use std::io::Write;

/// The parser/transformator. Given a `token` and a `suffix`, adds the `suffix`
/// after the `token` to any byte buffer given to [`Parser::process`].
///
/// For example, with a `Parser::new(b"foo", b"bar"), calling
/// `parser.process(b"is foo a bar?", &mut writer)` will result in
/// `"is foobar a bar?"` being written to `writer`.
//
// NOTE: Feel free to modify this struct as needed.
pub struct Parser {
    token: &'static [u8],
    suffix: &'static [u8],
    /// NORA: Records the position in the token across calls to `process`.
    pos: usize,
}

impl Parser {
    /// Create a new `Parser` which will append `suffix` after `token` whenever
    /// it appears in input passed to `process`.
    //
    // NOTE: This function signature should stay the same.
    pub fn new(token: &'static [u8], suffix: &'static [u8]) -> Self {
        Self {
            token,
            suffix,
            pos: 0,
        }
    }

    /// Write the bytes given in `input` to `output`, plus the bytes in `suffix`
    /// immediately after `token`, including across call boundaries.
    ///
    /// # Examples
    ///
    /// If the token is present in the input, the writer gets the input plus
    /// the suffix right after the token. For instance, here,
    /// `does this foo go bar?` is transformed into
    /// `does this foobar go bar?`.
    ///
    /// ```rust
    /// # use code_challenge::Parser;
    /// let mut parser = Parser::new(b"foo", b"bar");
    /// let mut buffer = Vec::new();
    /// parser.process(b"does this foo go bar?", &mut buffer).unwrap();
    /// assert_eq!(b"does this foobar go bar?", buffer.as_slice());
    /// ```
    ///
    /// This works even if the token is split across multiple calls to the
    /// `process` method on the same instance of `Parser`.
    /// For instance, this is exactly the same as the previous example,
    /// but splits the input across multiple calls to `parser`.
    ///  
    /// ```rust
    /// # use code_challenge::Parser;
    /// let mut parser = Parser::new(b"foo", b"bar");
    /// let mut buffer = Vec::new();
    /// parser.process(b"does this f", &mut buffer).unwrap();
    /// parser.process(b"oo go bar?", &mut buffer).unwrap();
    /// assert_eq!(b"does this foobar go bar?", buffer.as_slice());
    /// ```
    //
    // NOTE: This function signature should stay the same.
    pub fn process(&mut self, input: &[u8], output: &mut dyn Write) -> Result<(), std::io::Error> {
        // NORA: Create a buffer to work in for just this call.
        // We'll write the whole buffer at the end, unconditionally;
        // the only question is what's in it.
        let mut buffer = Vec::with_capacity(input.len() + self.suffix.len());

        // NORA: Look at each byte in the input.
        for byte in input {
            // NORA: Unconditionally, write that byte into our buffer.
            // No requirement calls for losing any bytes.
            buffer.push(*byte);

            // NORA: Compare the current byte in the input to the current byte
            // in the token.
            if *byte == self.token[self.pos] {
                // NORA: If this byte is a match, progress to the next token byte.
                self.pos += 1;
                // NORA: If we've reached the end of the token,
                // that's a successful match.
                if self.pos == self.token.len() {
                    // NORA: Write the whole suffix into the output buffer.
                    for suffix_byte in self.suffix {
                        buffer.push(*suffix_byte);
                    }
                    // NORA: And reset to zero, so we can look for a new match.
                    self.pos = 0;
                }
            } else {
                // NORA: If the current input byte isn't a match for the current
                // token byte, we have failed to match the token;
                // reset to the beginning.
                // This prevents e.g. "oat meal" from matching "oatmeal".
                self.pos = 0;
            }
        }

        output.write_all(&buffer)
    }
}

#[test]
fn test_output_unmodified() {
    let mut parser = Parser::new(b"lalala", b"");
    let mut buffer = Vec::new();
    parser
        .process(b"does not contain the token", &mut buffer)
        .expect("couldn't write to buffer");
    assert_eq!(
        "does not contain the token",
        String::from_utf8_lossy(&buffer)
    )
}

#[test]
fn test_output_onetoken() {
    let mut parser = Parser::new(b"token", b"xxx");
    let mut buffer = Vec::new();
    parser
        .process(b"does contain the token", &mut buffer)
        .expect("couldn't write to buffer");
    assert_eq!(
        "does contain the tokenxxx",
        String::from_utf8_lossy(&buffer)
    )
}

#[test]
fn test_output_onetoken_multi() {
    let mut parser = Parser::new(b"token", b"xxx");
    let mut buffer = Vec::new();
    parser
        .process(b"does contain ", &mut buffer)
        .expect("couldn't write to buffer");
    parser
        .process(b"the tok", &mut buffer)
        .expect("couldn't write to buffer");
    parser
        .process(b"en", &mut buffer)
        .expect("couldn't write to buffer");
    assert_eq!(
        "does contain the tokenxxx",
        String::from_utf8_lossy(&buffer)
    )
}

#[test]
fn test_output_splittoken() {
    let mut parser = Parser::new(b"token", b"xxx");
    let mut buffer = Vec::new();
    parser
        .process(b"doesn't contain the tok en", &mut buffer)
        .expect("couldn't write to buffer");
    assert_eq!(
        "doesn't contain the tok en",
        String::from_utf8_lossy(&buffer)
    )
}