use memmap::MmapOptions;
use std::fs::File;
use std::thread;
use std::error::Error;

pub struct TailReader<T> {
    file_path: String,
    processor: T,
}

impl<T> TailReader<T> where T: Fn(String) {
    pub fn new(file_path: String, processor: T) -> TailReader<T> {
        TailReader {
            file_path,
            processor,
        }
    }

    pub fn tail(self) -> Result<(), Box<Error>> {
        let mut offset: usize = 0;
        let stop_char: u8 = "\n".as_bytes()[0];
        let mut current_line = Vec::new();
        let mut file = File::open(self.file_path.clone())?;
        let mut mmap = unsafe { MmapOptions::new().map(&file)? };
        loop {
            let mmap_len = mmap.len();
            if mmap_len > offset {
                let bytes = match mmap.get(offset..mmap_len) {
                    Some(n) => n,
                    _ => continue
                };
                for byte in bytes {
                    if *byte == stop_char {
                        let line = String::from_utf8(current_line)
                            .unwrap_or(String::from("error parsing line"));
                        (self.processor)(line);
                        current_line = Vec::new();
                    } else {
                        current_line.push(*byte);
                    }
                    offset += 1;
                }
            } else {
                file = File::open(self.file_path.clone())?;
                mmap = unsafe { MmapOptions::new().map(&file)? };
            }
            thread::sleep_ms(1000)
        }
    }
}