package router

import (
	"net/http"

	"github.com/gin-gonic/gin"
)

// LimitMiddleware caps the number of requests executing the routes it guards
// at limit. A request blocks until a slot frees up; if the request context is
// cancelled or expires while waiting, it answers 503 Service Unavailable.
//
// The semaphore is owned by the returned handler, so sharing a single instance
// across several routes serialises those routes against each other. The engine
// relies on this to serialise every operation that mutates the canonical game
// state file, which must never run concurrently against one storage directory.
func LimitMiddleware(limit int) gin.HandlerFunc {
	if limit <= 0 {
		panic("limit must be greater than 0")
	}
	semaphore := make(chan struct{}, limit)

	return func(c *gin.Context) {
		select {
		case semaphore <- struct{}{}:
			defer func() { <-semaphore }()
			c.Next()
		case <-c.Request.Context().Done():
			c.AbortWithStatus(http.StatusServiceUnavailable)
		}
	}
}