// Package metrics implements Prometheus-compatible metrics for applications.
//
// This package is lightweight alternative to https://github.com/prometheus/client_golang
// with simpler API and smaller dependencies.
//
// Usage:
//
//     1. Register the required metrics via New* functions.
//     2. Expose them to `/metrics` page via WritePrometheus.
//     3. Update the registered metrics during application lifetime.
package metrics

import (
	"fmt"
	"io"
	"runtime"
	"sort"
	"sync"

	"github.com/valyala/histogram"
)

var (
	metricsMapLock sync.Mutex
	metricsList    []*namedMetric
	metricsMap     = make(map[string]*namedMetric)
)

type namedMetric struct {
	name   string
	metric metric
}

type metric interface {
	marshalTo(prefix string, w io.Writer)
}

// WritePrometheus writes all the registered metrics in Prometheus format to w.
//
// If exposeProcessMetrics is true, then various `go_*` metrics are exposed
// for the current process.
//
// The WritePrometheus func is usually called inside "/metrics" handler:
//
//     http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
//         metrics.WritePrometheus(w, true)
//     })
//
func WritePrometheus(w io.Writer, exposeProcessMetrics bool) {
	lessFunc := func(i, j int) bool {
		return metricsList[i].name < metricsList[j].name
	}
	metricsMapLock.Lock()
	if !sort.SliceIsSorted(metricsList, lessFunc) {
		sort.Slice(metricsList, lessFunc)
	}
	for _, nm := range metricsList {
		nm.metric.marshalTo(nm.name, w)
	}
	metricsMapLock.Unlock()
	if exposeProcessMetrics {
		writeProcessMetrics(w)
	}
}

func writeProcessMetrics(w io.Writer) {
	var ms runtime.MemStats
	runtime.ReadMemStats(&ms)
	fmt.Fprintf(w, `go_memstats_alloc_bytes %d`+"\n", ms.Alloc)
	fmt.Fprintf(w, `go_memstats_alloc_bytes_total %d`+"\n", ms.TotalAlloc)
	fmt.Fprintf(w, `go_memstats_buck_hash_sys_bytes %d`+"\n", ms.BuckHashSys)
	fmt.Fprintf(w, `go_memstats_frees_total %d`+"\n", ms.Frees)
	fmt.Fprintf(w, `go_memstats_gc_cpu_fraction %f`+"\n", ms.GCCPUFraction)
	fmt.Fprintf(w, `go_memstats_gc_sys_bytes %d`+"\n", ms.GCSys)
	fmt.Fprintf(w, `go_memstats_heap_alloc_bytes %d`+"\n", ms.HeapAlloc)
	fmt.Fprintf(w, `go_memstats_heap_idle_bytes %d`+"\n", ms.HeapIdle)
	fmt.Fprintf(w, `go_memstats_heap_inuse_bytes %d`+"\n", ms.HeapInuse)
	fmt.Fprintf(w, `go_memstats_heap_objects %d`+"\n", ms.HeapObjects)
	fmt.Fprintf(w, `go_memstats_heap_released_bytes %d`+"\n", ms.HeapReleased)
	fmt.Fprintf(w, `go_memstats_heap_sys_bytes %d`+"\n", ms.HeapSys)
	fmt.Fprintf(w, `go_memstats_last_gc_time_seconds %f`+"\n", float64(ms.LastGC)/1e9)
	fmt.Fprintf(w, `go_memstats_lookups_total %d`+"\n", ms.Lookups)
	fmt.Fprintf(w, `go_memstats_mallocs_total %d`+"\n", ms.Mallocs)
	fmt.Fprintf(w, `go_memstats_mcache_inuse_bytes %d`+"\n", ms.MCacheInuse)
	fmt.Fprintf(w, `go_memstats_mcache_sys_bytes %d`+"\n", ms.MCacheSys)
	fmt.Fprintf(w, `go_memstats_mspan_inuse_bytes %d`+"\n", ms.MSpanInuse)
	fmt.Fprintf(w, `go_memstats_mspan_sys_bytes %d`+"\n", ms.MSpanSys)
	fmt.Fprintf(w, `go_memstats_next_gc_bytes %d`+"\n", ms.NextGC)
	fmt.Fprintf(w, `go_memstats_other_sys_bytes %d`+"\n", ms.OtherSys)
	fmt.Fprintf(w, `go_memstats_stack_inuse_bytes %d`+"\n", ms.StackInuse)
	fmt.Fprintf(w, `go_memstats_stack_sys_bytes %d`+"\n", ms.StackSys)
	fmt.Fprintf(w, `go_memstats_sys_bytes %d`+"\n", ms.Sys)

	fmt.Fprintf(w, `go_cgo_calls_count %d`+"\n", runtime.NumCgoCall())
	fmt.Fprintf(w, `go_cpu_count %d`+"\n", runtime.NumCPU())

	gcPauses := histogram.NewFast()
	for _, pauseNs := range ms.PauseNs[:] {
		gcPauses.Update(float64(pauseNs) / 1e9)
	}
	phis := []float64{0, 0.25, 0.5, 0.75, 1}
	quantiles := make([]float64, 0, len(phis))
	for i, q := range gcPauses.Quantiles(quantiles[:0], phis) {
		fmt.Fprintf(w, `go_gc_duration_seconds{quantile="%g"} %f`+"\n", phis[i], q)
	}
	fmt.Fprintf(w, `go_gc_duration_seconds_sum %f`+"\n", float64(ms.PauseTotalNs)/1e9)
	fmt.Fprintf(w, `go_gc_duration_seconds_count %d`+"\n", ms.NumGC)
	fmt.Fprintf(w, `go_gc_forced_count %d`+"\n", ms.NumForcedGC)

	fmt.Fprintf(w, `go_gomaxprocs %d`+"\n", runtime.GOMAXPROCS(0))
	fmt.Fprintf(w, `go_goroutines %d`+"\n", runtime.NumGoroutine())
	numThread, _ := runtime.ThreadCreateProfile(nil)
	fmt.Fprintf(w, `go_threads %d`+"\n", numThread)

	// Export build details.
	fmt.Fprintf(w, "go_info{version=%q} 1\n", runtime.Version())
	fmt.Fprintf(w, "go_info_ext{compiler=%q, GOARCH=%q, GOOS=%q, GOROOT=%q} 1\n",
		runtime.Compiler, runtime.GOARCH, runtime.GOOS, runtime.GOROOT())
}

func registerMetric(name string, m metric) {
	if err := validateMetric(name); err != nil {
		panic(fmt.Errorf("BUG: invalid metric name %q: %s", name, err))
	}
	metricsMapLock.Lock()
	nm, ok := metricsMap[name]
	if !ok {
		nm = &namedMetric{
			name:   name,
			metric: m,
		}
		metricsMap[name] = nm
		metricsList = append(metricsList, nm)
	}
	metricsMapLock.Unlock()
	if ok {
		panic(fmt.Errorf("BUG: metric %q is already registered", name))
	}
}