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diff --git a/third_party/rust/minimal-lexical/src/rounding.rs b/third_party/rust/minimal-lexical/src/rounding.rs
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+//! Defines rounding schemes for floating-point numbers.
+
+#![doc(hidden)]
+
+use crate::extended_float::ExtendedFloat;
+use crate::mask::{lower_n_halfway, lower_n_mask};
+use crate::num::Float;
+
+// ROUNDING
+// --------
+
+/// Round an extended-precision float to the nearest machine float.
+///
+/// Shifts the significant digits into place, adjusts the exponent,
+/// so it can be easily converted to a native float.
+#[cfg_attr(not(feature = "compact"), inline)]
+pub fn round<F, Cb>(fp: &mut ExtendedFloat, cb: Cb)
+where
+    F: Float,
+    Cb: Fn(&mut ExtendedFloat, i32),
+{
+    let fp_inf = ExtendedFloat {
+        mant: 0,
+        exp: F::INFINITE_POWER,
+    };
+
+    // Calculate our shift in significant digits.
+    let mantissa_shift = 64 - F::MANTISSA_SIZE - 1;
+
+    // Check for a denormal float, if after the shift the exponent is negative.
+    if -fp.exp >= mantissa_shift {
+        // Have a denormal float that isn't a literal 0.
+        // The extra 1 is to adjust for the denormal float, which is
+        // `1 - F::EXPONENT_BIAS`. This works as before, because our
+        // old logic rounded to `F::DENORMAL_EXPONENT` (now 1), and then
+        // checked if `exp == F::DENORMAL_EXPONENT` and no hidden mask
+        // bit was set. Here, we handle that here, rather than later.
+        //
+        // This might round-down to 0, but shift will be at **max** 65,
+        // for halfway cases rounding towards 0.
+        let shift = -fp.exp + 1;
+        debug_assert!(shift <= 65);
+        cb(fp, shift.min(64));
+        // Check for round-up: if rounding-nearest carried us to the hidden bit.
+        fp.exp = (fp.mant >= F::HIDDEN_BIT_MASK) as i32;
+        return;
+    }
+
+    // The float is normal, round to the hidden bit.
+    cb(fp, mantissa_shift);
+
+    // Check if we carried, and if so, shift the bit to the hidden bit.
+    let carry_mask = F::CARRY_MASK;
+    if fp.mant & carry_mask == carry_mask {
+        fp.mant >>= 1;
+        fp.exp += 1;
+    }
+
+    // Handle if we carried and check for overflow again.
+    if fp.exp >= F::INFINITE_POWER {
+        // Exponent is above largest normal value, must be infinite.
+        *fp = fp_inf;
+        return;
+    }
+
+    // Remove the hidden bit.
+    fp.mant &= F::MANTISSA_MASK;
+}
+
+/// Shift right N-bytes and round towards a direction.
+///
+/// Callback should take the following parameters:
+///     1. is_odd
+///     1. is_halfway
+///     1. is_above
+#[cfg_attr(not(feature = "compact"), inline)]
+pub fn round_nearest_tie_even<Cb>(fp: &mut ExtendedFloat, shift: i32, cb: Cb)
+where
+    // is_odd, is_halfway, is_above
+    Cb: Fn(bool, bool, bool) -> bool,
+{
+    // Ensure we've already handled denormal values that underflow.
+    debug_assert!(shift <= 64);
+
+    // Extract the truncated bits using mask.
+    // Calculate if the value of the truncated bits are either above
+    // the mid-way point, or equal to it.
+    //
+    // For example, for 4 truncated bytes, the mask would be 0b1111
+    // and the midway point would be 0b1000.
+    let mask = lower_n_mask(shift as u64);
+    let halfway = lower_n_halfway(shift as u64);
+    let truncated_bits = fp.mant & mask;
+    let is_above = truncated_bits > halfway;
+    let is_halfway = truncated_bits == halfway;
+
+    // Bit shift so the leading bit is in the hidden bit.
+    // This optimixes pretty well:
+    //  ```text
+    //   mov     ecx, esi
+    //   shr     rdi, cl
+    //   xor     eax, eax
+    //   cmp     esi, 64
+    //   cmovne  rax, rdi
+    //   ret
+    //  ```
+    fp.mant = match shift == 64 {
+        true => 0,
+        false => fp.mant >> shift,
+    };
+    fp.exp += shift;
+
+    // Extract the last bit after shifting (and determine if it is odd).
+    let is_odd = fp.mant & 1 == 1;
+
+    // Calculate if we need to roundup.
+    // We need to roundup if we are above halfway, or if we are odd
+    // and at half-way (need to tie-to-even). Avoid the branch here.
+    fp.mant += cb(is_odd, is_halfway, is_above) as u64;
+}
+
+/// Round our significant digits into place, truncating them.
+#[cfg_attr(not(feature = "compact"), inline)]
+pub fn round_down(fp: &mut ExtendedFloat, shift: i32) {
+    // Might have a shift greater than 64 if we have an error.
+    fp.mant = match shift == 64 {
+        true => 0,
+        false => fp.mant >> shift,
+    };
+    fp.exp += shift;
+}