// 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 "source.monogon.dev/metropolis/pkg/logbuffer"

// entry is a journal entry, representing a single log event (encompassed in a Payload) at a given DN.
// See the journal struct for more information about the global/local linked lists.
type entry struct {
	// origin is the DN at which the log entry was recorded, or conversely, in which DN it will be available at.
	origin DN
	// journal is the parent journal of this entry. An entry can belong only to a single journal. This pointer is used
	// to mutate the journal's head/tail pointers when unlinking an entry.
	journal *journal
	// leveled is the leveled log entry for this entry, if this log entry was emitted by leveled logging. Otherwise it
	// is nil.
	leveled *LeveledPayload
	// raw is the raw log entry for this entry, if this log entry was emitted by raw logging. Otherwise it is nil.
	raw *logbuffer.Line

	// prevGlobal is the previous entry in the global linked list, or nil if this entry is the oldest entry in the
	// global linked list.
	prevGlobal *entry
	// nextGlobal is the next entry in the global linked list, or nil if this entry is the newest entry in the global
	// linked list.
	nextGlobal *entry

	// prevLocal is the previous entry in this entry DN's local linked list, or nil if this entry is the oldest entry in
	// this local linked list.
	prevLocal *entry
	// prevLocal is the next entry in this entry DN's local linked list, or nil if this entry is the newest entry in
	// this local linked list.
	nextLocal *entry

	// seqLocal is a counter within a local linked list that increases by one each time a new log entry is added. It is
	// used to quickly establish local linked list sizes (by subtracting seqLocal from both ends). This setup allows for
	// O(1) length calculation for local linked lists as long as entries are only unlinked from the head or tail (which
	// is the case in the current implementation).
	seqLocal uint64
}

// external returns a LogEntry object for this entry, ie. the public version of this object, without fields relating to
// the parent journal, linked lists, sequences, etc. These objects are visible to library consumers.
func (e *entry) external() *LogEntry {
	return &LogEntry{
		DN:      e.origin,
		Leveled: e.leveled,
		Raw:     e.raw,
	}
}

// unlink removes this entry from both global and local linked lists, updating the journal's head/tail pointers if
// needed.
// journal.mu must be taken as RW
func (e *entry) unlink() {
	// Unlink from the global linked list.
	if e.prevGlobal != nil {
		e.prevGlobal.nextGlobal = e.nextGlobal
	}
	if e.nextGlobal != nil {
		e.nextGlobal.prevGlobal = e.prevGlobal
	}
	// Update journal head/tail pointers.
	if e.journal.head == e {
		e.journal.head = e.prevGlobal
	}
	if e.journal.tail == e {
		e.journal.tail = e.nextGlobal
	}

	// Unlink from the local linked list.
	if e.prevLocal != nil {
		e.prevLocal.nextLocal = e.nextLocal
	}
	if e.nextLocal != nil {
		e.nextLocal.prevLocal = e.prevLocal
	}
	// Update journal head/tail pointers.
	if e.journal.heads[e.origin] == e {
		e.journal.heads[e.origin] = e.prevLocal
	}
	if e.journal.tails[e.origin] == e {
		e.journal.tails[e.origin] = e.nextLocal
	}
}

// quota describes the quota policy for logging at a given DN.
type quota struct {
	// origin is the exact DN that this quota applies to.
	origin DN
	// max is the maximum count of log entries permitted for this DN - ie, the maximum size of the local linked list.
	max uint64
}

// append adds an entry at the head of the global and local linked lists.
func (j *journal) append(e *entry) {
	j.mu.Lock()
	defer j.mu.Unlock()

	e.journal = j

	// Insert at head in global linked list, set pointers.
	e.nextGlobal = nil
	e.prevGlobal = j.head
	if j.head != nil {
		j.head.nextGlobal = e
	}
	j.head = e
	if j.tail == nil {
		j.tail = e
	}

	// Create quota if necessary.
	if _, ok := j.quota[e.origin]; !ok {
		j.quota[e.origin] = &quota{origin: e.origin, max: 8192}
	}

	// Insert at head in local linked list, calculate seqLocal, set pointers.
	e.nextLocal = nil
	e.prevLocal = j.heads[e.origin]
	if j.heads[e.origin] != nil {
		j.heads[e.origin].nextLocal = e
		e.seqLocal = e.prevLocal.seqLocal + 1
	} else {
		e.seqLocal = 0
	}
	j.heads[e.origin] = e
	if j.tails[e.origin] == nil {
		j.tails[e.origin] = e
	}

	// Apply quota to the local linked list that this entry got inserted to, ie. remove elements in excess of the
	// quota.max count.
	quota := j.quota[e.origin]
	count := (j.heads[e.origin].seqLocal - j.tails[e.origin].seqLocal) + 1
	if count > quota.max {
		// Keep popping elements off the tail of the local linked list until quota is not violated.
		left := count - quota.max
		cur := j.tails[e.origin]
		for {
			// This shouldn't happen if quota.max >= 1.
			if cur == nil {
				break
			}
			if left == 0 {
				break
			}
			el := cur
			cur = el.nextLocal
			// Unlinking the entry unlinks it from both the global and local linked lists.
			el.unlink()
			left -= 1
		}
	}
}