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