// 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 model

import (
	"bytes"
	"hash"
	"hash/fnv"
	"sort"
	"sync"
)

// SeparatorByte is a byte that cannot occur in valid UTF-8 sequences and is
// used to separate label names, label values, and other strings from each other
// when calculating their combined hash value (aka signature aka fingerprint).
const SeparatorByte byte = 255

var (
	// cache the signature of an empty label set.
	emptyLabelSignature = fnv.New64a().Sum64()

	hashAndBufPool sync.Pool
)

type hashAndBuf struct {
	h hash.Hash64
	b bytes.Buffer
}

func getHashAndBuf() *hashAndBuf {
	hb := hashAndBufPool.Get()
	if hb == nil {
		return &hashAndBuf{h: fnv.New64a()}
	}
	return hb.(*hashAndBuf)
}

func putHashAndBuf(hb *hashAndBuf) {
	hb.h.Reset()
	hb.b.Reset()
	hashAndBufPool.Put(hb)
}

// LabelsToSignature returns a quasi-unique signature (i.e., fingerprint) for a
// given label set. (Collisions are possible but unlikely if the number of label
// sets the function is applied to is small.)
func LabelsToSignature(labels map[string]string) uint64 {
	if len(labels) == 0 {
		return emptyLabelSignature
	}

	labelNames := make([]string, 0, len(labels))
	for labelName := range labels {
		labelNames = append(labelNames, labelName)
	}
	sort.Strings(labelNames)

	hb := getHashAndBuf()
	defer putHashAndBuf(hb)

	for _, labelName := range labelNames {
		hb.b.WriteString(labelName)
		hb.b.WriteByte(SeparatorByte)
		hb.b.WriteString(labels[labelName])
		hb.b.WriteByte(SeparatorByte)
		hb.h.Write(hb.b.Bytes())
		hb.b.Reset()
	}
	return hb.h.Sum64()
}

// metricToFingerprint works exactly as LabelsToSignature but takes a Metric as
// parameter (rather than a label map) and returns a Fingerprint.
func metricToFingerprint(m Metric) Fingerprint {
	if len(m) == 0 {
		return Fingerprint(emptyLabelSignature)
	}

	labelNames := make(LabelNames, 0, len(m))
	for labelName := range m {
		labelNames = append(labelNames, labelName)
	}
	sort.Sort(labelNames)

	hb := getHashAndBuf()
	defer putHashAndBuf(hb)

	for _, labelName := range labelNames {
		hb.b.WriteString(string(labelName))
		hb.b.WriteByte(SeparatorByte)
		hb.b.WriteString(string(m[labelName]))
		hb.b.WriteByte(SeparatorByte)
		hb.h.Write(hb.b.Bytes())
		hb.b.Reset()
	}
	return Fingerprint(hb.h.Sum64())
}

// metricToFastFingerprint works similar to metricToFingerprint but uses a
// faster and less allocation-heavy hash function, which is more susceptible to
// create hash collisions. Therefore, collision detection should be applied.
func metricToFastFingerprint(m Metric) Fingerprint {
	if len(m) == 0 {
		return Fingerprint(emptyLabelSignature)
	}

	var result uint64
	hb := getHashAndBuf()
	defer putHashAndBuf(hb)

	for labelName, labelValue := range m {
		hb.b.WriteString(string(labelName))
		hb.b.WriteByte(SeparatorByte)
		hb.b.WriteString(string(labelValue))
		hb.h.Write(hb.b.Bytes())
		result ^= hb.h.Sum64()
		hb.h.Reset()
		hb.b.Reset()
	}
	return Fingerprint(result)
}

// SignatureForLabels works like LabelsToSignature but takes a Metric as
// parameter (rather than a label map) and only includes the labels with the
// specified LabelNames into the signature calculation. The labels passed in
// will be sorted by this function.
func SignatureForLabels(m Metric, labels LabelNames) uint64 {
	if len(m) == 0 || len(labels) == 0 {
		return emptyLabelSignature
	}

	sort.Sort(labels)

	hb := getHashAndBuf()
	defer putHashAndBuf(hb)

	for _, label := range labels {
		hb.b.WriteString(string(label))
		hb.b.WriteByte(SeparatorByte)
		hb.b.WriteString(string(m[label]))
		hb.b.WriteByte(SeparatorByte)
		hb.h.Write(hb.b.Bytes())
		hb.b.Reset()
	}
	return hb.h.Sum64()
}

// SignatureWithoutLabels works like LabelsToSignature but takes a Metric as
// parameter (rather than a label map) and excludes the labels with any of the
// specified LabelNames from the signature calculation.
func SignatureWithoutLabels(m Metric, labels map[LabelName]struct{}) uint64 {
	if len(m) == 0 {
		return emptyLabelSignature
	}

	labelNames := make(LabelNames, 0, len(m))
	for labelName := range m {
		if _, exclude := labels[labelName]; !exclude {
			labelNames = append(labelNames, labelName)
		}
	}
	if len(labelNames) == 0 {
		return emptyLabelSignature
	}
	sort.Sort(labelNames)

	hb := getHashAndBuf()
	defer putHashAndBuf(hb)

	for _, labelName := range labelNames {
		hb.b.WriteString(string(labelName))
		hb.b.WriteByte(SeparatorByte)
		hb.b.WriteString(string(m[labelName]))
		hb.b.WriteByte(SeparatorByte)
		hb.h.Write(hb.b.Bytes())
		hb.b.Reset()
	}
	return hb.h.Sum64()
}
