Function int_to_decimal

Lib/_pylong.py:139–176

Asymptotically fast conversion of an 'int' to Decimal.

(n)

_unbounded_dec_context.traps[decimal.Inexact] = 1 # sanity check

def int_to_decimal(n):
    """Asymptotically fast conversion of an 'int' to Decimal."""

    # Function due to Tim Peters.  See GH issue #90716 for details.
    # https://github.com/python/cpython/issues/90716
    #
    # The implementation in longobject.c of base conversion algorithms
    # between power-of-2 and non-power-of-2 bases are quadratic time.
    # This function implements a divide-and-conquer algorithm that is
    # faster for large numbers.  Builds an equal decimal.Decimal in a
    # "clever" recursive way.  If we want a string representation, we
    # apply str to _that_.

    from decimal import Decimal as D
    BITLIM = 200

    # Don't bother caching the "lo" mask in this; the time to compute it is
    # tiny compared to the multiply.
    def inner(n, w):
        if w <= BITLIM:
            return D(n)
        w2 = w >> 1
        hi = n >> w2
        lo = n & ((1 << w2) - 1)
        return inner(lo, w2) + inner(hi, w - w2) * w2pow[w2]

    with decimal.localcontext(_unbounded_dec_context):
        nbits = n.bit_length()
        w2pow = compute_powers(nbits, D(2), BITLIM)
        if n < 0:
            negate = True
            n = -n
        else:
            negate = False
        result = inner(n, nbits)
        if negate:
            result = -result
    return result

def int_to_decimal_string(n):
    """Asymptotically fast conversion of an 'int' to a decimal string."""