// Copyright 2017 The Prometheus 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 chunkenc

import (
	"math"
	"sync"

	"github.com/pkg/errors"
	"github.com/prometheus/prometheus/pkg/histogram"
)

// Encoding is the identifier for a chunk encoding.
type Encoding uint8

func (e Encoding) String() string {
	switch e {
	case EncNone:
		return "none"
	case EncXOR:
		return "XOR"
	case EncSHS:
		return "SHS"
	}
	return "<unknown>"
}

func IsValidEncoding(e Encoding) bool {
	switch e {
	case EncXOR, EncSHS:
		return true
	}
	return false
}

// The different available chunk encodings.
const (
	EncNone Encoding = iota
	EncXOR
	EncSHS
)

// Chunk holds a sequence of sample pairs that can be iterated over and appended to.
type Chunk interface {
	// Bytes returns the underlying byte slice of the chunk.
	Bytes() []byte

	// Encoding returns the encoding type of the chunk.
	Encoding() Encoding

	// Appender returns an appender to append samples to the chunk.
	Appender() (Appender, error)

	// The iterator passed as argument is for re-use.
	// Depending on implementation, the iterator can
	// be re-used or a new iterator can be allocated.
	Iterator(Iterator) Iterator

	// NumSamples returns the number of samples in the chunk.
	NumSamples() int

	// Compact is called whenever a chunk is expected to be complete (no more
	// samples appended) and the underlying implementation can eventually
	// optimize the chunk.
	// There's no strong guarantee that no samples will be appended once
	// Compact() is called. Implementing this function is optional.
	Compact()
}

// Appender adds sample pairs to a chunk.
type Appender interface {
	Append(int64, float64)
	AppendHistogram(t int64, h histogram.SparseHistogram)
}

// Iterator is a simple iterator that can only get the next value.
// Iterator iterates over the samples of a time series, in timestamp-increasing order.
type Iterator interface {
	// Next advances the iterator by one.
	Next() bool
	// Seek advances the iterator forward to the first sample with the timestamp equal or greater than t.
	// If current sample found by previous `Next` or `Seek` operation already has this property, Seek has no effect.
	// Seek returns true, if such sample exists, false otherwise.
	// Iterator is exhausted when the Seek returns false.
	Seek(t int64) bool
	// At returns the current timestamp/value pair.
	// Before the iterator has advanced At behaviour is unspecified.
	At() (int64, float64)
	// AtHistogram returns the current timestamp/histogram pair.
	// Before the iterator has advanced AtHistogram behaviour is unspecified.
	AtHistogram() (int64, histogram.SparseHistogram)
	// Err returns the current error. It should be used only after iterator is
	// exhausted, that is `Next` or `Seek` returns false.
	Err() error
	// ChunkEncoding returns the encoding of the chunk that it is iterating.
	ChunkEncoding() Encoding
}

// NewNopIterator returns a new chunk iterator that does not hold any data.
func NewNopIterator() Iterator {
	return nopIterator{}
}

type nopIterator struct{}

func (nopIterator) Seek(int64) bool      { return false }
func (nopIterator) At() (int64, float64) { return math.MinInt64, 0 }
func (nopIterator) AtHistogram() (int64, histogram.SparseHistogram) {
	return math.MinInt64, histogram.SparseHistogram{}
}
func (nopIterator) Next() bool              { return false }
func (nopIterator) Err() error              { return nil }
func (nopIterator) ChunkEncoding() Encoding { return EncNone }

// Pool is used to create and reuse chunk references to avoid allocations.
type Pool interface {
	Put(Chunk) error
	Get(e Encoding, b []byte) (Chunk, error)
}

// pool is a memory pool of chunk objects.
type pool struct {
	xor sync.Pool
	shs sync.Pool
}

// NewPool returns a new pool.
func NewPool() Pool {
	return &pool{
		xor: sync.Pool{
			New: func() interface{} {
				return &XORChunk{b: bstream{}}
			},
		},
		shs: sync.Pool{
			New: func() interface{} {
				return &HistoChunk{b: bstream{}}
			},
		},
	}
}

func (p *pool) Get(e Encoding, b []byte) (Chunk, error) {
	switch e {
	case EncXOR:
		c := p.xor.Get().(*XORChunk)
		c.b.stream = b
		c.b.count = 0
		return c, nil
	case EncSHS:
		// TODO: update metadata
		c := p.shs.Get().(*HistoChunk)
		c.b.stream = b
		c.b.count = 0
		return c, nil
	}
	return nil, errors.Errorf("invalid chunk encoding %q", e)
}

func (p *pool) Put(c Chunk) error {
	switch c.Encoding() {
	case EncXOR:
		xc, ok := c.(*XORChunk)
		// This may happen often with wrapped chunks. Nothing we can really do about
		// it but returning an error would cause a lot of allocations again. Thus,
		// we just skip it.
		if !ok {
			return nil
		}
		xc.b.stream = nil
		xc.b.count = 0
		p.xor.Put(c)
	case EncSHS:
		// TODO: update metadata
		sh, ok := c.(*HistoChunk)
		// This may happen often with wrapped chunks. Nothing we can really do about
		// it but returning an error would cause a lot of allocations again. Thus,
		// we just skip it.
		if !ok {
			return nil
		}
		sh.b.stream = nil
		sh.b.count = 0
		p.shs.Put(c)
	default:
		return errors.Errorf("invalid chunk encoding %q", c.Encoding())
	}
	return nil
}

// FromData returns a chunk from a byte slice of chunk data.
// This is there so that users of the library can easily create chunks from
// bytes.
func FromData(e Encoding, d []byte) (Chunk, error) {
	switch e {
	case EncXOR:
		return &XORChunk{b: bstream{count: 0, stream: d}}, nil
	case EncSHS:
		// TODO: update metadata
		return &HistoChunk{b: bstream{count: 0, stream: d}}, nil
	}
	return nil, errors.Errorf("invalid chunk encoding %q", e)
}

// NewEmptyChunk returns an empty chunk for the given encoding.
func NewEmptyChunk(e Encoding) (Chunk, error) {
	switch e {
	case EncXOR:
		return NewXORChunk(), nil
	case EncSHS:
		return NewHistoChunk(), nil
	}
	return nil, errors.Errorf("invalid chunk encoding %q", e)
}