FizzBuzz Constant-Time Python

#!/usr/bin/env python
import math
import argparse
import sys

parser = argparse.ArgumentParser(description="Generate a fizzbuzz print function")
parser.add_argument('--file', metavar='filename', nargs='?', type=argparse.FileType(mode="w"),
                     help='The file to write to', default = "autogen.c")
args = vars(parser.parse_args())

number_bits = 27
int_bits = 27

# characters in the integer is given by the base 10 log of the maximum number
# but since we're two's complement, we subtract 1 power of two
int_chars = int(math.ceil(math.log(2**(int_bits-1),10)))
frac_chars = 0

int_loc = 0
length = int_chars

########

bits = [2**i for i in range(0, int_bits)]

# Ensure the patterns are up to snuff
fmt = "%%0%d.%df"%(length, frac_chars)
dec_patterns = [fmt%(b) for b in bits]

#generate the list of contributory things
fraction_patterns = [s[int_chars:] for s in dec_patterns]
int_patterns = [s[:int_chars] for s in dec_patterns]

# make a dictionary of dictionaries for fraction patterns.
# Top level key: character in fraction [0, frac_chars)
# Second level key: contributory bit
# Second level value: resulting decimal digit
ints =  {i: {bit: int_patterns[bit][i]      for bit in range(len(int_patterns))}      for i in range(int_chars)}

def int_poly(n):
    patterns = [(k,v) for k, v in ints[n].iteritems()]
    terms = ["(%c * bit%d)"%(p, bit) for bit, p in patterns if p != "0"]
    return " + ".join(terms)

with args["file"] as f:
  f.write(
"""
#include <stdint.h>

/*********
 * This file is autogenerated by generate_fizzbuzz.py.
 * Please don't modify it manually.
 *********/

void fizzbuzz(char* buffer, int f) {
  uint8_t isfizz = (f % 3) == 0;
  uint8_t isbuzz = (f % 5) == 0;
  uint8_t isfizzonly = isfizz & (!isbuzz);
  uint8_t isbuzzonly = isbuzz & (!isfizz);
  uint8_t isfizzbuzz = isfizz | isbuzz;

  uint32_t carry = 0;
  uint32_t scratch = 0;

""")

  f.write("\n".join(["  uint8_t bit%d = (((f) >> (%d))&1);"%(i,i) for i in range(0,number_bits)]))
  f.write("\n")

  extra_polynomials = {
          0: " + (isfizz * %d) + (isbuzzonly * %d)"%(ord('F'), ord(' ')),
          1: " + (isfizz * %d) + (isbuzzonly * %d)"%(ord('i'), ord(' ')),
          2: " + (isfizz * %d) + (isbuzzonly * %d)"%(ord('z'), ord(' ')),
          3: " + (isfizz * %d) + (isbuzzonly * %d)"%(ord('z'), ord(' ')),
          4: " + (isfizzonly * %d) + (isbuzz * %d)"%(ord(' '), ord('B')),
          5: " + (isfizzonly * %d) + (isbuzz * %d)"%(ord(' '), ord('u')),
          6: " + (isfizzonly * %d) + (isbuzz * %d)"%(ord(' '), ord('z')),
          7: " + (isfizzonly * %d) + (isbuzz * %d)"%(ord(' '), ord('z')),
          }
  def get_extra_poly(n):
      return extra_polynomials.get(n, "+ (isfizzbuzz * %d)"%(ord(' ')))

  for position in reversed(range(int_chars)):
      poly = int_poly(position)
      f.write("  scratch = %s + carry;\n"%(poly))
      f.write("  buffer[%d] = ((%d + (scratch %% 10)) * (1-(isfizz | isbuzz))) %s;\n"%(int_loc + position, ord('0'),
          get_extra_poly(int_loc+position)))
      f.write("carry = scratch / 10;\n")

  f.write("  buffer[%d] = '\\0';"%(length))

  f.write("""
}""")