/*
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 naming

import (
	"bytes"
	"fmt"
	"regexp"
	"text/template"

	"k8s.io/apimachinery/pkg/runtime/schema"

	pmodel "github.com/prometheus/common/model"
)

// labelGroupResExtractor extracts schema.GroupResources from series labels.
type labelGroupResExtractor struct {
	regex *regexp.Regexp

	resourceInd int
	groupInd    *int
}

// newLabelGroupResExtractor creates a new labelGroupResExtractor for labels whose form
// matches the given template.  It does so by creating a regular expression from the template,
// so anything in the template which limits resource or group name length will cause issues.
func newLabelGroupResExtractor(labelTemplate *template.Template) (*labelGroupResExtractor, error) {
	labelRegexBuff := new(bytes.Buffer)
	if err := labelTemplate.Execute(labelRegexBuff, schema.GroupResource{
		Group:    "(?P<group>.+?)",
		Resource: "(?P<resource>.+?)"},
	); err != nil {
		return nil, fmt.Errorf("unable to convert label template to matcher: %v", err)
	}
	if labelRegexBuff.Len() == 0 {
		return nil, fmt.Errorf("unable to convert label template to matcher: empty template")
	}
	labelRegexRaw := "^" + labelRegexBuff.String() + "$"
	labelRegex, err := regexp.Compile(labelRegexRaw)
	if err != nil {
		return nil, fmt.Errorf("unable to convert label template to matcher: %v", err)
	}

	var groupInd *int
	var resInd *int

	for i, name := range labelRegex.SubexpNames() {
		switch name {
		case "group":
			ind := i // copy to avoid iteration variable reference
			groupInd = &ind
		case "resource":
			ind := i // copy to avoid iteration variable reference
			resInd = &ind
		}
	}

	if resInd == nil {
		return nil, fmt.Errorf("must include at least `{{.Resource}}` in the label template")
	}

	return &labelGroupResExtractor{
		regex:       labelRegex,
		resourceInd: *resInd,
		groupInd:    groupInd,
	}, nil
}

// GroupResourceForLabel extracts a schema.GroupResource from the given label, if possible.
// The second return value indicates whether or not a potential group-resource was found in this label.
func (e *labelGroupResExtractor) GroupResourceForLabel(lbl pmodel.LabelName) (schema.GroupResource, bool) {
	matchGroups := e.regex.FindStringSubmatch(string(lbl))
	if matchGroups != nil {
		group := ""
		if e.groupInd != nil {
			group = matchGroups[*e.groupInd]
		}

		return schema.GroupResource{
			Group:    group,
			Resource: matchGroups[e.resourceInd],
		}, true
	}

	return schema.GroupResource{}, false
}