package transaction

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
)

// InspectionStatus 表示外部系统中某一步的实际结果。
type InspectionStatus string

const (
	InspectionApplied    InspectionStatus = "APPLIED"
	InspectionNotApplied InspectionStatus = "NOT_APPLIED"
	InspectionUnknown    InspectionStatus = "UNKNOWN"
)

// Inspection 是执行器通过 inspect、摘要、健康检查等方式得到的实际状态。
type Inspection struct {
	Status InspectionStatus
	Result json.RawMessage
}

// Operation 是一个可核对实际结果的外部副作用。
// Apply 返回成功只代表调用完成；最终成功必须由 Inspect 确认。
type Operation interface {
	Apply(context.Context) error
	Inspect(context.Context) (Inspection, error)
}

// UncertainStepError 表示当前无法确认外部副作用是否已经发生。
// 这种错误必须保留 INTENT_RECORDED，等待恢复流程再次核对。
type UncertainStepError struct {
	TransactionID string
	StepKey       string
	Cause         error
}

func (e *UncertainStepError) Error() string {
	return fmt.Sprintf("external step result is uncertain: transaction=%s step=%s: %v", e.TransactionID, e.StepKey, e.Cause)
}

func (e *UncertainStepError) Unwrap() error {
	return e.Cause
}

// Coordinator 串行化单机更新，并实现“先记录意图、再执行、最后 inspect”的步骤协议。
type Coordinator struct {
	store  *Store
	logger *slog.Logger
	permit chan struct{}
}

func NewCoordinator(store *Store, logger *slog.Logger) (*Coordinator, error) {
	if store == nil {
		return nil, errors.New("transaction store is required")
	}
	if logger == nil {
		logger = slog.Default()
	}
	permit := make(chan struct{}, 1)
	permit <- struct{}{}
	return &Coordinator{store: store, logger: logger, permit: permit}, nil
}

// RunExclusive 在一个进程内只允许一个完整更新流程进入执行区。
func (c *Coordinator) RunExclusive(ctx context.Context, run func(context.Context) error) error {
	if run == nil {
		return errors.New("exclusive update function is required")
	}
	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-c.permit:
	}
	defer func() { c.permit <- struct{}{} }()
	return run(ctx)
}

// ExecuteStep 执行或恢复一个外部步骤。
// 相同 step key 再次调用时先核对现场，禁止直接重复 Apply。
func (c *Coordinator) ExecuteStep(ctx context.Context, transactionID string, intent StepIntent, operation Operation) (Step, error) {
	if operation == nil {
		return Step{}, errors.New("external operation is required")
	}
	step, created, err := c.store.RecordStepIntent(ctx, transactionID, intent)
	if err != nil {
		return Step{}, err
	}
	if step.Status == StepSucceeded {
		return step, nil
	}
	if step.Status == StepFailed {
		return step, fmt.Errorf("external step already failed: %s", step.Error)
	}

	if !created {
		inspection, err := operation.Inspect(ctx)
		if err != nil {
			return step, c.uncertain(ctx, transactionID, intent.Key, err)
		}
		switch inspection.Status {
		case InspectionApplied:
			return c.completeApplied(ctx, transactionID, intent.Key, inspection)
		case InspectionUnknown:
			return step, c.uncertain(ctx, transactionID, intent.Key, errors.New("inspect returned UNKNOWN"))
		case InspectionNotApplied:
			// 现场明确未发生副作用后，才允许恢复流程重新 Apply。
		default:
			return step, c.uncertain(ctx, transactionID, intent.Key, fmt.Errorf("inspect returned invalid status %q", inspection.Status))
		}
	}

	c.logger.InfoContext(ctx, "external step apply started",
		"transaction_id", transactionID,
		"step_key", intent.Key,
		"step_name", intent.Name,
	)
	applyErr := operation.Apply(ctx)
	inspection, inspectErr := operation.Inspect(ctx)
	if inspectErr != nil {
		return step, c.uncertain(ctx, transactionID, intent.Key, errors.Join(applyErr, inspectErr))
	}
	switch inspection.Status {
	case InspectionApplied:
		completed, err := c.completeApplied(ctx, transactionID, intent.Key, inspection)
		if err == nil {
			c.logger.InfoContext(ctx, "external step applied",
				"transaction_id", transactionID,
				"step_key", intent.Key,
			)
		}
		return completed, err
	case InspectionUnknown:
		return step, c.uncertain(ctx, transactionID, intent.Key, errors.Join(applyErr, errors.New("inspect returned UNKNOWN")))
	case InspectionNotApplied:
		failure := applyErr
		if failure == nil {
			failure = errors.New("operation completed without reaching the expected external state")
		}
		completed, err := c.store.CompleteStep(ctx, transactionID, intent.Key, StepFailed, inspection.Result, failure.Error())
		if err != nil {
			return Step{}, errors.Join(failure, err)
		}
		c.logger.ErrorContext(ctx, "external step failed",
			"transaction_id", transactionID,
			"step_key", intent.Key,
			"error", failure,
		)
		return completed, failure
	default:
		return step, c.uncertain(ctx, transactionID, intent.Key, fmt.Errorf("inspect returned invalid status %q", inspection.Status))
	}
}

func (c *Coordinator) completeApplied(ctx context.Context, transactionID, stepKey string, inspection Inspection) (Step, error) {
	return c.store.CompleteStep(ctx, transactionID, stepKey, StepSucceeded, inspection.Result, "")
}

func (c *Coordinator) uncertain(ctx context.Context, transactionID, stepKey string, cause error) error {
	c.logger.WarnContext(ctx, "external step result is uncertain",
		"transaction_id", transactionID,
		"step_key", stepKey,
		"error", cause,
	)
	return &UncertainStepError{TransactionID: transactionID, StepKey: stepKey, Cause: cause}
}