// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

// Package metrics exposes Prometheus metrics for maglevd.
//
// Gauge-type metrics (backend state, health counter, weights) are collected
// on demand when Prometheus scrapes /metrics via the Collector. Counter and
// histogram metrics (probe totals, probe duration, transitions) are updated
// inline from the probe loop.
package metrics

import (
	"git.ipng.ch/ipng/vpp-maglev/internal/config"
	"git.ipng.ch/ipng/vpp-maglev/internal/health"

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

// BackendInfo holds the health and config state needed by the collector.
type BackendInfo struct {
	Health  *health.Backend
	Enabled bool
	HCName  string // healthcheck name from config
}

// StateSource provides read-only access to the running checker state.
type StateSource interface {
	ListBackends() []string
	GetBackendInfo(name string) (BackendInfo, bool)
	ListFrontends() []string
	GetFrontend(name string) (config.Frontend, bool)
}

// ---- inline metrics (updated per probe) ------------------------------------

var (
	ProbeTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Namespace: "maglev",
		Subsystem: "probe",
		Name:      "total",
		Help:      "Total number of health-check probes executed.",
	}, []string{"backend", "type", "result", "code"})

	ProbeDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
		Namespace: "maglev",
		Subsystem: "probe",
		Name:      "duration_seconds",
		Help:      "Health-check probe duration in seconds.",
		Buckets:   []float64{.001, .0025, .005, .01, .025, .05, .1, .25, .5, 1, 2.5},
	}, []string{"backend", "type"})

	TransitionTotal = prometheus.NewCounterVec(prometheus.CounterOpts{
		Namespace: "maglev",
		Subsystem: "backend",
		Name:      "transitions_total",
		Help:      "Total number of backend state transitions.",
	}, []string{"backend", "from", "to"})
)

// ---- collector (scraped on demand) -----------------------------------------

// Collector implements prometheus.Collector by querying the running checker
// on each scrape. This avoids stale label sets when backends are added or
// removed by a config reload.
type Collector struct {
	src StateSource

	backendState   *prometheus.Desc
	backendHealth  *prometheus.Desc
	backendEnabled *prometheus.Desc
	poolWeight     *prometheus.Desc
}

// NewCollector creates a Collector backed by the given StateSource.
func NewCollector(src StateSource) *Collector {
	return &Collector{
		src: src,
		backendState: prometheus.NewDesc(
			"maglev_backend_state",
			"Current backend state (1 = active for the given state label).",
			[]string{"backend", "address", "healthcheck", "state"}, nil,
		),
		backendHealth: prometheus.NewDesc(
			"maglev_backend_health",
			"Current health counter value.",
			[]string{"backend"}, nil,
		),
		backendEnabled: prometheus.NewDesc(
			"maglev_backend_enabled",
			"Whether the backend is enabled (1) or disabled (0).",
			[]string{"backend"}, nil,
		),
		poolWeight: prometheus.NewDesc(
			"maglev_frontend_pool_backend_weight",
			"Configured weight of a backend in a frontend pool (0-100).",
			[]string{"frontend", "pool", "backend"}, nil,
		),
	}
}

// Describe implements prometheus.Collector.
func (c *Collector) Describe(ch chan<- *prometheus.Desc) {
	ch <- c.backendState
	ch <- c.backendHealth
	ch <- c.backendEnabled
	ch <- c.poolWeight
}

// Collect implements prometheus.Collector.
func (c *Collector) Collect(ch chan<- prometheus.Metric) {
	states := []health.State{
		health.StateUnknown,
		health.StateUp,
		health.StateDown,
		health.StatePaused,
		health.StateRemoved,
	}

	for _, name := range c.src.ListBackends() {
		info, ok := c.src.GetBackendInfo(name)
		if !ok {
			continue
		}
		addr := info.Health.Address.String()

		// One time-series per possible state; the current state is 1, rest 0.
		for _, s := range states {
			val := 0.0
			if info.Health.State == s {
				val = 1.0
			}
			ch <- prometheus.MustNewConstMetric(
				c.backendState, prometheus.GaugeValue, val,
				name, addr, info.HCName, s.String(),
			)
		}

		ch <- prometheus.MustNewConstMetric(
			c.backendHealth, prometheus.GaugeValue,
			float64(info.Health.Counter.Health), name,
		)

		enabled := 0.0
		if info.Enabled {
			enabled = 1.0
		}
		ch <- prometheus.MustNewConstMetric(
			c.backendEnabled, prometheus.GaugeValue, enabled, name,
		)
	}

	for _, feName := range c.src.ListFrontends() {
		fe, ok := c.src.GetFrontend(feName)
		if !ok {
			continue
		}
		for _, pool := range fe.Pools {
			for beName, pb := range pool.Backends {
				ch <- prometheus.MustNewConstMetric(
					c.poolWeight, prometheus.GaugeValue,
					float64(pb.Weight), feName, pool.Name, beName,
				)
			}
		}
	}
}

// Register registers all metrics with the given registry.
func Register(reg prometheus.Registerer, src StateSource) *Collector {
	coll := NewCollector(src)
	reg.MustRegister(coll)
	reg.MustRegister(ProbeTotal)
	reg.MustRegister(ProbeDuration)
	reg.MustRegister(TransitionTotal)
	return coll
}