DriftLedger/src/domain/currency.ts

/**
 * 多币种支持（plan.md「5.8 多币种支持」）。
 * 参考 BeeCount v30 的交易级多币种（nativeAmount + 汇率快照）。
 */

import { compareDecimals, divideDecimals, multiplyDecimals, parseDecimal } from './decimal';

export interface ExchangeRate {
  from: string;
  to: string;
  rate: string;
  date: string;
}

/** 手动汇率覆盖（用户优先）。 */
export interface ExchangeRateOverride {
  from: string;
  to: string;
  rate: string;
  enabled: boolean;
}

/** 交易级多币种快照。 */
export interface TransactionCurrency {
  currencyCode: string;      // 交易货币（如 USD）
  nativeAmount: string;      // 原始金额
  exchangeRate: string;      // 记账时的汇率快照
  baseCurrency: string;      // 基础货币（如 CNY）
}

/** 货币转换。 */
export function convertCurrency(
  amount: string,
  from: string,
  to: string,
  rates: ExchangeRate[],
): string {
  if (from === to) return amount;
  const rate = rates.find(r => r.from === from && r.to === to);
  if (!rate) throw new Error(`无 ${from} → ${to} 的汇率`);
  const result = multiplyDecimals(amount, rate.rate);
  // 货币结果保留两位小数（使用 string decimal 精确截断，避免 parseFloat 精度风险）
  const parsed = parseDecimal(result);
  // 乘以 100 取整再除以 100，等效于截断到 2 位小数
  const scaled = (parsed.coefficient * 100n) / (10n ** BigInt(parsed.scale));
  // 手动格式化，保留尾随零（货币显示惯例：720 → 720.00）
  const sign = scaled < 0n ? '-' : '';
  const abs = scaled < 0n ? -scaled : scaled;
  const intPart = abs / 100n;
  const fracPart = abs % 100n;
  return `${sign}${intPart}.${fracPart.toString().padStart(2, '0')}`;
}

/**
 * 获取有效汇率（优先手动覆盖）。
 */
export function getEffectiveRate(
  from: string,
  to: string,
  autoRates: ExchangeRate[],
  overrides: ExchangeRateOverride[],
): ExchangeRate | null {
  // 手动覆盖优先
  const override = overrides.find(o => o.enabled && o.from === from && o.to === to);
  if (override) {
    return { from, to, rate: override.rate, date: new Date().toISOString().slice(0, 10) };
  }
  return autoRates.find(r => r.from === from && r.to === to) ?? null;
}

/** 构造交易级多币种快照（记账时调用）。 */
export function buildTransactionCurrency(
  nativeAmount: string,
  currencyCode: string,
  baseCurrency: string,
  rates: ExchangeRate[],
): TransactionCurrency {
  if (currencyCode === baseCurrency) {
    return { currencyCode, nativeAmount, exchangeRate: '1', baseCurrency };
  }
  const rate = rates.find(r => r.from === currencyCode && r.to === baseCurrency)
    ?? rates.find(r => r.from === baseCurrency && r.to === currencyCode);
  if (!rate) throw new Error(`无 ${currencyCode}/${baseCurrency} 汇率`);
  // 正向汇率直接用，反向取倒数
  const effectiveRate = rate.from === currencyCode ? rate.rate : divideDecimals('1', rate.rate);
  return { currencyCode, nativeAmount, exchangeRate: effectiveRate, baseCurrency };
}

/** 校验汇率列表完整性（基础货币对所有其他货币都有汇率）。 */
export function validateRateCoverage(
  baseCurrency: string,
  currencies: string[],
  rates: ExchangeRate[],
): string[] {
  const missing: string[] = [];
  for (const cur of currencies) {
    if (cur === baseCurrency) continue;
    const has = rates.some(r =>
      (r.from === cur && r.to === baseCurrency) || (r.from === baseCurrency && r.to === cur)
    );
    if (!has) missing.push(cur);
  }
  return missing;
}