use nom::IResult;

#[test]
fn test_tag() {
    use nom::bytes::complete::tag;
    fn parser(s: &str) -> IResult<&str, &str> {
        tag("#command")(s)
    }
    let input = "#command";
    let result = parser(input);
    assert_eq!(result, Ok(("", "#command")));
}

#[test]
fn test_string() {
    use nom::branch::alt;
    use nom::bytes::streaming::{is_not, take_while_m_n};
    use nom::character::streaming::{char, multispace1};
    use nom::combinator::{map, map_opt, map_res, value, verify};
    use nom::error::{FromExternalError, ParseError};
    use nom::multi::fold;
    use nom::sequence::{delimited, preceded};
    use nom::{IResult, Parser};

    // parser combinators are constructed from the bottom up:
    // first we write parsers for the smallest elements (escaped characters),
    // then combine them into larger parsers.

    /// Parse a unicode sequence, of the form u{XXXX}, where XXXX is 1 to 6
    /// hexadecimal numerals. We will combine this later with parse_escaped_char
    /// to parse sequences like \u{00AC}.
    fn parse_unicode<'a, E>(input: &'a str) -> IResult<&'a str, char, E>
    where
        E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
    {
        // `take_while_m_n` parses between `m` and `n` bytes (inclusive) that match
        // a predicate. `parse_hex` here parses between 1 and 6 hexadecimal numerals.
        let parse_hex = take_while_m_n(1, 6, |c: char| c.is_ascii_hexdigit());

        // `preceded` takes a prefix parser, and if it succeeds, returns the result
        // of the body parser. In this case, it parses u{XXXX}.
        let parse_delimited_hex = preceded(
            char('u'),
            // `delimited` is like `preceded`, but it parses both a prefix and a suffix.
            // It returns the result of the middle parser. In this case, it parses
            // {XXXX}, where XXXX is 1 to 6 hex numerals, and returns XXXX
            delimited(char('{'), parse_hex, char('}')),
        );

        // `map_res` takes the result of a parser and applies a function that returns
        // a Result. In this case we take the hex bytes from parse_hex and attempt to
        // convert them to a u32.
        let parse_u32 = map_res(parse_delimited_hex, move |hex| u32::from_str_radix(hex, 16));

        // map_opt is like map_res, but it takes an Option instead of a Result. If
        // the function returns None, map_opt returns an error. In this case, because
        // not all u32 values are valid unicode code points, we have to fallibly
        // convert to char with from_u32.
        map_opt(parse_u32, std::char::from_u32).parse(input)
    }

    /// Parse an escaped character: \n, \t, \r, \u{00AC}, etc.
    fn parse_escaped_char<'a, E>(input: &'a str) -> IResult<&'a str, char, E>
    where
        E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
    {
        preceded(
            char('\\'),
            // `alt` tries each parser in sequence, returning the result of
            // the first successful match
            alt((
                parse_unicode,
                // The `value` parser returns a fixed value (the first argument) if its
                // parser (the second argument) succeeds. In these cases, it looks for
                // the marker characters (n, r, t, etc) and returns the matching
                // character (\n, \r, \t, etc).
                value('\n', char('n')),
                value('\r', char('r')),
                value('\t', char('t')),
                value('\u{08}', char('b')),
                value('\u{0C}', char('f')),
                value('\\', char('\\')),
                value('/', char('/')),
                value('"', char('"')),
            )),
        )
        .parse(input)
    }

    /// Parse a backslash, followed by any amount of whitespace. This is used later
    /// to discard any escaped whitespace.
    fn parse_escaped_whitespace<'a, E: ParseError<&'a str>>(
        input: &'a str,
    ) -> IResult<&'a str, &'a str, E> {
        preceded(char('\\'), multispace1).parse(input)
    }

    /// Parse a non-empty block of text that doesn't include \ or "
    fn parse_literal<'a, E: ParseError<&'a str>>(input: &'a str) -> IResult<&'a str, &'a str, E> {
        // `is_not` parses a string of 0 or more characters that aren't one of the
        // given characters.
        let not_quote_slash = is_not("\"\\");

        // `verify` runs a parser, then runs a verification function on the output of
        // the parser. The verification function accepts out output only if it
        // returns true. In this case, we want to ensure that the output of is_not
        // is non-empty.
        verify(not_quote_slash, |s: &str| !s.is_empty()).parse(input)
    }

    /// A string fragment contains a fragment of a string being parsed: either
    /// a non-empty Literal (a series of non-escaped characters), a single
    /// parsed escaped character, or a block of escaped whitespace.
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    enum StringFragment<'a> {
        Literal(&'a str),
        EscapedChar(char),
        EscapedWS,
    }

    /// Combine parse_literal, parse_escaped_whitespace, and parse_escaped_char
    /// into a StringFragment.
    fn parse_fragment<'a, E>(input: &'a str) -> IResult<&'a str, StringFragment<'a>, E>
    where
        E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
    {
        alt((
            // The `map` combinator runs a parser, then applies a function to the output
            // of that parser.
            map(parse_literal, StringFragment::Literal),
            map(parse_escaped_char, StringFragment::EscapedChar),
            value(StringFragment::EscapedWS, parse_escaped_whitespace),
        ))
        .parse(input)
    }

    /// Parse a string. Use a loop of parse_fragment and push all of the fragments
    /// into an output string.
    fn parse_string<'a, E>(input: &'a str) -> IResult<&'a str, String, E>
    where
        E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
    {
        // fold is the equivalent of iterator::fold. It runs a parser in a loop,
        // and for each output value, calls a folding function on each output value.
        let build_string = fold(
            0..,
            // Our parser function – parses a single string fragment
            parse_fragment,
            // Our init value, an empty string
            String::new,
            // Our folding function. For each fragment, append the fragment to the
            // string.
            |mut string, fragment| {
                match fragment {
                    StringFragment::Literal(s) => string.push_str(s),
                    StringFragment::EscapedChar(c) => string.push(c),
                    StringFragment::EscapedWS => {}
                }
                string
            },
        );

        // Finally, parse the string. Note that, if `build_string` could accept a raw
        // " character, the closing delimiter " would never match. When using
        // `delimited` with a looping parser (like fold), be sure that the
        // loop won't accidentally match your closing delimiter!
        delimited(char('"'), build_string, char('"')).parse(input)
    }

    let data = "\"abc\"";
    println!("EXAMPLE 1:\nParsing a simple input string: {}", data);
    let result = parse_string::<()>(data);
    assert_eq!(result, Ok(("", String::from("abc"))));
    println!("Result: {}\n\n", result.unwrap().1);

    let data = "\"tab:\\tafter tab, newline:\\nnew line, quote: \\\", emoji: \\u{1F602}, newline:\\nescaped whitespace: \\    abc\"";
    println!(
        "EXAMPLE 2:\nParsing a string with escape sequences, newline literal, and escaped whitespace:\n\n{}\n",
        data
    );
    let result = parse_string::<()>(data);
    assert_eq!(
        result,
        Ok((
            "",
            String::from(
                "tab:\tafter tab, newline:\nnew line, quote: \", emoji: 😂, newline:\nescaped whitespace: abc"
            )
        ))
    );
    println!("Result:\n\n{}", result.unwrap().1);
}
