// 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

import (
	"errors"
	"math"
	"sync/atomic"

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

// Counter 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.
//
// A Counter is typically used to count requests served, tasks completed, errors
// occurred, etc.
//
// To create Counter instances, use NewCounter.
type Counter interface {
	Metric
	Collector

	// Inc increments the counter by 1. Use Add to increment it by arbitrary
	// non-negative values.
	Inc()
	// Add adds the given value to the counter. It panics if the value is <
	// 0.
	Add(float64)
}

// CounterOpts is an alias for Opts. See there for doc comments.
type CounterOpts Opts

// NewCounter creates a new Counter based on the provided CounterOpts.
//
// The returned implementation tracks the counter value in two separate
// variables, a float64 and a uint64. The latter is used to track calls of the
// Inc method and calls of the Add method with a value that can be represented
// as a uint64. This allows atomic increments of the counter with optimal
// performance. (It is common to have an Inc call in very hot execution paths.)
// Both internal tracking values are added up in the Write method. This has to
// be taken into account when it comes to precision and overflow behavior.
func NewCounter(opts CounterOpts) Counter {
	desc := NewDesc(
		BuildFQName(opts.Namespace, opts.Subsystem, opts.Name),
		opts.Help,
		nil,
		opts.ConstLabels,
	)
	result := &counter{desc: desc, labelPairs: desc.constLabelPairs}
	result.init(result) // Init self-collection.
	return result
}

type counter struct {
	// valBits contains the bits of the represented float64 value, while
	// valInt stores values that are exact integers. Both have to go first
	// in the struct to guarantee alignment for atomic operations.
	// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
	valBits uint64
	valInt  uint64

	selfCollector
	desc *Desc

	labelPairs []*dto.LabelPair
}

func (c *counter) Desc() *Desc {
	return c.desc
}

func (c *counter) Add(v float64) {
	if v < 0 {
		panic(errors.New("counter cannot decrease in value"))
	}
	ival := uint64(v)
	if float64(ival) == v {
		atomic.AddUint64(&c.valInt, ival)
		return
	}

	for {
		oldBits := atomic.LoadUint64(&c.valBits)
		newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
		if atomic.CompareAndSwapUint64(&c.valBits, oldBits, newBits) {
			return
		}
	}
}

func (c *counter) Inc() {
	atomic.AddUint64(&c.valInt, 1)
}

func (c *counter) Write(out *dto.Metric) error {
	fval := math.Float64frombits(atomic.LoadUint64(&c.valBits))
	ival := atomic.LoadUint64(&c.valInt)
	val := fval + float64(ival)

	return populateMetric(CounterValue, val, c.labelPairs, out)
}

// CounterVec is a Collector that bundles a set of Counters that all share the
// same Desc, but have different values for their variable labels. This is used
// if you want to count the same thing partitioned by various dimensions
// (e.g. number of HTTP requests, partitioned by response code and
// method). Create instances with NewCounterVec.
type CounterVec struct {
	*metricVec
}

// NewCounterVec creates a new CounterVec based on the provided CounterOpts and
// partitioned by the given label names.
func NewCounterVec(opts CounterOpts, labelNames []string) *CounterVec {
	desc := NewDesc(
		BuildFQName(opts.Namespace, opts.Subsystem, opts.Name),
		opts.Help,
		labelNames,
		opts.ConstLabels,
	)
	return &CounterVec{
		metricVec: newMetricVec(desc, func(lvs ...string) Metric {
			if len(lvs) != len(desc.variableLabels) {
				panic(makeInconsistentCardinalityError(desc.fqName, desc.variableLabels, lvs))
			}
			result := &counter{desc: desc, labelPairs: makeLabelPairs(desc, lvs)}
			result.init(result) // Init self-collection.
			return result
		}),
	}
}

// GetMetricWithLabelValues returns the Counter for the given slice of label
// values (same order as the VariableLabels in Desc). If that combination of
// label values is accessed for the first time, a new Counter is created.
//
// It is possible to call this method without using the returned Counter to only
// create the new Counter but leave it at its starting value 0. See also the
// SummaryVec example.
//
// Keeping the Counter for later use is possible (and should be considered if
// performance is critical), but keep in mind that Reset, DeleteLabelValues and
// Delete can be used to delete the Counter from the CounterVec. In that case,
// the Counter will still exist, but it will not be exported anymore, even if a
// Counter with the same label values is created later.
//
// An error is returned if the number of label values is not the same as the
// number of VariableLabels in Desc (minus any curried labels).
//
// Note that for more than one label value, this method is prone to mistakes
// caused by an incorrect order of arguments. Consider GetMetricWith(Labels) as
// an alternative to avoid that type of mistake. For higher label numbers, the
// latter has a much more readable (albeit more verbose) syntax, but it comes
// with a performance overhead (for creating and processing the Labels map).
// See also the GaugeVec example.
func (v *CounterVec) GetMetricWithLabelValues(lvs ...string) (Counter, error) {
	metric, err := v.metricVec.getMetricWithLabelValues(lvs...)
	if metric != nil {
		return metric.(Counter), err
	}
	return nil, err
}

// GetMetricWith returns the Counter for the given Labels map (the label names
// must match those of the VariableLabels in Desc). If that label map is
// accessed for the first time, a new Counter is created. Implications of
// creating a Counter without using it and keeping the Counter for later use are
// the same as for GetMetricWithLabelValues.
//
// An error is returned if the number and names of the Labels are inconsistent
// with those of the VariableLabels in Desc (minus any curried labels).
//
// This method is used for the same purpose as
// GetMetricWithLabelValues(...string). See there for pros and cons of the two
// methods.
func (v *CounterVec) GetMetricWith(labels Labels) (Counter, error) {
	metric, err := v.metricVec.getMetricWith(labels)
	if metric != nil {
		return metric.(Counter), err
	}
	return nil, err
}

// WithLabelValues works as GetMetricWithLabelValues, but panics where
// GetMetricWithLabelValues would have returned an error. Not returning an
// error allows shortcuts like
//     myVec.WithLabelValues("404", "GET").Add(42)
func (v *CounterVec) WithLabelValues(lvs ...string) Counter {
	c, err := v.GetMetricWithLabelValues(lvs...)
	if err != nil {
		panic(err)
	}
	return c
}

// With works as GetMetricWith, but panics where GetMetricWithLabels would have
// returned an error. Not returning an error allows shortcuts like
//     myVec.With(prometheus.Labels{"code": "404", "method": "GET"}).Add(42)
func (v *CounterVec) With(labels Labels) Counter {
	c, err := v.GetMetricWith(labels)
	if err != nil {
		panic(err)
	}
	return c
}

// CurryWith returns a vector curried with the provided labels, i.e. the
// returned vector has those labels pre-set for all labeled operations performed
// on it. The cardinality of the curried vector is reduced accordingly. The
// order of the remaining labels stays the same (just with the curried labels
// taken out of the sequence – which is relevant for the
// (GetMetric)WithLabelValues methods). It is possible to curry a curried
// vector, but only with labels not yet used for currying before.
//
// The metrics contained in the CounterVec are shared between the curried and
// uncurried vectors. They are just accessed differently. Curried and uncurried
// vectors behave identically in terms of collection. Only one must be
// registered with a given registry (usually the uncurried version). The Reset
// method deletes all metrics, even if called on a curried vector.
func (v *CounterVec) CurryWith(labels Labels) (*CounterVec, error) {
	vec, err := v.curryWith(labels)
	if vec != nil {
		return &CounterVec{vec}, err
	}
	return nil, err
}

// MustCurryWith works as CurryWith but panics where CurryWith would have
// returned an error.
func (v *CounterVec) MustCurryWith(labels Labels) *CounterVec {
	vec, err := v.CurryWith(labels)
	if err != nil {
		panic(err)
	}
	return vec
}

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

// NewCounterFunc creates a new CounterFunc based on the provided
// CounterOpts. The value reported is determined by calling the given function
// from within the Write method. Take into account that metric collection may
// happen concurrently. If that results in concurrent calls to Write, like in
// the case where a CounterFunc is directly registered with Prometheus, the
// provided function must be concurrency-safe. The function should also honor
// the contract for a Counter (values only go up, not down), but compliance will
// not be checked.
func NewCounterFunc(opts CounterOpts, function func() float64) CounterFunc {
	return newValueFunc(NewDesc(
		BuildFQName(opts.Namespace, opts.Subsystem, opts.Name),
		opts.Help,
		nil,
		opts.ConstLabels,
	), CounterValue, function)
}
