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//! This module contains the `ByteIterator` struct, which is used to iterate
//! over the bytes of a file. This allows us to read a file byte by byte,
//! instead of reading the entire file into memory at once.

use std::{
    fs::File,
    io::{self, BufReader, Read},
};


/// This struct is used to iterate over the bytes of a file.
///
///
/// # Fields
///
/// * `reader` - A `BufReader` that reads the file.
pub struct ByteIterator {
    reader: BufReader<File>,
}

impl ByteIterator {
    /// Creates a new `ByteIterator` from a file. This is used to iterate over
    /// the bytes of a file.
    ///
    /// # Arguments
    ///
    /// * `filename` - The name of the file.
    ///
    /// # Errors
    ///
    /// * If the file cannot be opened.
    pub fn new(filename: &str) -> io::Result<Self> {
        let file = File::open(filename)?;
        let reader = BufReader::new(file);
        Ok(Self { reader })
    }

    /// Returns the next character of the file.
    pub fn next_char(&mut self) -> Option<char> {
        self.next().unwrap().unwrap().chars().next()
    }
}

impl Iterator for ByteIterator {
    type Item = io::Result<String>;

    /// Returns the next byte of the file.
    fn next(&mut self) -> Option<Self::Item> {
        let mut buffer = [0; 1];
        match self.reader.read_exact(&mut buffer) {
            Ok(_) => Some(Ok(String::from_utf8_lossy(&buffer).into_owned())),
            Err(error) if error.kind() == io::ErrorKind::UnexpectedEof => None,
            Err(error) => Some(Err(error)),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_byte_iter_new_instance_accepts_valid_filename() {
        let bytes_iter = ByteIterator::new("src/readers/mod.rs");
        assert!(bytes_iter.is_ok());
    }

    #[test]
    #[should_panic]
    fn test_byte_iter_new_instance_panics_on_invalid_filename() {
        let bytes_iter = ByteIterator::new("bad_filename");
        assert!(bytes_iter.is_ok());
    }

    #[test]
    fn test_byte_iter_can_iterate_over_bytes() {
        let bytes_iter = ByteIterator::new("src/readers/mod.rs").unwrap();
        let mut bytes = String::new();

        for byte in bytes_iter {
            bytes.push_str(&byte.unwrap());
        }

        assert_eq!(bytes, include_str!("mod.rs"));
    }
}