/*
Copyright 2019 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package metrics

import (
	"errors"

	"github.com/prometheus/client_golang/prometheus"
	dto "github.com/prometheus/client_model/go"
)

// This file contains a series of interfaces which we explicitly define for
// integrating with prometheus. We redefine the interfaces explicitly here
// so that we can prevent breakage if methods are ever added to prometheus
// variants of them.

// Collector defines a subset of prometheus.Collector interface methods
type Collector interface {
	Describe(chan<- *prometheus.Desc)
	Collect(chan<- prometheus.Metric)
}

// Metric defines a subset of prometheus.Metric interface methods
type Metric interface {
	Desc() *prometheus.Desc
	Write(*dto.Metric) error
}

// CounterMetric is a Metric that represents a single numerical value that only ever
// goes up. That implies that it cannot be used to count items whose number can
// also go down, e.g. the number of currently running goroutines. Those
// "counters" are represented by Gauges.

// CounterMetric is an interface which defines a subset of the interface provided by prometheus.Counter
type CounterMetric interface {
	Inc()
	Add(float64)
}

// CounterVecMetric is an interface which prometheus.CounterVec satisfies.
type CounterVecMetric interface {
	WithLabelValues(...string) CounterMetric
	With(prometheus.Labels) CounterMetric
}

// GaugeMetric is an interface which defines a subset of the interface provided by prometheus.Gauge
type GaugeMetric interface {
	Set(float64)
	Inc()
	Dec()
	Add(float64)
	Write(out *dto.Metric) error
	SetToCurrentTime()
}

// GaugeVecMetric is a collection of Gauges that differ only in label values.
type GaugeVecMetric interface {
	// Default Prometheus Vec behavior is that member extraction results in creation of a new element
	// if one with the unique label values is not found in the underlying stored metricMap.
	// This means  that if this function is called but the underlying metric is not registered
	// (which means it will never be exposed externally nor consumed), the metric would exist in memory
	// for perpetuity (i.e. throughout application lifecycle).
	//
	// For reference: https://github.com/prometheus/client_golang/blob/v0.9.2/prometheus/gauge.go#L190-L208
	//
	// In contrast, the Vec behavior in this package is that member extraction before registration
	// returns a permanent noop object.

	// WithLabelValuesChecked, if called before this vector has been registered in
	// at least one registry, will return a noop gauge and
	// an error that passes ErrIsNotRegistered.
	// If called on a hidden vector,
	// will return a noop gauge and a nil error.
	// If called with a syntactic problem in the labels, will
	// return a noop gauge and an error about the labels.
	// If none of the above apply, this method will return
	// the appropriate vector member and a nil error.
	WithLabelValuesChecked(labelValues ...string) (GaugeMetric, error)

	// WithLabelValues calls WithLabelValuesChecked
	// and handles errors as follows.
	// An error that passes ErrIsNotRegistered is ignored
	// and the noop gauge is returned;
	// all other errors cause a panic.
	WithLabelValues(labelValues ...string) GaugeMetric

	// WithChecked, if called before this vector has been registered in
	// at least one registry, will return a noop gauge and
	// an error that passes ErrIsNotRegistered.
	// If called on a hidden vector,
	// will return a noop gauge and a nil error.
	// If called with a syntactic problem in the labels, will
	// return a noop gauge and an error about the labels.
	// If none of the above apply, this method will return
	// the appropriate vector member and a nil error.
	WithChecked(labels map[string]string) (GaugeMetric, error)

	// With calls WithChecked and handles errors as follows.
	// An error that passes ErrIsNotRegistered is ignored
	// and the noop gauge is returned;
	// all other errors cause a panic.
	With(labels map[string]string) GaugeMetric

	// Delete asserts that the vec should have no member for the given label set.
	// The returned bool indicates whether there was a change.
	// The return will certainly be `false` if the given label set has the wrong
	// set of label names.
	Delete(map[string]string) bool

	// Reset removes all the members
	Reset()
}

// ObserverMetric captures individual observations.
type ObserverMetric interface {
	Observe(float64)
}

// PromRegistry is an interface which implements a subset of prometheus.Registerer and
// prometheus.Gatherer interfaces
type PromRegistry interface {
	Register(prometheus.Collector) error
	MustRegister(...prometheus.Collector)
	Unregister(prometheus.Collector) bool
	Gather() ([]*dto.MetricFamily, error)
}

// Gatherer is the interface for the part of a registry in charge of gathering
// the collected metrics into a number of MetricFamilies.
type Gatherer interface {
	prometheus.Gatherer
}

// GaugeFunc is a Gauge whose value is determined at collect time by calling a
// provided function.
//
// To create GaugeFunc instances, use NewGaugeFunc.
type GaugeFunc interface {
	Metric
	Collector
}

func ErrIsNotRegistered(err error) bool {
	return err == errNotRegistered
}

var errNotRegistered = errors.New("metric vec is not registered yet")
