package handler

import (
	"fmt"
	"net/http"
	"runtime"
	"time"

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

// MetricsHandler exposes Prometheus-compatible metrics in text exposition format.
// Reference: Chatwoot uses Prometheus exporter for Sidekiq, Rails metrics
// This provides Go runtime + application metrics on a dedicated port.

// PrometheusHandler returns metrics in Prometheus text format.
func PrometheusHandler(startTime time.Time) gin.HandlerFunc {
	return func(c *gin.Context) {
		var m runtime.MemStats
		runtime.ReadMemStats(&m)

		uptime := time.Since(startTime).Seconds()

		metrics := []string{
			// Go runtime metrics
			formatGauge("gochat_go_goroutines", uint64(runtime.NumGoroutine())),
			formatGauge("gochat_go_memory_alloc_bytes", m.Alloc),
			formatGauge("gochat_go_memory_sys_bytes", m.Sys),
			formatGauge("gochat_go_memory_total_alloc_bytes", m.TotalAlloc),
			formatGauge("gochat_go_gc_pause_total_ns", m.PauseTotalNs),
			formatCounter("gochat_go_gc_count", uint64(m.NumGC)),
			// Application metrics
			formatGauge("gochat_uptime_seconds", uint64(uptime)),
			formatGauge("gochat_threads_count", uint64(runtime.NumCPU())),
		}

		// HELP and TYPE annotations
		help := []string{
			"# HELP gochat_go_goroutines Number of goroutines currently running",
			"# TYPE gochat_go_goroutines gauge",
			"# HELP gochat_go_memory_alloc_bytes Bytes of allocated heap objects",
			"# TYPE gochat_go_memory_alloc_bytes gauge",
			"# HELP gochat_go_memory_sys_bytes Bytes obtained from system",
			"# TYPE gochat_go_memory_sys_bytes gauge",
			"# HELP gochat_uptime_seconds Application uptime in seconds",
			"# TYPE gochat_uptime_seconds gauge",
		}

		output := ""
		for _, h := range help {
			output += h + "\n"
		}
		for _, m := range metrics {
			output += m + "\n"
		}

		c.Header("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
		c.String(http.StatusOK, output)
	}
}

func formatGauge(name string, value uint64) string {
	return name + " " + formatValue(value)
}

func formatCounter(name string, value uint64) string {
	return name + " " + formatValue(value)
}

func formatValue(v uint64) string {
	// Simple uint64 formatting without strconv dependency
	if v == 0 {
		return "0"
	}
	result := ""
	for v > 0 {
		digit := v % 10
		result = fmt.Sprintf("%d%s", digit, result)
		v /= 10
	}
	return result
}