minimal_lexical/
rounding.rs

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1//! Defines rounding schemes for floating-point numbers.
2
3#![doc(hidden)]
4
5use crate::extended_float::ExtendedFloat;
6use crate::mask::{lower_n_halfway, lower_n_mask};
7use crate::num::Float;
8
9// ROUNDING
10// --------
11
12/// Round an extended-precision float to the nearest machine float.
13///
14/// Shifts the significant digits into place, adjusts the exponent,
15/// so it can be easily converted to a native float.
16#[cfg_attr(not(feature = "compact"), inline)]
17pub fn round<F, Cb>(fp: &mut ExtendedFloat, cb: Cb)
18where
19    F: Float,
20    Cb: Fn(&mut ExtendedFloat, i32),
21{
22    let fp_inf = ExtendedFloat {
23        mant: 0,
24        exp: F::INFINITE_POWER,
25    };
26
27    // Calculate our shift in significant digits.
28    let mantissa_shift = 64 - F::MANTISSA_SIZE - 1;
29
30    // Check for a denormal float, if after the shift the exponent is negative.
31    if -fp.exp >= mantissa_shift {
32        // Have a denormal float that isn't a literal 0.
33        // The extra 1 is to adjust for the denormal float, which is
34        // `1 - F::EXPONENT_BIAS`. This works as before, because our
35        // old logic rounded to `F::DENORMAL_EXPONENT` (now 1), and then
36        // checked if `exp == F::DENORMAL_EXPONENT` and no hidden mask
37        // bit was set. Here, we handle that here, rather than later.
38        //
39        // This might round-down to 0, but shift will be at **max** 65,
40        // for halfway cases rounding towards 0.
41        let shift = -fp.exp + 1;
42        debug_assert!(shift <= 65);
43        cb(fp, shift.min(64));
44        // Check for round-up: if rounding-nearest carried us to the hidden bit.
45        fp.exp = (fp.mant >= F::HIDDEN_BIT_MASK) as i32;
46        return;
47    }
48
49    // The float is normal, round to the hidden bit.
50    cb(fp, mantissa_shift);
51
52    // Check if we carried, and if so, shift the bit to the hidden bit.
53    let carry_mask = F::CARRY_MASK;
54    if fp.mant & carry_mask == carry_mask {
55        fp.mant >>= 1;
56        fp.exp += 1;
57    }
58
59    // Handle if we carried and check for overflow again.
60    if fp.exp >= F::INFINITE_POWER {
61        // Exponent is above largest normal value, must be infinite.
62        *fp = fp_inf;
63        return;
64    }
65
66    // Remove the hidden bit.
67    fp.mant &= F::MANTISSA_MASK;
68}
69
70/// Shift right N-bytes and round towards a direction.
71///
72/// Callback should take the following parameters:
73///     1. is_odd
74///     1. is_halfway
75///     1. is_above
76#[cfg_attr(not(feature = "compact"), inline)]
77pub fn round_nearest_tie_even<Cb>(fp: &mut ExtendedFloat, shift: i32, cb: Cb)
78where
79    // is_odd, is_halfway, is_above
80    Cb: Fn(bool, bool, bool) -> bool,
81{
82    // Ensure we've already handled denormal values that underflow.
83    debug_assert!(shift <= 64);
84
85    // Extract the truncated bits using mask.
86    // Calculate if the value of the truncated bits are either above
87    // the mid-way point, or equal to it.
88    //
89    // For example, for 4 truncated bytes, the mask would be 0b1111
90    // and the midway point would be 0b1000.
91    let mask = lower_n_mask(shift as u64);
92    let halfway = lower_n_halfway(shift as u64);
93    let truncated_bits = fp.mant & mask;
94    let is_above = truncated_bits > halfway;
95    let is_halfway = truncated_bits == halfway;
96
97    // Bit shift so the leading bit is in the hidden bit.
98    // This optimixes pretty well:
99    //  ```text
100    //   mov     ecx, esi
101    //   shr     rdi, cl
102    //   xor     eax, eax
103    //   cmp     esi, 64
104    //   cmovne  rax, rdi
105    //   ret
106    //  ```
107    fp.mant = match shift == 64 {
108        true => 0,
109        false => fp.mant >> shift,
110    };
111    fp.exp += shift;
112
113    // Extract the last bit after shifting (and determine if it is odd).
114    let is_odd = fp.mant & 1 == 1;
115
116    // Calculate if we need to roundup.
117    // We need to roundup if we are above halfway, or if we are odd
118    // and at half-way (need to tie-to-even). Avoid the branch here.
119    fp.mant += cb(is_odd, is_halfway, is_above) as u64;
120}
121
122/// Round our significant digits into place, truncating them.
123#[cfg_attr(not(feature = "compact"), inline)]
124pub fn round_down(fp: &mut ExtendedFloat, shift: i32) {
125    // Might have a shift greater than 64 if we have an error.
126    fp.mant = match shift == 64 {
127        true => 0,
128        false => fp.mant >> shift,
129    };
130    fp.exp += shift;
131}
minimal_lexical/rounding.rs

minimal_lexical/
rounding.rs
