: "10Y", years: 10,    seriesId: "DGS10" },
  { maturity: "20Y", years: 20,    seriesId: "DGS20" },
  { maturity: "30Y", years: 30,    seriesId: "DGS30" },
];

export type YieldCurveLoader = () => Promise<YieldPoint[]>;

export async function loadYieldCurve(
  loader: YieldCurveLoader = () => apiClient.getCloudYieldCurve(),
): Promise<YieldPoint[]> {
  return loader();
}

export function parseYieldPoints(points: YieldPoint[]): YieldPoint[] {
  return points
    .filter((p) => p.yield != null && Number.isFinite(p.yield))
    .sort((a, b) => a.maturityYears - b.maturityYears);
}

/**
 * A curve has one as-of date only when all its available tenors agree.
 */
export function curveAsOf(points: readonly YieldPoint[]): string | null {
  const available = points.filter((point) => point.yield != null && Number.isFinite(point.yield));
  const date = available[0]?.asOf;
  return isYieldObservationDate(date) && available.every((point) => point.asOf === date) ? date : null;
}

export function spreadBasisPoints(points: readonly YieldPoint[]): number | null {
  const y2 = points.find((point) => point.maturity === "2Y");
  const y10 = points.find((point) => point.maturity === "10Y");
  if (y2?.yield == null || y10?.yield == null || !Number.isFinite(y2.yield) || !Number.isFinite(y10.yield)
    || !isYieldObservationDate(y2.asOf) || y2.asOf !== y10.asOf) return null;
  return Math.round((y10.yield - y2.yield) * 100);
}

export function isInverted(points: readonly YieldPoint[]): boolean | null {
  const spread = spreadBasisPoints(points);
  return spread == null ? null : spread < 0;
}