f.py

#!/usr/bin/env python

'''
utils to pack and unpack single-precision floats in IEEE 754

I wrote this to get used to bit twiddling with IEEE

this is free and unencumbered software released into the public domain
'''

import sys
import struct
import collections


def tof(x):
  ''' reinterpret integer as float '''
  b = struct.pack('I', x)
  return struct.unpack('f', b)[0]


def toi(f):
  ''' reinterpret float as integer '''
  b = struct.pack('f', f)
  return struct.unpack('I', b)[0]


def encode(sign, exp, mantissa):
  ''' encode sign, exp, mantissa as a float '''
  sign = not not int(sign, 0)
  exp = int(exp, 0) + 127
  mantissa = int(mantissa, 0)
  return tof(
    ((sign     << 31) & 0x80000000) |
    ((exp      << 23) & 0x7F800000) |
    ((mantissa <<  0) & 0x007FFFFF)
  )


FloatComponents = (
  collections.namedtuple( "FloatComponents", ["sign", "exp", "mantissa"])
)


def decode(f):
  ''' decode a float into sign, exp, mantissa '''
  x = toi(f)
  sign = (x >> 31) & 1
  exp = ((x >> 23) & 0xFF) - 127
  mantissa = x & 0x007FFFFF
  return FloatComponents(int(sign), int(exp), int(mantissa))


def convert(x):
  ''' detect type of x and use toi/tof accordingly '''
  if isinstance(x, str):
    if x.startswith("0x"):
      x = int(x, 0)
    else:
      x = float(x)
  if not isinstance(x, float):
    f = tof(x)
    return (f, decode(f))
  return (toi(x), decode(x))


COMPFMT = " | sign=%d, exp=%d, mantissa=%08X"


def swap(x):
  ''' detect type of x, use toi/tof accordingly and format to string '''
  y, components = convert(x)
  if isinstance(y, int) or isinstance(y, long):
    return str(x) + (" -> 0x%08X" % int(y)) + COMPFMT % components
  return ("0x%08X -> " % int(x, 0)) + str(y) + COMPFMT % components


if __name__ == '__main__':
  if len(sys.argv) == 2:
    print(swap(sys.argv[1]))
  elif len(sys.argv) == 4:
    print(encode(*sys.argv[1:]))
  else:
    sys.stderr.write('''usage: %s number

converts between hex and single-precision float
number can be
- float: 123.123
- hex integer: 0xAABBCCDD
- sign exp mantissa: 0 2 0x00776655\n
''' % sys.argv[0])
    sys.exit(1)