// SPDX-License-Identifier: AGPL-3.0-or-later

//! Nom parsers used within the parsing steps.

use nom::{
	bytes::complete::{tag, take_while},
	character::complete::multispace0,
	combinator::map_res,
	sequence::{pair, preceded},
	IResult,
};

/// Determines if the provided character is an ascii digit.
const fn is_decimal_digit(c: char) -> bool {
	c.is_ascii_digit()
}

/// Parses a string slice into a [`u16`].
///
/// # Errors
///
/// This function will return an error if the string cannot be parsed.
fn from_decimal(input: &str) -> Result<u16, std::num::ParseIntError> {
	input.parse::<u16>()
}

/// Retrieves all the digits from a CRN and maps them to a [`u16`].
///
/// # Errors
///
/// This function will return an error if nom cannot parse the input.
fn crn_digits(input: &str) -> IResult<&str, u16> {
	map_res(take_while(is_decimal_digit), from_decimal)(input)
}

/// Parses a course reference number.
///
/// # Errors
///
/// This function will return an error if nom cannot parse the input.
pub fn course_reference_number(input: &str) -> IResult<&str, u16> {
	preceded(pair(tag("CRN"), multispace0), crn_digits)(input)
}

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

	#[test]
	fn crn_parser_basic() {
		assert_eq!(course_reference_number("CRN 5912").unwrap().1, 5912);
		assert_eq!(course_reference_number("CRN 17146").unwrap().1, 17146);
	}

	#[test]
	fn crn_parser_postfix() {
		assert_eq!(
			course_reference_number("CRN 331 [Distance]").unwrap().1,
			331
		);
	}

	#[test]
	fn crn_parser_extra_whitespace() {
		assert_eq!(course_reference_number("CRN 8913 ").unwrap().1, 8913);
		assert_eq!(course_reference_number("CRN  61151").unwrap().1, 61151);
	}

	#[test]
	fn crn_parser_no_whitespace() {
		assert_eq!(course_reference_number("CRN615").unwrap().1, 615);
	}
}