package exec

import (
	"context"
	"fmt"
)

// LimitingBackend 用有界信号量限制底层 agent 并发，保护 GPU/API 资源。
type LimitingBackend struct {
	inner Backend
	sem   chan struct{}
}

// WrapWithConcurrencyLimit 包装 backend；max<=0 时默认 8。
func WrapWithConcurrencyLimit(inner Backend, max int) Backend {
	if inner == nil {
		return nil
	}
	if max <= 0 {
		max = 8
	}
	return &LimitingBackend{
		inner: inner,
		sem:   make(chan struct{}, max),
	}
}

func (l *LimitingBackend) Name() string {
	if l.inner == nil {
		return "limiting"
	}
	return l.inner.Name()
}

func (l *LimitingBackend) Execute(ctx context.Context, req Request) (*Result, error) {
	select {
	case l.sem <- struct{}{}:
		defer func() { <-l.sem }()
	case <-ctx.Done():
		return &Result{
			Success:     false,
			ErrorReason: fmt.Sprintf("agent concurrency wait canceled: %v", ctx.Err()),
		}, ctx.Err()
	}
	return l.inner.Execute(ctx, req)
}