44 lines
1.3 KiB
Rust
44 lines
1.3 KiB
Rust
/// Multiply unsigned 128 bit integers, return upper 128 bits of the result
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#[inline]
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fn u128_mulhi(x: u128, y: u128) -> u128 {
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let x_lo = x as u64;
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let x_hi = (x >> 64) as u64;
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let y_lo = y as u64;
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let y_hi = (y >> 64) as u64;
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// handle possibility of overflow
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let carry = (x_lo as u128 * y_lo as u128) >> 64;
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let m = x_lo as u128 * y_hi as u128 + carry;
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let high1 = m >> 64;
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let m_lo = m as u64;
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let high2 = (x_hi as u128 * y_lo as u128 + m_lo as u128) >> 64;
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x_hi as u128 * y_hi as u128 + high1 + high2
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}
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/// Divide `n` by 1e19 and return quotient and remainder
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///
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/// Integer division algorithm is based on the following paper:
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///
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/// T. Granlund and P. Montgomery, “Division by Invariant Integers Using Multiplication”
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/// in Proc. of the SIGPLAN94 Conference on Programming Language Design and
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/// Implementation, 1994, pp. 61–72
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///
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#[inline]
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pub fn udivmod_1e19(n: u128) -> (u128, u64) {
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let d = 10_000_000_000_000_000_000_u64; // 10^19
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let quot = if n < 1 << 83 {
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((n >> 19) as u64 / (d >> 19)) as u128
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} else {
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u128_mulhi(n, 156927543384667019095894735580191660403) >> 62
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};
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let rem = (n - quot * d as u128) as u64;
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debug_assert_eq!(quot, n / d as u128);
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debug_assert_eq!(rem as u128, n % d as u128);
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(quot, rem)
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}
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