package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"strings"
)

func Grep(reader io.Reader, str string) error {
	var buf strings.Builder
	r := bufio.NewReaderSize(reader, 64*1024)

	for {
		frag, err := r.ReadSlice('\n')
		if err == bufio.ErrBufferFull {
			buf.Write(frag)
			continue // partial line
		}
		if err == io.EOF {
			if len(frag) == 0 {
				break
			}
		} else if err != nil {
			return errors.New("index parse error: " + err.Error())
		}
		buf.Write(frag)
		line := buf.String()

		pos := strings.Index(line, str)
		if pos != -1 {
			fmt.Printf(line)
		}
		buf.Reset()
	}
	return nil
}

func main() {
	r := strings.NewReader("line 1\nline 2 test\nline3\nline test4")
	_ = Grep(r, "test")
}