// Copyright 2018 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 policy contains implementation of triggering policy functions.
package policy

import (
	"fmt"
	"time"

	"google.golang.org/protobuf/proto"
	"google.golang.org/protobuf/types/known/durationpb"

	"go.chromium.org/luci/common/errors"

	"go.chromium.org/luci/scheduler/appengine/internal"
	"go.chromium.org/luci/scheduler/appengine/messages"
	"go.chromium.org/luci/scheduler/appengine/task"
)

// Environment is used by the triggering policy for getting transient
// information about the environment and for logging.
//
// TODO(vadimsh): This is intentionally mostly empty for now. Will be extended
// on as-needed basis.
type Environment interface {
	// DebugLog appends a line to the triage text log.
	DebugLog(format string, args ...any)
}

// In contains parameters for a triggering policy function.
type In struct {
	// Now is the time when the policy function was called.
	Now time.Time
	// ActiveInvocations is a set of currently running invocations of the job.
	ActiveInvocations []int64
	// Triggers is a list of pending triggers sorted by time, more recent last.
	Triggers []*internal.Trigger
}

// Out contains the decision of a triggering policy function.
type Out struct {
	// Requests is a list of requests to start new invocations (if any).
	//
	// Each request contains parameters that will be passed to a new invocation by
	// the engine. The policy is responsible for filling them in.
	//
	// Triggers specified in the each request will be removed from the set of
	// pending triggers (they are consumed).
	Requests []task.Request

	// Discard is a list of triggers that the policy decided aren't needed anymore
	// and should be discarded by the engine.
	//
	// A trigger associated with a request must not be also slated for discard.
	Discard []*internal.Trigger
}

// Func is the concrete implementation of a triggering policy.
//
// It looks at the current state of the job and its pending triggers list and
// (optionally) emits a bunch of requests to start new invocations.
//
// It is a pure function without any side effects (except, perhaps, logging into
// the given environment).
type Func func(Environment, In) Out

// New is a factory that takes TriggeringPolicy proto and returns a concrete
// function that implements this policy.
//
// The returned function will be used only during one triage round and then
// discarded.
//
// The caller is responsible for filling in all default values in the proto if
// necessary. Use UnmarshalDefinition to deserialize TriggeringPolicy filling in
// the defaults.
//
// Returns an error if the TriggeringPolicy message can't be
// understood (for example, it references an undefined policy kind).
func New(p *messages.TriggeringPolicy) (Func, error) {
	// There will be more kinds here, so this should technically be a switch, even
	// though currently it is not a very interesting switch.
	switch p.Kind {
	case messages.TriggeringPolicy_GREEDY_BATCHING:
		return GreedyBatchingPolicy(int(p.MaxConcurrentInvocations), int(p.MaxBatchSize))
	case messages.TriggeringPolicy_LOGARITHMIC_BATCHING:
		return LogarithmicBatchingPolicy(int(p.MaxConcurrentInvocations), int(p.MaxBatchSize), float64(p.LogBase))
	case messages.TriggeringPolicy_NEWEST_FIRST:
		return NewestFirstPolicy(int(p.MaxConcurrentInvocations), p.PendingTimeout.AsDuration())
	default:
		return nil, errors.Reason("unrecognized triggering policy kind %d", p.Kind).Err()
	}
}

var defaultPolicy = messages.TriggeringPolicy{
	Kind:                     messages.TriggeringPolicy_GREEDY_BATCHING,
	MaxConcurrentInvocations: 1,
	MaxBatchSize:             1000,
	PendingTimeout:           durationpb.New(defaultPendingTimeout),
}

const defaultPendingTimeout = 7 * 24 * time.Hour // 7 days.

// Default instantiates default triggering policy function.
//
// Is is used if jobs do not define a triggering policy or it can't be
// understood.
func Default() Func {
	f, err := New(&defaultPolicy)
	if err != nil {
		panic(fmt.Errorf("failed to instantiate default triggering policy - %s", err))
	}
	return f
}

// UnmarshalDefinition deserializes TriggeringPolicy, filling in defaults.
func UnmarshalDefinition(b []byte) (*messages.TriggeringPolicy, error) {
	msg := &messages.TriggeringPolicy{}
	if len(b) != 0 {
		if err := proto.Unmarshal(b, msg); err != nil {
			return nil, errors.Annotate(err, "failed to unmarshal TriggeringPolicy").Err()
		}
	}
	if msg.Kind == 0 {
		msg.Kind = defaultPolicy.Kind
	}
	if msg.MaxConcurrentInvocations == 0 {
		msg.MaxConcurrentInvocations = defaultPolicy.MaxConcurrentInvocations
	}
	if msg.MaxBatchSize == 0 {
		msg.MaxBatchSize = defaultPolicy.MaxBatchSize
	}
	if msg.Kind == messages.TriggeringPolicy_NEWEST_FIRST && msg.PendingTimeout.AsDuration() == 0 {
		msg.PendingTimeout = defaultPolicy.PendingTimeout
	}
	return msg, nil
}