// Package employeecollection 收口员工代收款账单的金额、状态与预占不变量。
// 只依赖标准库、领域常量和稳定错误，不依赖传输、持久化或外部 SDK。
package employeecollection

import (
	"strings"
	"unicode/utf8"

	"github.com/break/junhong_cmp_fiber/pkg/constants"
	"github.com/break/junhong_cmp_fiber/pkg/errors"
)

// BillAmounts 描述一张员工代收款账单的应收、已核销、审批中预占与关闭事实。
type BillAmounts struct {
	// Receivable 表示应收金额（分），来源成功事务判定后不允许为负。
	Receivable int64
	// Received 表示企业微信最终通过后累计的已核销金额（分）。
	Received int64
	// Reserved 表示审批中分摊预占的金额（分）。
	Reserved int64
	// Closed 表示账单是否已关闭；已关闭账单的可核销余额为 0。
	Closed bool
}

// NewBillAmounts 依据来源应收金额构造初始账单金额事实。
// 应收金额必须大于零，避免零元账单立即成为已核销。
func NewBillAmounts(receivable int64) (BillAmounts, error) {
	amounts := BillAmounts{Receivable: receivable}
	if err := amounts.Validate(); err != nil {
		return BillAmounts{}, err
	}
	return amounts, nil
}

// Validate 校验账单金额不变量：应收为正，已核销与预占非负且合计不超过应收。
func (a BillAmounts) Validate() error {
	if a.Receivable <= 0 {
		return errors.New(errors.CodeInvalidParam, "账单应收金额必须大于零")
	}
	if a.Received < 0 || a.Reserved < 0 {
		return errors.New(errors.CodeInvalidParam, "账单已核销与预占金额不能为负")
	}
	if a.Received+a.Reserved > a.Receivable {
		return errors.New(errors.CodeInvalidParam, "账单已核销与预占金额合计不能超过应收金额")
	}
	return nil
}

// Available 返回账单当前可被新分摊占用的金额；已关闭账单始终返回 0。
func (a BillAmounts) Available() int64 {
	if a.Closed {
		return 0
	}
	available := a.Receivable - a.Received - a.Reserved
	if available < 0 {
		return 0
	}
	return available
}

// DerivedStatus 依据金额推导未关闭账单的核销状态。
// 调用方必须自行区分已关闭账单，关闭状态不可由金额推导。
func (a BillAmounts) DerivedStatus() int {
	switch {
	case a.Received <= 0:
		return constants.EmployeeCollectionBillStatusPending
	case a.Received >= a.Receivable:
		return constants.EmployeeCollectionBillStatusSettled
	default:
		return constants.EmployeeCollectionBillStatusPartial
	}
}

// Reserve 在审批中预占指定金额，返回预占后的新金额事实。
// 分摊金额必须大于零且不超过当前可核销余额。
func (a BillAmounts) Reserve(amount int64) (BillAmounts, error) {
	if amount <= 0 {
		return BillAmounts{}, errors.New(errors.CodeEmployeeCollectionAllocationAmountInvalid)
	}
	if err := a.ensureSettleable(); err != nil {
		return BillAmounts{}, err
	}
	if amount > a.Available() {
		return BillAmounts{}, errors.New(errors.CodeEmployeeCollectionAllocationExceeded)
	}
	a.Reserved += amount
	return a, nil
}

// Release 释放指定金额的审批中预占，返回释放后的新金额事实。
func (a BillAmounts) Release(amount int64) (BillAmounts, error) {
	if amount <= 0 {
		return BillAmounts{}, errors.New(errors.CodeEmployeeCollectionAllocationAmountInvalid)
	}
	if amount > a.Reserved {
		return BillAmounts{}, errors.New(errors.CodeInternalError, "释放的预占金额超过账单当前预占")
	}
	a.Reserved -= amount
	return a, nil
}

// Approve 将指定金额从审批中预占转入已核销，返回通过后的新金额事实。
func (a BillAmounts) Approve(amount int64) (BillAmounts, error) {
	if amount <= 0 {
		return BillAmounts{}, errors.New(errors.CodeEmployeeCollectionAllocationAmountInvalid)
	}
	if amount > a.Reserved {
		return BillAmounts{}, errors.New(errors.CodeInternalError, "通过的分摊金额超过账单当前预占")
	}
	a.Reserved -= amount
	a.Received += amount
	return a, nil
}

// ReduceReceivable 按来源订单退款金额冲减应收，仅在账单不存在任何已通过或审批中分摊时允许。
func (a BillAmounts) ReduceReceivable(amount int64) (BillAmounts, error) {
	if amount <= 0 {
		return BillAmounts{}, errors.New(errors.CodeInvalidParam, "冲减金额必须大于零")
	}
	if a.Received > 0 || a.Reserved > 0 {
		return BillAmounts{}, errors.New(errors.CodeEmployeeCollectionBillNotSettleable, "账单存在分摊，不能冲减应收")
	}
	if amount >= a.Receivable {
		return BillAmounts{}, errors.New(errors.CodeInvalidParam, "冲减金额必须小于账单应收金额")
	}
	a.Receivable -= amount
	return a, nil
}

// ensureSettleable 校验账单允许产生新的审批中分摊。
func (a BillAmounts) ensureSettleable() error {
	if a.Closed {
		return errors.New(errors.CodeEmployeeCollectionBillClosed)
	}
	if a.DerivedStatus() == constants.EmployeeCollectionBillStatusSettled {
		return errors.New(errors.CodeEmployeeCollectionBillNotSettleable)
	}
	return nil
}

// BillCloseInput 描述关闭一张账单前的事实。
type BillCloseInput struct {
	// Status 表示账单当前持久化状态。
	Status int
	// PendingAllocations 表示账单上仍处于审批中（预占）的分摊数量。
	PendingAllocations int64
	// Reason 表示关闭原因，必填。
	Reason string
}

// ValidateBillClose 校验关闭账单的前置条件。
// 已关闭账单返回账单已关闭，已核销账单不允许关闭，存在审批中分摊时拒绝关闭，关闭原因必填。
func ValidateBillClose(input BillCloseInput) error {
	if input.Status == constants.EmployeeCollectionBillStatusClosed {
		return errors.New(errors.CodeEmployeeCollectionBillClosed)
	}
	if input.Status == constants.EmployeeCollectionBillStatusSettled {
		return errors.New(errors.CodeEmployeeCollectionBillNotSettleable, "已核销账单没有未核销余额，不能关闭")
	}
	if input.Status != constants.EmployeeCollectionBillStatusPending &&
		input.Status != constants.EmployeeCollectionBillStatusPartial {
		return errors.New(errors.CodeConflict, "账单当前状态不允许关闭")
	}
	if input.PendingAllocations > 0 {
		return errors.New(errors.CodeEmployeeCollectionApplicationPending)
	}
	if trimmed := strings.TrimSpace(input.Reason); trimmed == "" {
		return errors.New(errors.CodeInvalidParam, "关闭原因必填")
	} else if utf8.RuneCountInString(trimmed) > constants.EmployeeCollectionRemarkMaxLength {
		return errors.New(errors.CodeInvalidParam, "关闭原因长度超出限制")
	}
	return nil
}