/**
 * 后台按份数撤销拼单份额：校验并算出保留下来的份数。
 *
 * 口径（已确认）：
 *   - 只能部分撤销，撤到 0 份走「单人撤单」，所以 `1 ≤ 撤掉份数 ≤ 当前份数 − 1`；
 *   - 押金不随份数减少而退还（保持原冻结额），这里只算数量。
 */
export function resolveShareReduceTargetQuantity(input: {
  currentQuantity: number
  reduceQuantity: number
}): { nextQuantity: number; failureReason: 'invalid_quantity' | null } {
  const current = Math.floor(Number(input.currentQuantity) || 0)
  const reduce = Math.floor(Number(input.reduceQuantity) || 0)
  if (!Number.isFinite(current) || !Number.isFinite(reduce)) {
    return { nextQuantity: 0, failureReason: 'invalid_quantity' }
  }
  if (current < 2 || reduce < 1 || reduce > current - 1) {
    return { nextQuantity: 0, failureReason: 'invalid_quantity' }
  }
  return { nextQuantity: current - reduce, failureReason: null }
}