package disruptor

import (
	"encoding/binary"
	"log"
	"runtime"
	"sync/atomic"
	"time"
)

const (
	RingBufferSize = 8 * 64 // 8 64-byte cache lines
	SizeOfInt      = 8
)

type Offset = uint64

type ring struct {
	// Make sure 64-byte alignment
	buf [RingBufferSize]byte

	dirty   Offset
	clean   Offset
	datumSz Offset // max 264 bytes (4*64)
	retries int64
}

func New(datumSize Offset) *ring {
	return &ring{datumSz: datumSize}
}

// commit should be inlined by the compiler
func (r *ring) commit(oldDirty, newOffset Offset) bool {
	clean := atomic.LoadUint64(&r.clean)
	return atomic.CompareAndSwapUint64(&r.dirty, oldDirty, newOffset) &&
		atomic.CompareAndSwapUint64(&r.clean, clean, newOffset)
}

func (r *ring) write(data []byte) int {
	sz := Offset(len(data))
	if sz > r.datumSz {
		panic("unsupported data chunk size")
	}
	// Try until we succeed
	for {
		// Load stored area
		dirty := atomic.LoadUint64(&r.dirty)
		// Calculate new offset
		off := (dirty + sz) % r.datumSz
		// Copy data to the new slot
		_ = copy(r.buf[off:], data[:r.datumSz])

		// Break if we are able to commit
		if r.commit(dirty, off) {
			break
		}
		atomic.AddInt64(&r.retries, 1)
	}
	return int(sz)
}

// Read returns data chunk available to read. It may return zeroed memory if there is no available
// data in the buffer.
func (r *ring) Read() []byte {
	return r.buf[atomic.LoadUint64(&r.clean):r.datumSz]
}

func TestDisruptor(samples int, wait *int64) *ring {
	data := make([]byte, SizeOfInt)
	r := New(SizeOfInt) //
	go func(t0 time.Time) {
		for i := 0; i < samples; i++ {
			binary.LittleEndian.PutUint64(data, uint64(i))
			r.write(data)
		}
		elapsed := time.Since(t0)
		log.Printf("writer:     finished writing %dM samples in %v [%.2f samples/s] (with %d contentions)",
			samples/1000000, elapsed, float64(samples)/elapsed.Seconds(), atomic.LoadInt64(&r.retries))
		atomic.AddInt64(wait, -1)
	}(time.Now())
	return r
}

func WriteN(n int, wait *int64) *ring {
	data := make([]byte, SizeOfInt)
	r := New(SizeOfInt) //
	go func() {
		for i := 10; i < 10+n; i++ {
			binary.LittleEndian.PutUint64(data, uint64(i))
			r.write(data)
			runtime.Gosched()
		}
		atomic.AddInt64(wait, -1)
	}()
	return r
}