import type { Asset } from './types';
import { parseDecimalToUnits } from './amounts';
import { RATE_DECIMALS } from './peg';

const COINGECKO_IDS: Record<Asset, string> = { USDT: 'tether', USDC: 'usd-coin' };

export interface RateQuote {
  /** Fiat per 1 whole token, scaled by 1e6 (see RATE_DECIMALS). */
  rateUnits: bigint;
  source: string;
  at: Date;
}

export interface RateServiceOptions {
  fetchFn?: typeof fetch;
  /** Cache TTL, default 60s per the spec. */
  ttlMs?: number;
  now?: () => number;
  baseUrl?: string;
}

/** Convert a JSON price number into integer rate units without float arithmetic on money paths. */
export function priceToRateUnits(price: number): bigint {
  if (!Number.isFinite(price) || price <= 0) throw new Error(`invalid price: ${price}`);
  let text = String(price);
  if (/e/i.test(text)) text = price.toFixed(RATE_DECIMALS + 2);
  const [whole = '0', frac = ''] = text.split('.');
  const truncatedFrac = frac.slice(0, RATE_DECIMALS);
  return parseDecimalToUnits(truncatedFrac ? `${whole}.${truncatedFrac}` : whole, RATE_DECIMALS);
}

export class RateService {
  private readonly fetchFn: typeof fetch;
  private readonly ttlMs: number;
  private readonly now: () => number;
  private readonly baseUrl: string;
  private readonly cache = new Map<string, { fetchedAt: number; quote: RateQuote }>();

  constructor(options: RateServiceOptions = {}) {
    this.fetchFn = options.fetchFn ?? fetch;
    this.ttlMs = options.ttlMs ?? 60_000;
    this.now = options.now ?? Date.now;
    this.baseUrl = options.baseUrl ?? 'https://api.coingecko.com/api/v3';
  }

  async getRate(asset: Asset, fiatCurrency: string): Promise<RateQuote> {
    const fiat = fiatCurrency.toLowerCase();
    const key = `${asset}:${fiat}`;
    const nowMs = this.now();
    const cached = this.cache.get(key);
    if (cached && nowMs - cached.fetchedAt < this.ttlMs) return cached.quote;

    const id = COINGECKO_IDS[asset];
    const url = `${this.baseUrl}/simple/price?ids=${id}&vs_currencies=${encodeURIComponent(fiat)}`;
    const res = await this.fetchFn(url, { headers: { accept: 'application/json' } });
    if (!res.ok) throw new Error(`rate provider returned HTTP ${res.status}`);
    const body = (await res.json()) as Record<string, Record<string, number>>;
    const price = body?.[id]?.[fiat];
    if (typeof price !== 'number' || !(price > 0)) {
      throw new Error(`rate provider returned no ${fiatCurrency} price for ${asset}`);
    }
    const quote: RateQuote = { rateUnits: priceToRateUnits(price), source: 'coingecko', at: new Date(nowMs) };
    this.cache.set(key, { fetchedAt: nowMs, quote });
    return quote;
  }
}