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Implement rational() function for floats #11125

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Dec 30, 2013
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61 changes: 60 additions & 1 deletion src/libextra/num/rational.rs
Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@
use std::cmp;
use std::from_str::FromStr;
use std::num::{Zero,One,ToStrRadix,FromStrRadix,Round};
use super::bigint::BigInt;
use super::bigint::{BigInt, BigUint, Sign, Plus, Minus};

/// Represents the ratio between 2 numbers.
#[deriving(Clone)]
Expand Down Expand Up @@ -107,6 +107,27 @@ impl<T: Clone + Integer + Ord>
}
}

impl Ratio<BigInt> {
/// Converts a float into a rational number
pub fn from_float<T: Float>(f: T) -> Option<BigRational> {
if !f.is_finite() {
return None;
}
let (mantissa, exponent, sign) = f.integer_decode();
let bigint_sign: Sign = if sign == 1 { Plus } else { Minus };
if exponent < 0 {
let one: BigInt = One::one();
let denom: BigInt = one << ((-exponent) as uint);
let numer: BigUint = FromPrimitive::from_u64(mantissa).unwrap();
Some(Ratio::new(BigInt::from_biguint(bigint_sign, numer), denom))
} else {
let mut numer: BigUint = FromPrimitive::from_u64(mantissa).unwrap();
numer = numer << (exponent as uint);
Some(Ratio::from_integer(BigInt::from_biguint(bigint_sign, numer)))
}
}
}

/* Comparisons */

// comparing a/b and c/d is the same as comparing a*d and b*c, so we
Expand Down Expand Up @@ -621,4 +642,42 @@ mod test {
test(s);
}
}

#[test]
fn test_from_float() {
fn test<T: Float>(given: T, (numer, denom): (&str, &str)) {
let ratio: BigRational = Ratio::from_float(given).unwrap();
assert_eq!(ratio, Ratio::new(
FromStr::from_str(numer).unwrap(),
FromStr::from_str(denom).unwrap()));
}

// f32
test(3.14159265359f32, ("13176795", "4194304"));
test(2f32.pow(&100.), ("1267650600228229401496703205376", "1"));
test(-2f32.pow(&100.), ("-1267650600228229401496703205376", "1"));
test(1.0 / 2f32.pow(&100.), ("1", "1267650600228229401496703205376"));
test(684729.48391f32, ("1369459", "2"));
test(-8573.5918555f32, ("-4389679", "512"));

// f64
test(3.14159265359f64, ("3537118876014453", "1125899906842624"));
test(2f64.pow(&100.), ("1267650600228229401496703205376", "1"));
test(-2f64.pow(&100.), ("-1267650600228229401496703205376", "1"));
test(684729.48391f64, ("367611342500051", "536870912"));
test(-8573.5918555, ("-4713381968463931", "549755813888"));
test(1.0 / 2f64.pow(&100.), ("1", "1267650600228229401496703205376"));
}

#[test]
fn test_from_float_fail() {
use std::{f32, f64};

assert_eq!(Ratio::from_float(f32::NAN), None);
assert_eq!(Ratio::from_float(f32::INFINITY), None);
assert_eq!(Ratio::from_float(f32::NEG_INFINITY), None);
assert_eq!(Ratio::from_float(f64::NAN), None);
assert_eq!(Ratio::from_float(f64::INFINITY), None);
assert_eq!(Ratio::from_float(f64::NEG_INFINITY), None);
}
}