package sarama

import (
	"hash"
	"hash/fnv"
	"math/rand"
	"time"
)

// Partitioner is anything that, given a Kafka message key and a number of partitions indexed [0...numPartitions-1],
// decides to which partition to send the message. RandomPartitioner, RoundRobinPartitioner and HashPartitioner are provided
// as simple default implementations.
type Partitioner interface {
	Partition(key Encoder, numPartitions int32) int32
}

// RandomPartitioner implements the Partitioner interface by choosing a random partition each time.
type RandomPartitioner struct {
	generator *rand.Rand
}

func NewRandomPartitioner() *RandomPartitioner {
	p := new(RandomPartitioner)
	p.generator = rand.New(rand.NewSource(time.Now().UTC().UnixNano()))
	return p
}

func (p *RandomPartitioner) Partition(key Encoder, numPartitions int32) int32 {
	return int32(p.generator.Intn(int(numPartitions)))
}

// RoundRobinPartitioner implements the Partitioner interface by walking through the available partitions one at a time.
type RoundRobinPartitioner struct {
	partition int32
}

func (p *RoundRobinPartitioner) Partition(key Encoder, numPartitions int32) int32 {
	if p.partition >= numPartitions {
		p.partition = 0
	}
	ret := p.partition
	p.partition++
	return ret
}

// HashPartitioner implements the Partitioner interface. If the key is nil, or fails to encode, then a random partition
// is chosen. Otherwise the FNV-1a hash of the encoded bytes is used modulus the number of partitions. This ensures that messages
// with the same key always end up on the same partition.
type HashPartitioner struct {
	random *RandomPartitioner
	hasher hash.Hash32
}

func NewHashPartitioner() *HashPartitioner {
	p := new(HashPartitioner)
	p.random = NewRandomPartitioner()
	p.hasher = fnv.New32a()
	return p
}

func (p *HashPartitioner) Partition(key Encoder, numPartitions int32) int32 {
	if key == nil {
		return p.random.Partition(key, numPartitions)
	}
	bytes, err := key.Encode()
	if err != nil {
		return p.random.Partition(key, numPartitions)
	}
	p.hasher.Reset()
	p.hasher.Write(bytes)
	hash := int32(p.hasher.Sum32())
	if hash < 0 {
		hash = -hash
	}
	return hash % numPartitions
}