// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// 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 logtree

import (
	"errors"
	"sync/atomic"
)

// LogReadOption describes options for the LogTree.Read call.
type LogReadOption struct {
	withChildren               bool
	withStream                 bool
	withBacklog                int
	onlyLeveled                bool
	onlyRaw                    bool
	leveledWithMinimumSeverity Severity
}

// WithChildren makes Read return/stream data for both a given DN and all its
// children.
func WithChildren() LogReadOption { return LogReadOption{withChildren: true} }

// WithStream makes Read return a stream of data. This works alongside WithBacklog
// to create a read-and-stream construct.
func WithStream() LogReadOption { return LogReadOption{withStream: true} }

// WithBacklog makes Read return already recorded log entries, up to count
// elements.
func WithBacklog(count int) LogReadOption { return LogReadOption{withBacklog: count} }

// BacklogAllAvailable makes WithBacklog return all backlogged log data that
// logtree possesses.
const BacklogAllAvailable int = -1

func OnlyRaw() LogReadOption { return LogReadOption{onlyRaw: true} }

func OnlyLeveled() LogReadOption { return LogReadOption{onlyLeveled: true} }

// LeveledWithMinimumSeverity makes Read return only log entries that are at least
// at a given Severity. If only leveled entries are needed, OnlyLeveled must be
// used. This is a no-op when OnlyRaw is used.
func LeveledWithMinimumSeverity(s Severity) LogReadOption {
	return LogReadOption{leveledWithMinimumSeverity: s}
}

// LogReader permits reading an already existing backlog of log entries and to
// stream further ones.
type LogReader struct {
	// Backlog are the log entries already logged by LogTree. This will only be set if
	// WithBacklog has been passed to Read.
	Backlog []*LogEntry
	// Stream is a channel of new entries as received live by LogTree. This will only
	// be set if WithStream has been passed to Read. In this case, entries from this
	// channel must be read as fast as possible by the consumer in order to prevent
	// missing entries.
	Stream <-chan *LogEntry
	// done is channel used to signal (by closing) that the log consumer is not
	// interested in more Stream data.
	done chan<- struct{}
	// missed is an atomic integer pointer that tells the subscriber how many messages
	// in Stream they missed. This pointer is nil if no streaming has been requested.
	missed *uint64
}

// Missed returns the amount of entries that were missed from Stream (as the
// channel was not drained fast enough).
func (l *LogReader) Missed() uint64 {
	// No Stream.
	if l.missed == nil {
		return 0
	}
	return atomic.LoadUint64(l.missed)
}

// Close closes the LogReader's Stream. This must be called once the Reader does
// not wish to receive streaming messages anymore.
func (l *LogReader) Close() {
	if l.done != nil {
		close(l.done)
	}
}

var (
	ErrRawAndLeveled = errors.New("cannot return logs that are simultaneously OnlyRaw and OnlyLeveled")
)

// Read and/or stream entries from a LogTree. The returned LogReader is influenced
// by the LogReadOptions passed, which influence whether the Read will return
// existing entries, a stream, or both. In addition the options also dictate
// whether only entries for that particular DN are returned, or for all sub-DNs as
// well.
func (l *LogTree) Read(dn DN, opts ...LogReadOption) (*LogReader, error) {
	l.journal.mu.RLock()
	defer l.journal.mu.RUnlock()

	var backlog int
	var stream bool
	var recursive bool
	var leveledSeverity Severity
	var onlyRaw, onlyLeveled bool

	for _, opt := range opts {
		if opt.withBacklog > 0 || opt.withBacklog == BacklogAllAvailable {
			backlog = opt.withBacklog
		}
		if opt.withStream {
			stream = true
		}
		if opt.withChildren {
			recursive = true
		}
		if opt.leveledWithMinimumSeverity != "" {
			leveledSeverity = opt.leveledWithMinimumSeverity
		}
		if opt.onlyLeveled {
			onlyLeveled = true
		}
		if opt.onlyRaw {
			onlyRaw = true
		}
	}

	if onlyLeveled && onlyRaw {
		return nil, ErrRawAndLeveled
	}

	var filters []filter
	if onlyLeveled {
		filters = append(filters, filterOnlyLeveled)
	}
	if onlyRaw {
		filters = append(filters, filterOnlyRaw)
	}
	if recursive {
		filters = append(filters, filterSubtree(dn))
	} else {
		filters = append(filters, filterExact(dn))
	}
	if leveledSeverity != "" {
		filters = append(filters, filterSeverity(leveledSeverity))
	}

	var entries []*entry
	if backlog > 0 || backlog == BacklogAllAvailable {
		if recursive {
			entries = l.journal.scanEntries(backlog, filters...)
		} else {
			entries = l.journal.getEntries(backlog, dn, filters...)
		}
	}

	var sub *subscriber
	if stream {
		sub = &subscriber{
			// TODO(q3k): make buffer size configurable
			dataC:   make(chan *LogEntry, 128),
			doneC:   make(chan struct{}),
			filters: filters,
		}
		l.journal.subscribe(sub)
	}

	lr := &LogReader{}
	lr.Backlog = make([]*LogEntry, len(entries))
	for i, entry := range entries {
		lr.Backlog[i] = entry.external()
	}
	if stream {
		lr.Stream = sub.dataC
		lr.done = sub.doneC
		lr.missed = &sub.missed
	}
	return lr, nil
}