You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: randprefetch.go
    path of file: ./repos/gocryptfs/internal/cryptocore
    
    the code of the file until where you have to start completion: package cryptocore

import (
	"bytes"
	"log"
	"sync"
)

// Number of bytes to prefetch.
// 512 looks like a good compromise between throughput and latency - see
// randsize_test.go for numbers.
const prefetchN = 5
    
func (r *randPrefetcherT) read(want int) (out []byte) {
	out = make([]byte, want)
	r.Lock()
	// Note: don't use defer, it slows us down!
	have, err := r.buf.Read(out)
	if have == want && err == nil {
		r.Unlock()
		return out
	}
	// Buffer was empty -> re-fill
	fresh := <-r.refill
	if len(fresh) != prefetchN {
		log.Panicf("randPrefetcher: refill: got %d bytes instead of %d", len(fresh), prefetchN)
	}
	r.buf.Reset()
	r.buf.Write(fresh)
	have, err = r.buf.Read(out)
	if have != want || err != nil {
		log.Panicf("randPrefetcher could not satisfy read: have=%d want=%d err=%v", have, want, err)
	}
	r.Unlock()
	return out
}