define and implement trait NarrowingDiv for unsigned integer division

Author: quaternic

libm/src/math/support/big/tests.rs

@@ -3,6 +3,7 @@ use std::string::String;
 use std::{eprintln, format};
 
 use super::{HInt, MinInt, i256, u256};
+use crate::support::{Int as _, NarrowingDiv};
 
 const LOHI_SPLIT: u128 = 0xaaaaaaaaaaaaaaaaffffffffffffffff;
 
@@ -336,3 +337,28 @@ fn i256_shifts() {
         x = y;
     }
 }
+#[test]
+fn div_u256_by_u128() {
+    for j in i8::MIN..=i8::MAX {
+        let y: u128 = (j as i128).rotate_right(4).unsigned();
+        if y == 0 {
+            continue;
+        }
+        for i in i8::MIN..=i8::MAX {
+            let x: u128 = (i as i128).rotate_right(4).unsigned();
+            let xy = x.widen_mul(y);
+            assert_eq!(xy.checked_narrowing_div_rem(y), Some((x, 0)));
+            if y != 1 {
+                assert_eq!((xy + u256::ONE).checked_narrowing_div_rem(y), Some((x, 1)));
+            }
+            if x != 0 {
+                assert_eq!(
+                    (xy - u256::ONE).checked_narrowing_div_rem(y),
+                    Some((x - 1, y - 1))
+                );
+            }
+            let r = ((y as f64) * 0.12345) as u128;
+            assert_eq!((xy + r.widen()).checked_narrowing_div_rem(y), Some((x, r)));
+        }
+    }
+}

libm/src/math/support/int_traits.rs

@@ -1,5 +1,8 @@
 use core::{cmp, fmt, ops};
 
+mod narrowing_div;
+pub use narrowing_div::NarrowingDiv;
+
 /// Minimal integer implementations needed on all integer types, including wide integers.
 #[allow(dead_code)] // Some constants are only used with tests
 pub trait MinInt:

libm/src/math/support/int_traits/narrowing_div.rs

@@ -0,0 +1,162 @@
+use crate::support::{DInt, HInt, Int, MinInt, u256};
+
+/// Trait for unsigned division of a double-wide integer
+/// when the quotient doesn't overflow.
+///
+/// This is the inverse of widening multiplication:
+///  - for any `x` and nonzero `y`: `x.widen_mul(y).checked_narrowing_div_rem(y) == Some((x, 0))`,
+///  - and for any `r in 0..y`: `x.carrying_mul(y, r).checked_narrowing_div_rem(y) == Some((x, r))`,
+pub trait NarrowingDiv: DInt + MinInt<Unsigned = Self> {
+    /// Computes `(self / n, self % n))`
+    ///
+    /// # Safety
+    /// The caller must ensure that `self.hi() < n`, or equivalently,
+    /// that the quotient does not overflow.
+    unsafe fn unchecked_narrowing_div_rem(self, n: Self::H) -> (Self::H, Self::H);
+
+    /// Returns `Some((self / n, self % n))` when `self.hi() < n`.
+    fn checked_narrowing_div_rem(self, n: Self::H) -> Option<(Self::H, Self::H)> {
+        if self.hi() < n {
+            Some(unsafe { self.unchecked_narrowing_div_rem(n) })
+        } else {
+            None
+        }
+    }
+}
+
+macro_rules! impl_narrowing_div_primitive {
+    ($D:ident) => {
+        impl NarrowingDiv for $D {
+            unsafe fn unchecked_narrowing_div_rem(self, n: Self::H) -> (Self::H, Self::H) {
+                if self.hi() >= n {
+                    unsafe { core::hint::unreachable_unchecked() }
+                }
+                ((self / n as $D) as Self::H, (self % n as $D) as Self::H)
+            }
+        }
+    };
+}
+
+// Extend division from `u2N / uN` to `u4N / u2N`
+// This is not the most efficient algorithm, but it is
+// relatively simple.
+macro_rules! impl_narrowing_div_recurse {
+    ($D:ident) => {
+        impl NarrowingDiv for $D {
+            unsafe fn unchecked_narrowing_div_rem(self, n: Self::H) -> (Self::H, Self::H) {
+                if self.hi() >= n {
+                    unsafe { core::hint::unreachable_unchecked() }
+                }
+
+                // Normalize the divisor by shifting the most significant one
+                // to the leading position. `n != 0` is implied by `self.hi() < n`
+                let lz = n.leading_zeros();
+                let a = self << lz;
+                let b = n << lz;
+
+                let ah = a.hi();
+                let (a0, a1) = a.lo().lo_hi();
+                // SAFETY: For both calls, `b.leading_zeros() == 0` by the above shift.
+                // SAFETY: `ah < b` follows from `self.hi() < n`
+                let (q1, r) = unsafe { div_three_digits_by_two(a1, ah, b) };
+                // SAFETY: `r < b` is given as the postcondition of the previous call
+                let (q0, r) = unsafe { div_three_digits_by_two(a0, r, b) };
+
+                // Undo the earlier normalization for the remainder
+                (Self::H::from_lo_hi(q0, q1), r >> lz)
+            }
+        }
+    };
+}
+
+impl_narrowing_div_primitive!(u16);
+impl_narrowing_div_primitive!(u32);
+impl_narrowing_div_primitive!(u64);
+impl_narrowing_div_primitive!(u128);
+impl_narrowing_div_recurse!(u256);
+
+/// Implement `u3N / u2N`-division on top of `u2N / uN`-division.
+///
+/// Returns the quotient and remainder of `(a * R + a0) / n`,
+/// where `R = (1 << U::BITS)` is the digit size.
+///
+/// # Safety
+/// Requires that `n.leading_zeros() == 0` and `a < n`.
+unsafe fn div_three_digits_by_two<U>(a0: U, a: U::D, n: U::D) -> (U, U::D)
+where
+    U: HInt,
+    U::D: Int + NarrowingDiv,
+{
+    if n.leading_zeros() > 0 || a >= n {
+        debug_assert!(false, "unsafe preconditions not met");
+        unsafe { core::hint::unreachable_unchecked() }
+    }
+
+    // n = n1R + n0
+    let (n0, n1) = n.lo_hi();
+    // a = a2R + a1
+    let (a1, a2) = a.lo_hi();
+
+    let mut q;
+    let mut r;
+    let mut wrap;
+    // `a < n` is guaranteed by the caller, but `a2 == n1 && a1 < n0` is possible
+    if let Some((q0, r1)) = a.checked_narrowing_div_rem(n1) {
+        q = q0;
+        // a = qn1 + r1, where 0 <= r1 < n1
+
+        // Include the remainder with the low bits:
+        // r = a0 + r1R
+        r = U::D::from_lo_hi(a0, r1);
+
+        // Subtract the contribution of the divisor low bits with the estimated quotient
+        let d = q.widen_mul(n0);
+        (r, wrap) = r.overflowing_sub(d);
+
+        // Since `q` is the quotient of dividing with a slightly smaller divisor,
+        // it may be an overapproximation, but is never too small, and similarly,
+        // `r` is now either the correct remainder ...
+        if !wrap {
+            return (q, r);
+        }
+        // ... or the remainder went "negative" (by as much as `d = qn0 < RR`)
+        // and we have to adjust.
+        q -= U::ONE;
+    } else {
+        debug_assert!(a2 == n1 && a1 < n0);
+        // Otherwise, `a2 == n1`, and the estimated quotient would be
+        // `R + (a1 % n1)`, but the correct quotient can't overflow.
+        // We'll start from `q = R = (1 << U::BITS)`,
+        // so `r = aR + a0 - qn = (a - n)R + a0`
+        r = U::D::from_lo_hi(a0, a1.wrapping_sub(n0));
+        // Since `a < n`, the first decrement is always needed:
+        q = U::MAX; /* R - 1 */
+    }
+
+    (r, wrap) = r.overflowing_add(n);
+    if wrap {
+        return (q, r);
+    }
+
+    // If the remainder still didn't wrap, we need another step.
+    q -= U::ONE;
+    (r, wrap) = r.overflowing_add(n);
+    // Since `n >= RR/2`, at least one of the two `r += n` must have wrapped.
+    debug_assert!(wrap, "estimated quotient should be off by at most two");
+    (q, r)
+}
+
+#[cfg(test)]
+mod test {
+    use super::{HInt, NarrowingDiv};
+
+    #[test]
+    fn inverse_mul() {
+        for x in 0..=u8::MAX {
+            for y in 1..=u8::MAX {
+                let xy = x.widen_mul(y);
+                assert_eq!(xy.checked_narrowing_div_rem(y), Some((x, 0)));
+            }
+        }
+    }
+}

libm/src/math/support/mod.rs

@@ -28,7 +28,7 @@ pub use hex_float::hf16;
 pub use hex_float::hf128;
 #[allow(unused_imports)]
 pub use hex_float::{hf32, hf64};
-pub use int_traits::{CastFrom, CastInto, DInt, HInt, Int, MinInt};
+pub use int_traits::{CastFrom, CastInto, DInt, HInt, Int, MinInt, NarrowingDiv};
 
 /// Hint to the compiler that the current path is cold.
 pub fn cold_path() {