package main

import (
	"io"
	"sync"

	"golang.org/x/exp/mmap"
)

const lf = byte('\n')
const null = byte(0)

type AsyncLineReader struct {
	offset    int64
	eof       bool
	file      *mmap.ReaderAt
	mu        *sync.Mutex
	remainder []byte
}

func NewAsyncLineReader(name string) (*AsyncLineReader, error) {
	var a = &AsyncLineReader{
		offset: 0,
		eof:    false,
		mu:     &sync.Mutex{},
	}

	var err error
	a.file, err = mmap.Open(name)

	return a, err
}

func (a *AsyncLineReader) Read(b []byte) (int, error) {
	if a.eof {
		return 0, io.EOF
	}

	a.mu.Lock()
	defer a.mu.Unlock()

	var n, err = a.file.ReadAt(b, a.offset)
	if err == io.EOF {
		a.eof = true
		return 0, err
	}
	if n == 0 {
		return 0, err
	}

	// find the last line feed in the batch
	for {
		n--

		if b[n] == lf {
			// selected character is a line feed, we're done
			break
		} else if n == 0 {
			break // result is at start of file
		}

		b[n] = null // Discard this byte so it doesn't get read multiple times
	}

	a.offset += int64(n)

	return n, err
}

func (a *AsyncLineReader) Close() error {
	return a.file.Close()
}