// Copyright 2014 The Prometheus 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 prometheus

// Collector is the interface implemented by anything that can be used by
// Prometheus to collect metrics. A Collector has to be registered for
// collection. See Register, MustRegister, RegisterOrGet, and MustRegisterOrGet.
//
// The stock metrics provided by this package (like Gauge, Counter, Summary) are
// also Collectors (which only ever collect one metric, namely itself). An
// implementer of Collector may, however, collect multiple metrics in a
// coordinated fashion and/or create metrics on the fly. Examples for collectors
// already implemented in this library are the metric vectors (i.e. collection
// of multiple instances of the same Metric but with different label values)
// like GaugeVec or SummaryVec, and the ExpvarCollector.
//
// (Two Collectors are considered equal if their
// Describe method yields the same set of descriptors.)
type Collector interface {
	// Describe sends the super-set of all possible descriptors of metrics
	// collected by this Collector to the provided channel and returns once
	// the last descriptor has been sent. The sent descriptors fulfill the
	// consistency and uniqueness requirements described in the Desc
	// documentation. (It is valid if one and the same Collector sends
	// duplicate descriptors. Those duplicates are simply ignored. However,
	// two different Collectors must not send duplicate descriptors.) This
	// method idempotently sends the same descriptors throughout the
	// lifetime of the Collector. If a Collector encounters an error while
	// executing this method, it must send an invalid descriptor (created
	// with NewInvalidDesc) to signal the error to the registry.
	Describe(chan<- *Desc)
	// Collect is called by Prometheus when collecting metrics. The
	// implementation sends each collected metric via the provided channel
	// and returns once the last metric has been sent. The descriptor of
	// each sent metric is one of those returned by Describe. Returned
	// metrics that share the same descriptor must differ in their variable
	// label values. This method may be called concurrently and must
	// therefore be implemented in a concurrency safe way. Blocking occurs
	// at the expense of total performance of rendering all registered
	// metrics. Ideally, Collector implementations support concurrent
	// readers. If a Collector finds itself unable to collect a metric, it
	// can signal the error to the registry by sending a Metric that will
	// return the error when its Write method is called.
	Collect(chan<- Metric)
}

// SelfCollector implements Collector for a single Metric so that that the
// Metric collects itself. Add it as an anonymous field to a struct that
// implements Metric, and call Init with the Metric itself as an argument.
type SelfCollector struct {
	self Metric
}

// Init provides the SelfCollector with a reference to the metric it is supposed
// to collect. It is usually called within the factory function to create a
// metric. See example.
func (c *SelfCollector) Init(self Metric) {
	c.self = self
}

// Describe implements Collector.
func (c *SelfCollector) Describe(ch chan<- *Desc) {
	ch <- c.self.Desc()
}

// Collect implements Collector.
func (c *SelfCollector) Collect(ch chan<- Metric) {
	ch <- c.self
}