dashu-int 0.6.0

//! Information about radixes.

use crate::{
    arch::word::Word,
    math::{max_exp_in_dword, max_exp_in_word},
};
use static_assertions::const_assert;

type FastDivideSmall = num_modular::PreMulInv1by1<Word>;
type FastDivideNormalized = num_modular::Normalized2by1Divisor<Word>;

/// Digit and radix type, it's always u8.
pub type Digit = u8;

/// Minimum supported radix.
pub const MIN_RADIX: Digit = 2;

/// Maximum supported radix.
pub const MAX_RADIX: Digit = 36;

/// Is a radix in valid range?
#[inline]
pub const fn is_radix_valid(radix: Digit) -> bool {
    MIN_RADIX <= radix && radix <= MAX_RADIX
}

const_assert!(b'a' > b'0' + 10 && b'A' > b'0' + 10);

/// u8 representation is: how much digits >= 10 should be offset by in ASCII.
#[derive(Clone, Copy, Eq, PartialEq)]
#[repr(u8)]
pub enum DigitCase {
    NoLetters = 0,
    Lower = b'a' - b'0' - 10,
    Upper = b'A' - b'0' - 10,
}

/// Converts a byte (ASCII) representation of a digit to its value.
///
/// The radix has to be in range 2..36 (inclusive), and the digit will
/// be parsed with insensitive case.
#[inline]
pub const fn digit_from_ascii_byte(byte: u8, radix: Digit) -> Option<Digit> {
    assert!(is_radix_valid(radix));

    let res = match byte {
        c @ b'0'..=b'9' => (c - b'0') as Digit,
        c @ b'a'..=b'z' => (c - b'a') as Digit + 10,
        c @ b'A'..=b'Z' => (c - b'A') as Digit + 10,
        _ => return None,
    };
    if res < radix {
        Some(res)
    } else {
        None
    }
}

/// Properties of a given radix.
#[derive(Clone, Copy)]
pub struct RadixInfo {
    /// The number of digits that can always fit in a `Word`.
    pub(crate) digits_per_word: usize,

    /// Radix to the power of `max_digits`.
    /// Only for non-power-of-2 radixes.
    pub(crate) range_per_word: Word,

    /// Faster division by `radix`.
    pub(crate) fast_div_radix: FastDivideSmall,

    /// Faster division by normalized range_per_word.
    /// Only for non-power-of-2 radixes.
    pub(crate) fast_div_range_per_word: FastDivideNormalized,
}

/// Radix info for base 10
pub const RADIX10_INFO: RadixInfo = RadixInfo::for_radix(10);

/// Maximum number of digits that a `Word` can ever have for any non-power-of-2 radix.
pub const MAX_WORD_DIGITS_NON_POW_2: usize = max_exp_in_word(3).0 + 1;
/// Maximum number of digits that a `DoubleWord` can ever have for any non-power-of-2 radix.
pub const MAX_DWORD_DIGITS_NON_POW_2: usize = max_exp_in_dword(3).0 + 1;

/// Get [RadixInfo] for a given radix.
///
/// This method is not specialized for power of two.
#[inline]
pub fn radix_info(radix: Digit) -> RadixInfo {
    debug_assert!(is_radix_valid(radix));

    match radix {
        10 => RADIX10_INFO,
        _ => RadixInfo::for_radix(radix),
    }
}

impl RadixInfo {
    const fn for_radix(radix: Digit) -> RadixInfo {
        let (digits_per_word, range_per_word) = max_exp_in_word(radix as Word);
        let shift = range_per_word.leading_zeros();
        let fast_div_radix = FastDivideSmall::new(radix as Word);
        let fast_div_range_per_word = FastDivideNormalized::new(range_per_word << shift);
        RadixInfo {
            digits_per_word,
            range_per_word,
            fast_div_radix,
            fast_div_range_per_word,
        }
    }
}

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

    #[test]
    fn test_digit_from_utf8_byte() {
        assert_eq!(digit_from_ascii_byte(b'7', 10), Some(7));
        assert_eq!(digit_from_ascii_byte(b'a', 16), Some(10));
        assert_eq!(digit_from_ascii_byte(b'z', 36), Some(35));
        assert_eq!(digit_from_ascii_byte(b'Z', 36), Some(35));
        assert_eq!(digit_from_ascii_byte(b'?', 10), None);
        assert_eq!(digit_from_ascii_byte(b'a', 10), None);
        assert_eq!(digit_from_ascii_byte(b'z', 35), None);
        assert_eq!(digit_from_ascii_byte(255, 35), None);
    }
}