// Copyright Istio 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 xds

import (
	discovery "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3"

	"istio.io/istio/pilot/pkg/model"
	"istio.io/istio/pilot/pkg/networking/core"
	"istio.io/istio/pilot/pkg/util/protoconv"
	"istio.io/istio/pkg/config/schema/kind"
	"istio.io/istio/pkg/util/sets"
)

// NdsGenerator generates config for Nds i.e. Name Discovery Service. Istio agents
// send NDS requests to istiod and istiod responds with a list of services and their
// associated IPs (including service entries).
// The agent then updates its internal DNS based on this data. If DNS capture is enabled
// in the pod the agent will capture all DNS requests and attempt to resolve locally before
// forwarding to upstream dns servers.
type NdsGenerator struct {
	ConfigGenerator core.ConfigGenerator
}

var _ model.XdsResourceGenerator = &NdsGenerator{}

// Map of all configs that do not impact NDS
var skippedNdsConfigs = sets.New(
	kind.Gateway,
	kind.VirtualService,
	kind.DestinationRule,
	kind.Secret,
	kind.Telemetry,
	kind.EnvoyFilter,
	kind.WorkloadEntry,
	kind.WorkloadGroup,
	kind.AuthorizationPolicy,
	kind.RequestAuthentication,
	kind.PeerAuthentication,
	kind.WasmPlugin,
	kind.ProxyConfig,
	kind.MeshConfig,

	kind.KubernetesGateway,
	kind.HTTPRoute,
	kind.TCPRoute,
	kind.TLSRoute,
	kind.GRPCRoute,
)

func ndsNeedsPush(req *model.PushRequest) bool {
	if req == nil {
		return true
	}
	if !req.Full {
		// NDS generally handles full push. We only allow partial pushes, when headless endpoints change.
		return headlessEndpointsUpdated(req)
	}
	// If none set, we will always push
	if len(req.ConfigsUpdated) == 0 {
		return true
	}
	for config := range req.ConfigsUpdated {
		if _, f := skippedNdsConfigs[config.Kind]; !f {
			return true
		}
	}
	return false
}

func headlessEndpointsUpdated(req *model.PushRequest) bool {
	return req.Reason.Has(model.HeadlessEndpointUpdate)
}

func (n NdsGenerator) Generate(proxy *model.Proxy, _ *model.WatchedResource, req *model.PushRequest) (model.Resources, model.XdsLogDetails, error) {
	if !ndsNeedsPush(req) {
		return nil, model.DefaultXdsLogDetails, nil
	}
	nt := n.ConfigGenerator.BuildNameTable(proxy, req.Push)
	if nt == nil {
		return nil, model.DefaultXdsLogDetails, nil
	}
	resources := model.Resources{&discovery.Resource{Resource: protoconv.MessageToAny(nt)}}
	return resources, model.DefaultXdsLogDetails, nil
}
