277 lines
10 KiB
Python
Executable File
277 lines
10 KiB
Python
Executable File
#!/usr/bin/python
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#
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# Native python routines for Galois field calculations.
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#
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# These calculations can be done much faster in C/C++ (and with vectorization), but
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# this code helps illustrate how Galois field math works. Also, it's helpful if
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# you want to do small amounts of calculation without working with native C/C++
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# code.
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#
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import random
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class GaloisException(Exception):
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def __init__ (self, value):
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self.value = value
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def __str__ (self):
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return repr(self.value)
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class GaloisNumber:
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'''
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Class to represent a number in a Galois (finite) field.
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The class supports "normal" syntax for the addition, subtraction, multiplication,
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division, additive inverse (-), and multiplicative inverse (~) operations.
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'''
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def __init__ (self, x, value=0):
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if isinstance(x, GaloisNumber):
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self.field = x.field
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self.value = x.value
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else:
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self.field = x
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self.assign (value)
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def assign (self, v):
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'''
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Assign a new integer value to this Galois field number. The number must be valid in
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the field with which the GaloisNumber instance was defined.
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'''
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if v > self.field.size:
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raise GaloisException ("Value {0} is outside field".format (v))
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self.value = v
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def __add__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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return GaloisNumber (self.field, self.value ^ other.value)
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def __iadd__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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self.value ^= other.value
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def __sub__ (self, other):
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return self + other
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def __isub__ (self, other):
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self += other
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def __invert__ (self):
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return self.field.invert (self)
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def __neg__ (self):
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return GaloisNumber (self)
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def __mul__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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return self.field.multiply (self, other)
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def __imul__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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self.value = self.field.direct_multiply (self.value, other.value)
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def __div__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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return self.field.divide (self, other)
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def __eq__ (self, other):
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if self.field != other.field:
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raise GaloisException ("Field elements from different fields")
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return self.value == other.value
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def __repr__ (self):
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return self.field.fmt (self.value)
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class GaloisFieldLog:
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'''
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Pure python implementation of Galois (finite) field arithmetic routines using log/antilog
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tables.
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There only needs to be one instantiation of the field for a given set of parameters,
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but elements from different field instances with the same parameters may be mixed.
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'''
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field_widths = (4, 8, 12, 16)
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poly_defaults = {4: 0x13, 8: 0x11d, 12:0x1053, 16: 0x1100b}
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multiply_test_size = 10000
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def __init__ (self, bits, primitive_polynomial = None, repr_prefix = 'G', alpha = 1):
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'''
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Create a Galois field using log/antilog tables for arithmetic.
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'''
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if bits not in self.field_widths:
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raise GaloisException ("Field widths supported: {0}".format (self.field_widths))
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self.bits = bits
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self.size = (1 << bits)
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self.prim = self.poly_defaults[bits] if not primitive_polynomial else primitive_polynomial
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self.value_format = repr_prefix + '{:0>' + str(bits / 4) + 'x}'
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self.alpha = alpha
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# Set up the log and anti-log tables
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self.log_tbl = [0] * self.size
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self.antilog_tbl = [0] * (self.size - 1)
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b = 1
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for i in range (self.size - 1):
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self.log_tbl[b] = i
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self.antilog_tbl[i] = b
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b <<= 1
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if b >= self.size:
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b ^= self.prim
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def __eq__ (self, other):
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return self.bits == other.bits and self.prim == other.prim and self.alpha == other.alpha
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def fmt (self, v):
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return self.value_format.format (v)
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def multiply (self, v1, v2):
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a = v1.value
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b = v2.value
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if a == 0 or b == 0:
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return GaloisNumber (self, 0)
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return GaloisNumber (self, self.antilog_tbl[(self.log_tbl[a] + self.log_tbl[b]) %
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(self.size - 1)])
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def invert (self, v):
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if v.value == 0:
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return GaloisNumber(self, 0)
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elif v.value == 1:
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return GaloisNumber (self, 1)
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else:
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return GaloisNumber (self, self.antilog_tbl[self.size - 1 - self.log_tbl[v.value]])
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def divide (self, v1, v2):
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return self.multiply (v1, self.invert(v2))
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def self_test (self):
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mul_identity = GaloisNumber (self, 1)
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v = GaloisNumber (self)
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g_0 = GaloisNumber (self, 0)
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g_1 = GaloisNumber (self, 1)
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for i in range (self.size):
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v.assign (i)
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if i == 0: continue
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assert v * ~v == mul_identity, "Multiplicative inverse failed at {}".format (i)
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assert g_0 - v == -v, "Additive inverse failed at {}".format (i)
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assert v * g_1 == v, "Multiplicative identity failed at {}".format (i)
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vb = GaloisNumber (self)
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for a in range (1, self.multiply_test_size):
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v.assign (random.randint (1, self.size - 1))
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vb.assign (random.randint (1, self.size - 1))
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product = v * vb
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assert product / v == vb, "Multiplication failed for {} * {}".format(v.value,vb.value)
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assert product / vb == v, "Multiplication failed for {} * {}".format(v.value,vb.value)
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return True
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class GaloisFieldDirect:
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'''
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Pure python implementation of Galois (finite) field arithmetic routines using direct
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arithmetic (no log tables).
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There only needs to be one instantiation of the field for a given set of parameters,
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but elements from different field instances with the same parameters may be mixed.
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'''
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field_widths = (4, 8, 12, 16, 32)
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poly_defaults = {4: 0x13, 8: 0x11d, 12:0x1053, 16: 0x1100b, 32: 0x1000000c5}
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max_test_size = 5000
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def __init__ (self, bits, primitive_polynomial = None, repr_prefix = 'G', alpha = 1):
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'''
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Create a Galois field using direct arithmetic. No log tables or inverses to
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precalculate, since the field might be too large to store them
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'''
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if bits not in self.field_widths:
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raise GaloisException ("Field widths supported: {0}".format (self.field_widths))
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self.bits = bits
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self.size = (1 << bits)
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self.prim = self.poly_defaults[bits] if not primitive_polynomial else primitive_polynomial
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self.value_format = repr_prefix + '{:0>' + str(bits / 4) + 'x}'
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self.alpha = alpha
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def __eq__ (self, other):
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return self.bits == other.bits and self.prim == other.prim and self.alpha == other.alpha
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def fmt (self, v):
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return self.value_format.format (v)
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def multiply (self, v1, v2):
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return GaloisNumber (self, self.direct_multiply (v1.value, v2.value))
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def direct_multiply (self, a, b):
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# Multiplication is commutative, and it's faster if we use the smaller value as the
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# multiplier since we can exit the while loop sooner.
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if b > a:
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a, b = b, a
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if a == 0:
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result = 0
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else:
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result = a if b & 1 else 0
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tmp = a
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b >>= 1
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while b != 0:
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a <<= 1
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if a >= self.size:
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a ^= self.prim
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if b & 1:
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result ^= a
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b >>= 1
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return result
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def invert (self, v):
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'''
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Calculate inverse(v) by computing v^(field_size-2).
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This is just v^2 * v^4 ... v^(field_size / 2), so calculation time is proportional
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to field width in bits.
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'''
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if v.value == 0:
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return GaloisNumber(self, 0)
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elif v.value == 1:
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return GaloisNumber (self, 1)
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inv = 1
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sq = v.value
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for i in range (1, self.bits):
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sq = self.direct_multiply (sq, sq)
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inv = self.direct_multiply (inv, sq)
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return GaloisNumber (self, inv)
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def divide (self, v1, v2):
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return self.multiply (v1, self.invert(v2))
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def self_test (self):
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mul_identity = GaloisNumber (self, 1)
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v = GaloisNumber (self)
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g_0 = GaloisNumber (self, 0)
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g_1 = GaloisNumber (self, 1)
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small_field = self.size < self.max_test_size
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n_tests = (self.size - 1) if small_field else self.max_test_size
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for i in range (0, n_tests):
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v.assign ((i if small_field else random.randint (0, self.size - 2)) + 1)
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assert v * ~v == mul_identity, "Multiplicative inverse failed at {}".format (i)
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assert g_0 - v == -v, "Additive inverse failed at {}".format (i)
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assert v * g_1 == v, "Multiplicative identity failed at {}".format (i)
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vb = GaloisNumber (self)
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for a in range (1, self.max_test_size):
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v.assign (random.randint (1, self.size - 1))
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vb.assign (random.randint (1, self.size - 1))
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product = v * vb
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assert product / v == vb, "Multiplication failed for {} * {}".format(v.value,vb.value)
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assert product / vb == v, "Multiplication failed for {} * {}".format(v.value,vb.value)
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return True
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if __name__ == '__main__':
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print '\nTesting direct fields...........'
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for width in GaloisFieldDirect.field_widths:
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field = GaloisFieldDirect (width)
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g0 = GaloisNumber (field, 2)
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g1 = GaloisNumber (field, 7)
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print '{0} + {1} = {2}'.format (g0, g1, g0 + g1)
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if field.self_test ():
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print "{0} bit field (direct) passed!".format (width)
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print '\nTesting log fields...........'
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for width in GaloisFieldLog.field_widths:
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field = GaloisFieldLog (width)
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g0 = GaloisNumber (field, 2)
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g1 = GaloisNumber (field, 7)
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print '{0} + {1} = {2}'.format (g0, g1, g0 + g1)
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if field.self_test ():
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print "{0} bit field (log) passed!".format (width)
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