package cache

import (
	"context"

	"github.com/envoyproxy/go-control-plane/pkg/cache/types"
	"github.com/envoyproxy/go-control-plane/pkg/server/stream/v3"
)

// groups together resource-related arguments for the createDeltaResponse function
type resourceContainer struct {
	resourceMap   map[string]types.Resource
	versionMap    map[string]string
	systemVersion string
}

func createDeltaResponse(ctx context.Context, req *DeltaRequest, state stream.StreamState, resources resourceContainer) *RawDeltaResponse {
	// variables to build our response with
	var nextVersionMap map[string]string
	var filtered []types.Resource
	var toRemove []string

	// If we are handling a wildcard request, we want to respond with all resources
	switch {
	case state.IsWildcard():
		if len(state.GetResourceVersions()) == 0 {
			filtered = make([]types.Resource, 0, len(resources.resourceMap))
		}
		nextVersionMap = make(map[string]string, len(resources.resourceMap))
		for name, r := range resources.resourceMap {
			// Since we've already precomputed the version hashes of the new snapshot,
			// we can just set it here to be used for comparison later
			version := resources.versionMap[name]
			nextVersionMap[name] = version
			prevVersion, found := state.GetResourceVersions()[name]
			if !found || (prevVersion != version) {
				filtered = append(filtered, r)
			}
		}

		// Compute resources for removal
		for name := range state.GetResourceVersions() {
			if _, ok := resources.resourceMap[name]; !ok {
				toRemove = append(toRemove, name)
			}
		}
	default:
		// Reply only with the requested resources
		nextVersionMap = make(map[string]string, len(state.GetResourceVersions()))
		for name, prevVersion := range state.GetResourceVersions() {
			if r, ok := resources.resourceMap[name]; ok {
				nextVersion := resources.versionMap[name]
				if prevVersion != nextVersion {
					filtered = append(filtered, r)
				}
				nextVersionMap[name] = nextVersion
			} else {
				// We track non-existent resources for non-wildcard streams until the client explicitly unsubscribes from them.
				nextVersionMap[name] = ""
				// The version check is to make sure we are only sending an update once right after removal.
				// If the client keeps the subscription, we skip the add for every subsequent response.
				if prevVersion != "" {
					toRemove = append(toRemove, name)
				}
			}
		}
	}

	return &RawDeltaResponse{
		DeltaRequest:      req,
		Resources:         filtered,
		RemovedResources:  toRemove,
		NextVersionMap:    nextVersionMap,
		SystemVersionInfo: resources.systemVersion,
		Ctx:               ctx,
	}
}
