export interface PositionSize {
  shares: number;
  riskPerShare: number;
  dollarRisk: number;
  positionValue: number;
  /** Position value exceeds the account → needs margin / not affordable in cash. */
  exceedsAccount: boolean;
}

/**
 * Risk-based position sizing. Risk a fixed % of the account per trade; the stop
 * distance sets how many shares that budget buys:
 *   shares = floor((account × risk%) / |entry − stop|)
 * Returns null when inputs are unusable (no account, no risk, zero stop width).
 */
export function positionSize(
  accountSize: number,
  riskPct: number,
  entry: number,
  stop: number,
): PositionSize | null {
  const riskPerShare = Math.abs(entry - stop);
  if (!(accountSize > 0) || !(riskPct > 0) || !(riskPerShare > 0) || !(entry > 0)) {
    return null;
  }
  const budget = accountSize * (riskPct / 100);
  const shares = Math.floor(budget / riskPerShare);
  const dollarRisk = shares * riskPerShare;
  const positionValue = shares * entry;
  return {
    shares,
    riskPerShare,
    dollarRisk,
    positionValue,
    exceedsAccount: positionValue > accountSize,
  };
}