package obs

import (
	"net/http"
	"runtime"
	"strings"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/collectors"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

const scrapesInFlight = 2

type Label struct {
	Name   string
	Values []string
}

type Registry struct {
	reg  *prometheus.Registry
	lazy lazy
}

func New() *Registry {
	reg := prometheus.NewRegistry()
	reg.MustRegister(
		collectors.NewGoCollector(),
		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
		buildInfo(),
	)
	return &Registry{reg: reg}
}

func buildInfo() prometheus.Collector {
	g := prometheus.NewGauge(prometheus.GaugeOpts{
		Name:        "tug_build_info",
		Help:        "The running tug binary: the Go version that built it.",
		ConstLabels: prometheus.Labels{"goversion": runtime.Version()},
	})
	g.Set(1)
	return g
}

func (r *Registry) Handler() http.Handler {
	return promhttp.HandlerFor(r.reg, promhttp.HandlerOpts{MaxRequestsInFlight: scrapesInFlight})
}

func (r *Registry) Prometheus() *prometheus.Registry {
	if r == nil {
		return nil
	}
	return r.reg
}

type Counter struct {
	children map[string]prometheus.Counter
}

const keySep = "\xff"

func (r *Registry) Counter(name, help string, labels ...Label) *Counter {
	if r == nil {
		return nil
	}
	names := make([]string, len(labels))
	for i, l := range labels {
		names[i] = l.Name
	}
	vec := prometheus.NewCounterVec(prometheus.CounterOpts{Name: name, Help: help}, names)
	r.reg.MustRegister(vec)
	return children(vec, product(labels))
}

func product(labels []Label) [][]string {
	combos := [][]string{nil}
	for _, l := range labels {
		var next [][]string
		for _, prefix := range combos {
			for _, v := range l.Values {
				next = append(next, append(append([]string(nil), prefix...), v))
			}
		}
		combos = next
	}
	return combos
}

func children(vec *prometheus.CounterVec, combos [][]string) *Counter {
	c := &Counter{children: map[string]prometheus.Counter{}}
	for _, values := range combos {
		c.children[strings.Join(values, keySep)] = vec.WithLabelValues(values...)
	}
	return c
}

func (c *Counter) Inc(values ...string) {
	if c == nil {
		return
	}
	if child, ok := c.children[strings.Join(values, keySep)]; ok {
		child.Inc()
	}
}