/*
Copyright 2014 Workiva, LLC

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.
*/

//go:generate msgp -tests=false -io=false

package btree

import "sync"

// context is used to keep track of the nodes in this mutable
// that have been created.  This is basically any node that had
// to be touched to perform mutations.  Further mutations will visit
// this context first so we don't have to constantly copy if
// we don't need to.
type context struct {
	lock      sync.RWMutex
	seenNodes map[string]*Node
}

func (c *context) nodeExists(id ID) bool {
	c.lock.RLock()
	defer c.lock.RUnlock()
	_, ok := c.seenNodes[string(id)]
	return ok
}

func (c *context) addNode(n *Node) {
	c.lock.Lock()
	defer c.lock.Unlock()
	c.seenNodes[string(n.ID)] = n
}

func (c *context) getNode(id ID) *Node {
	c.lock.RLock()
	defer c.lock.RUnlock()
	return c.seenNodes[string(id)]
}

func newContext() *context {
	return &context{
		seenNodes: make(map[string]*Node, 10),
	}
}

// Tr itself is exported so that the code generated for serialization/deserialization
// works on Tr.  Exported fields on Tr are those fields that need to be
// serialized.
type Tr struct {
	UUID      ID  `msg:"u"`
	Count     int `msg:"c"`
	config    Config
	Root      ID `msg:"r"`
	cacher    *cacher
	context   *context
	NodeWidth int `msg:"nw"`
	mutable   bool
}

func (t *Tr) createRoot() *Node {
	n := newNode()
	n.IsLeaf = true
	return n
}

// contextOrCachedNode is a convenience function for either fetching
// a node from the context or persistence.
func (t *Tr) contextOrCachedNode(id ID, cache bool) (*Node, error) {
	if t.context != nil {
		n := t.context.getNode(id)
		if n != nil {
			return n, nil
		}
	}

	return t.cacher.getNode(t, id, cache)
}

func (t *Tr) ID() ID {
	return t.UUID
}

// toBytes encodes this tree into a byte array.  Panics if unable
// as this error has to be fixed in code.
func (t *Tr) toBytes() []byte {
	buf, err := t.MarshalMsg(nil)
	if err != nil {
		panic(`unable to encode tree`)
	}

	return buf
}

// reset is called on a tree to empty the context and clear the cache.
func (t *Tr) reset() {
	t.cacher.clear()
	t.context = nil
}

// commit will gather up all created nodes and serialize them into
// items that can be persisted.
func (t *Tr) commit() []*Payload {
	items := make([]*Payload, 0, len(t.context.seenNodes))
	for _, n := range t.context.seenNodes {
		n.ChildValues, n.ChildKeys = n.flatten()
		buf, err := n.MarshalMsg(nil)
		if err != nil {
			panic(`unable to encode node`)
		}

		n.ChildValues, n.ChildKeys = nil, nil
		item := &Payload{n.ID, buf}
		items = append(items, item)
	}

	return items
}

func (t *Tr) copyNode(n *Node) *Node {
	if t.context.nodeExists(n.ID) {
		return n
	}

	cp := n.copy()
	t.context.addNode(cp)
	return cp
}

func (t *Tr) Len() int {
	return t.Count
}

func (t *Tr) AsMutable() MutableTree {
	return &Tr{
		Count:     t.Count,
		UUID:      newID(),
		Root:      t.Root,
		config:    t.config,
		cacher:    t.cacher,
		context:   newContext(),
		NodeWidth: t.NodeWidth,
		mutable:   true,
	}
}

func (t *Tr) Commit() (ReadableTree, error) {
	t.NodeWidth = t.config.NodeWidth
	items := make([]*Payload, 0, len(t.context.seenNodes))
	items = append(items, t.commit()...)

	// save self
	items = append(items, &Payload{t.ID(), t.toBytes()})

	err := t.config.Persister.Save(items...)
	if err != nil {
		return nil, err
	}

	t.reset()
	t.context = nil
	return t, nil
}

func treeFromBytes(p Persister, data []byte, comparator Comparator) (*Tr, error) {
	t := &Tr{}
	_, err := t.UnmarshalMsg(data)
	if err != nil {
		return nil, err
	}

	cfg := DefaultConfig(p, comparator)
	if t.NodeWidth > 0 {
		cfg.NodeWidth = t.NodeWidth
	}
	t.config = cfg
	t.cacher = newCacher(cfg.Persister)

	return t, nil
}

func newTree(cfg Config) *Tr {
	return &Tr{
		config: cfg,
		UUID:   newID(),
		cacher: newCacher(cfg.Persister),
	}
}

// New creates a new ReadableTree using the provided config.
func New(cfg Config) ReadableTree {
	return newTree(cfg)
}

// Load returns a ReadableTree from persistence.  The provided
// config should contain a persister that can be used for this purpose.
// An error is returned if the tree could not be found or an error
// occurred in the persistence layer.
func Load(p Persister, id []byte, comparator Comparator) (ReadableTree, error) {
	items, err := p.Load(id)
	if err != nil {
		return nil, err
	}

	if len(items) == 0 || items[0] == nil {
		return nil, ErrTreeNotFound
	}

	rt, err := treeFromBytes(p, items[0].Payload, comparator)
	if err != nil {
		return nil, err
	}

	return rt, nil
}