// Package monitor 运行监控（方案系统管理模块补足）。
//
// 提供 Prometheus 指标采集中间件，覆盖 4 金指标：
//   - Latency：请求延迟直方图
//   - Traffic：请求总量计数器
//   - Errors：错误响应计数器
//   - Saturation：并发在途请求 gauge
//
// 使用方式：
//   r := gin.Default()
//   r.Use(monitor.Middleware())
//   r.GET("/metrics", monitor.Handler())
package monitor

import (
	"strconv"
	"time"

	"github.com/gin-gonic/gin"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
	// httpDuration 请求延迟直方图（Latency）。
	httpDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
		Name:    "tcs_http_request_duration_seconds",
		Help:    "HTTP 请求延迟（秒）",
		Buckets: []float64{0.01, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10},
	}, []string{"method", "path", "status"})

	// httpRequests 请求总量计数器（Traffic）。
	httpRequests = promauto.NewCounterVec(prometheus.CounterOpts{
		Name: "tcs_http_requests_total",
		Help: "HTTP 请求总量",
	}, []string{"method", "path", "status"})

	// httpErrors 错误响应计数器（Errors）。
	httpErrors = promauto.NewCounterVec(prometheus.CounterOpts{
		Name: "tcs_http_errors_total",
		Help: "HTTP 错误响应总量（状态码 >= 400）",
	}, []string{"method", "path", "status"})

	// httpInFlight 并发在途请求（Saturation）。
	httpInFlight = promauto.NewGauge(prometheus.GaugeOpts{
		Name: "tcs_http_in_flight_requests",
		Help: "当前在途 HTTP 请求数",
	})
)

// Middleware 返回 Gin 中间件，采集 Prometheus 指标。
func Middleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		start := time.Now()
		httpInFlight.Inc()

		c.Next()

		httpInFlight.Dec()
		status := strconv.Itoa(c.Writer.Status())
		elapsed := time.Since(start).Seconds()
		path := c.FullPath()
		if path == "" {
			path = "unknown"
		}

		httpDuration.WithLabelValues(c.Request.Method, path, status).Observe(elapsed)
		httpRequests.WithLabelValues(c.Request.Method, path, status).Inc()
		if c.Writer.Status() >= 400 {
			httpErrors.WithLabelValues(c.Request.Method, path, status).Inc()
		}
	}
}

// Handler 返回 Prometheus metrics 暴露端点。
func Handler() gin.HandlerFunc {
	return gin.WrapH(promhttp.Handler())
}