Moving the query building logic, adding first unit test.

Moving it into its own type to better organize and separate concerns. Also makes unit testing easier.

pkg/custom-provider/external_metric_query_builder.go

@@ -0,0 +1,94 @@
+package provider
+
+import (
+	"fmt"
+	s "strings"
+
+	"k8s.io/apimachinery/pkg/labels"
+	"k8s.io/apimachinery/pkg/selection"
+)
+
+type ExternalMetricQueryBuilder interface {
+	BuildPrometheusQuery(namespace string, metricName string, metricSelector labels.Selector) string
+}
+
+type externalMetricQueryBuilder struct {
+}
+
+func NewExternalMetricQueryBuilder() ExternalMetricQueryBuilder {
+	return &externalMetricQueryBuilder{}
+}
+
+func (p *externalMetricQueryBuilder) BuildPrometheusQuery(namespace string, metricName string, metricSelector labels.Selector) string {
+	namespaceSelector := p.makeLabelFilter("namespace", "=", namespace)
+	otherSelectors := p.convertSelectors(metricSelector)
+
+	finalTargets := append([]string{namespaceSelector}, otherSelectors...)
+	joinedLabels := s.Join(finalTargets, ", ")
+	return fmt.Sprintf("%s{%s}", metricName, joinedLabels)
+}
+
+func (p *externalMetricQueryBuilder) makeLabelFilter(labelName string, operator string, targetValue string) string {
+	return fmt.Sprintf("%s%s\"%s\"", labelName, operator, targetValue)
+}
+
+func (p *externalMetricQueryBuilder) convertSelectors(metricSelector labels.Selector) []string {
+	requirements, _ := metricSelector.Requirements()
+
+	selectors := []string{}
+	for i := 0; i < len(requirements); i++ {
+		selector := p.convertRequirement(requirements[i])
+
+		selectors = append(selectors, selector)
+	}
+
+	return selectors
+}
+
+func (p *externalMetricQueryBuilder) convertRequirement(requirement labels.Requirement) string {
+	labelName := requirement.Key()
+	values := requirement.Values().List()
+
+	stringValues := values[0]
+
+	valueCount := len(values)
+	if valueCount > 1 {
+		stringValues = s.Join(values, "|")
+	}
+
+	operator := p.selectOperator(requirement.Operator(), valueCount)
+
+	return p.makeLabelFilter(labelName, operator, stringValues)
+}
+
+func (p *externalMetricQueryBuilder) selectOperator(operator selection.Operator, valueCount int) string {
+	if valueCount > 1 {
+		return p.selectRegexOperator(operator)
+	}
+
+	return p.selectSingleValueOperator(operator)
+}
+
+func (p *externalMetricQueryBuilder) selectRegexOperator(operator selection.Operator) string {
+	switch operator {
+	case selection.Equals:
+		return "=~"
+	case selection.NotEquals:
+		return "!~"
+	}
+
+	//TODO: Cover more cases, supply appropriate errors for any unhandled cases.
+	return "="
+}
+
+func (p *externalMetricQueryBuilder) selectSingleValueOperator(operator selection.Operator) string {
+	switch operator {
+	case selection.Equals:
+		return "="
+	case selection.NotEquals:
+		return "!="
+	}
+
+	//TODO: Cover more cases, supply appropriate errors for any unhandled cases.
+	return "="
+}