Convert BigFloat to BigUint (from num_bigint)

[Janmajayamall]

Thanks for the lib! It has been immensely helpful.

I am running into issues converting a BigFloat instance to BigUint (from num_bigint). My naive try is

let mut consts = Consts::new().unwrap();

let mut value = BigFloat::from(1.2);

let (sign, radix_repr, ex) = value
  .round(0, ROUNDING_MODE)
  .convert_to_radix(astro_float::Radix::Dec, ROUNDING_MODE, &mut consts)
  .unwrap();
  
// Assume sign is always positive
let value_biguint = BigUint::from_radix_be(&radix_repr, 10).unwrap();

I though that setting to n to 0 (i.e. fractional digits are decimal) will round the value to 1, hence mantissa will 1 and exp will be 0. But this isn't the case. Probably I am misreding the docs. Can you point me to a function using which I can round the value towards 0 and mantissa will drop rounded-off digits?

[stencillogic]

round is the function that can round fractional part of a number.
What's the value of this ROUNDING_MODE ?

[stencillogic]

Also, rounding will not change exponent of a number, unless there was overflow during rounding.

[Janmajayamall]

What's the value of this ROUNDING_MODE ?

It is set to none - RoundingMode::None

[Janmajayamall]

Ok, so seems like the only way is to read the mantissa after rounding as a string. Drop off digits after decimal as per the exponent. Then parse the resulting string of digits into BigUint. Do you think there's a better way to do this?

[Janmajayamall]

This is what I came up with:

let mut consts = Consts::new().unwrap();

let mut value = AstroBFloat::from(1.2);

let (sign, radix_repr, mut ex) = value
    .round(0, ROUNDING_MODE)
    .convert_to_radix(astro_float::Radix::Dec, ROUNDING_MODE, &mut consts)
    .unwrap();

if ex < 0 {
    ex = 0;
}

// Assume sign is always positive
let value_biguint = BigUint::from_radix_be(&radix_repr[..ex as usize], 10).unwrap();

[stencillogic]

round() with RoundingMode::None has no effect: docs.

The most efficient way in my opinion is to get raw parts of big float, and then construct big integer from the mantissa slice after accomplishing required bit shift.

[Janmajayamall]

Sorry for late reply!

Is this what you had in mind ?

// PRECISION = 256
// MANTISSA_LEN = 4
let value = value.0.clone();
let (raw, _, s, _, _) = value.as_raw_parts().unwrap();
let exponent = value.exponent().unwrap();

let mut bits = exponent;
let mut index = MATISSA_LEN - 1;
let mut res = BigInt::zero();
while bits > 0 {
    res += (BigInt::from_u64(raw[index as usize]).unwrap())
        << (astro_float::WORD_BIT_SIZE * index);
    bits -= astro_float::WORD_BIT_SIZE as i32;
    index -= 1;
}
res >>= (PRECISION as i32) - exponent;
res = if s.is_negative() { res.neg() } else { res }

[stencillogic]

Closing the issue as it is not a bug.