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//! Utility functions.

use crate::{format::try_parse_fmt_string, internals};
#[cfg(not(feature = "std"))]
use alloc::string::String;
use const_fn::const_fn;

/// Checks if a user-provided formatting string is valid. If it isn't, a
/// description of the error is returned.
pub fn validate_format_string(s: impl AsRef<str>) -> Result<(), String> {
    try_parse_fmt_string(s.as_ref()).map(|_| ())
}

/// The number of days in a month in both common and leap years.
const DAYS_IN_MONTH_COMMON_LEAP: [[u16; 12]; 2] = [
    [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
    [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
];

/// Get the number of days in the month of a given year.
pub(crate) const fn days_in_year_month(year: i32, month: u8) -> u8 {
    DAYS_IN_MONTH_COMMON_LEAP[is_leap_year(year) as usize][month as usize - 1] as u8
}

/// Returns if the provided year is a leap year in the proleptic Gregorian
/// calendar. Uses [astronomical year numbering](https://en.wikipedia.org/wiki/Astronomical_year_numbering).
///
/// ```rust
/// # use time::is_leap_year;
/// assert!(!is_leap_year(1900));
/// assert!(is_leap_year(2000));
/// assert!(is_leap_year(2004));
/// assert!(!is_leap_year(2005));
/// assert!(!is_leap_year(2100));
/// ```
pub const fn is_leap_year(year: i32) -> bool {
    (year % 4 == 0) & ((year % 100 != 0) | (year % 400 == 0))
}

/// Get the number of calendar days in a given year.
///
/// The returned value will always be either 365 or 366.
///
/// ```rust
/// # use time::days_in_year;
/// assert_eq!(days_in_year(1900), 365);
/// assert_eq!(days_in_year(2000), 366);
/// assert_eq!(days_in_year(2004), 366);
/// assert_eq!(days_in_year(2005), 365);
/// assert_eq!(days_in_year(2100), 365);
/// ```
pub const fn days_in_year(year: i32) -> u16 {
    365 + is_leap_year(year) as u16
}

/// Get the number of weeks in the ISO year.
///
/// The returned value will always be either 52 or 53.
///
/// ```rust
/// # use time::weeks_in_year;
/// assert_eq!(weeks_in_year(2019), 52);
/// assert_eq!(weeks_in_year(2020), 53);
/// ```
///
/// This function is `const fn` when using rustc >= 1.46.
#[const_fn("1.46")]
pub const fn weeks_in_year(year: i32) -> u8 {
    let weekday = internals::Date::from_yo_unchecked(year, 1).iso_weekday_number();

    if weekday == 4 || weekday == 3 && is_leap_year(year) {
        53
    } else {
        52
    }
}