// Adapted from
https://github.com/Alexhuszagh/rust-lexical.
//! Utilities to calculate exponents.
/// Convert usize into i32 without overflow.
///
/// This is needed to ensure when adjusting the exponent relative to
/// the mantissa we do not overflow for comically-long exponents.
#[inline]
fn into_i32(value: usize) -> i32 {
if value > i32::MAX as usize {
i32::MAX
} else {
value as i32
}
}
// EXPONENT CALCULATION
// Calculate the scientific notation exponent without overflow.
//
// For example, 0.1 would be -1, and 10 would be 1 in base 10.
#[inline]
pub(crate) fn scientific_exponent(
exponent: i32,
integer_digits: usize,
fraction_start: usize,
) -> i32 {
if integer_digits == 0 {
let fraction_start = into_i32(fraction_start);
exponent.saturating_sub(fraction_start).saturating_sub(1)
} else {
let integer_shift = into_i32(integer_digits - 1);
exponent.saturating_add(integer_shift)
}
}
// Calculate the mantissa exponent without overflow.
//
// Remove the number of digits that contributed to the mantissa past
// the dot, and add the number of truncated digits from the mantissa,
// to calculate the scaling factor for the mantissa from a raw exponent.
#[inline]
pub(crate) fn mantissa_exponent(exponent: i32, fraction_digits: usize, truncated: usize
) -> i32 {
if fraction_digits > truncated {
exponent.saturating_sub(into_i32(fraction_digits - truncated))
} else {
exponent.saturating_add(into_i32(truncated - fraction_digits))
}
}
