// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0

package aggregate // import "go.opentelemetry.io/otel/sdk/metric/internal/aggregate"

import "go.opentelemetry.io/otel/attribute"

// overflowSet is the attribute set used to record a measurement when adding
// another distinct attribute set to the aggregate would exceed the aggregate
// limit.
var overflowSet = attribute.NewSet(attribute.Bool("otel.metric.overflow", true))

// limiter limits aggregate values.
type limiter[V any] struct {
	// aggLimit is the maximum number of metric streams that can be aggregated.
	//
	// Any metric stream with attributes distinct from any set already
	// aggregated once the aggLimit will be meet will instead be aggregated
	// into an "overflow" metric stream. That stream will only contain the
	// "otel.metric.overflow"=true attribute.
	aggLimit int
}

// newLimiter returns a new Limiter with the provided aggregation limit.
func newLimiter[V any](aggregation int) limiter[V] {
	return limiter[V]{aggLimit: aggregation}
}

// Attributes checks if adding a measurement for attrs will exceed the
// aggregation cardinality limit for the existing measurements. If it will,
// overflowSet is returned. Otherwise, if it will not exceed the limit, or the
// limit is not set (limit <= 0), attr is returned.
func (l limiter[V]) Attributes(attrs attribute.Set, measurements map[attribute.Distinct]V) attribute.Set {
	if l.aggLimit > 0 {
		_, exists := measurements[attrs.Equivalent()]
		if !exists && len(measurements) >= l.aggLimit-1 {
			return overflowSet
		}
	}

	return attrs
}
