Decode a Posit-encoded number

posit.py

#!/usr/bin/env python3

# http://web.stanford.edu/class/ee380/Abstracts/170201-slides.pdf

from math import inf, log


NBITS = 8
ES = 1


def testbit(bits, pos):
    return bool(bits & 1 << pos)


def decode(bits):
    # zero is zero
    if bits == 0:
        return 0.0

    # decode inf
    if bits == 2 ** (NBITS - 1):
        return inf

    # decode sign
    if testbit(bits, NBITS - 1):
        sign = -1
        value = 2 ** NBITS - bits
    else:
        sign = 1
        value = bits

    # decode regime
    if testbit(value, NBITS - 2):
        for shift in range(NBITS - 1):
            if shift == NBITS - 2:
                break
            if not testbit(value, NBITS - 3 - shift):
                break
        regime = shift
    else:
        for shift in range(NBITS - 1):
            if shift == NBITS - 2:
                break
            if testbit(value, NBITS - 3 - shift):
                break
        regime = -(shift + 1)

    nfbits = NBITS - 3 - ES
    exponent_bitmask = 2 ** (NBITS - 3) - 1
    fraction_bitmask = 2 ** (NBITS - 3 - ES) - 1
    exponent = (value << shift & exponent_bitmask) >> nfbits
    fraction = value << shift & fraction_bitmask

    decoded = sign * (2 ** 2 ** ES) ** regime * 2 ** exponent * (1 + fraction / 2 ** nfbits)

    return decoded