// Copyright 2016 The LUCI 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 tsmon

import (
	"context"

	"go.chromium.org/luci/common/tsmon/types"
)

// Callback is a function that is run at metric collection time to set the
// values of one or more metrics.  A callback can be registered with
// RegisterCallback.
type Callback func(ctx context.Context)

// A GlobalCallback is a Callback with the list of metrics it affects, so those
// metrics can be reset after they are flushed.
type GlobalCallback struct {
	Callback
	metrics []types.Metric
}

// RegisterCallback registers a callback function that will be run at metric
// collection time to set the values of one or more metrics.  RegisterCallback
// should be called from an init() function in your module.
func RegisterCallback(f Callback) {
	RegisterCallbackIn(context.Background(), f)
}

// RegisterCallbackIn is like RegisterCallback but registers in a given context.
func RegisterCallbackIn(ctx context.Context, f Callback) {
	GetState(ctx).RegisterCallbacks(f)
}

// RegisterGlobalCallback registers a callback function that will be run once
// per minute on *one* instance of your application. It is supported primarily
// on GAE.
//
// You must specify the list of metrics that your callback affects - these
// metrics will be reset after flushing to ensure they are not sent by multiple
// instances.
//
// RegisterGlobalCallback should be called from an init() function in your
// module.
func RegisterGlobalCallback(f Callback, metrics ...types.Metric) {
	RegisterGlobalCallbackIn(context.Background(), f, metrics...)
}

// RegisterGlobalCallbackIn is like RegisterGlobalCallback but registers in a
// given context.
func RegisterGlobalCallbackIn(ctx context.Context, f Callback, metrics ...types.Metric) {
	if len(metrics) == 0 {
		panic("RegisterGlobalCallback called without any metrics")
	}

	GetState(ctx).RegisterGlobalCallbacks(GlobalCallback{f, metrics})
}