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fractions.py
# Originally contributed by Sjoerd Mullender. # Significantly modified by Jeffrey Yasskin <jyasskin at gmail.com>. """Fraction, infinite-precision, rational numbers.""" from decimal import Decimal import math import numbers import operator import re import sys __all__ = ['Fraction'] # Constants related to the hash implementation; hash(x) is based # on the reduction of x modulo the prime _PyHASH_MODULUS. _PyHASH_MODULUS = sys.hash_info.modulus # Value to be used for rationals that reduce to infinity modulo # _PyHASH_MODULUS. _PyHASH_INF = sys.hash_info.inf _RATIONAL_FORMAT = re.compile(r""" \A\s* # optional whitespace at the start, (?P<sign>[-+]?) # an optional sign, then (?=\d|\.\d) # lookahead for digit or .digit (?P<num>\d*|\d+(_\d+)*) # numerator (possibly empty) (?: # followed by (?:/(?P<denom>\d+(_\d+)*))? # an optional denominator | # or (?:\.(?P<decimal>\d*|\d+(_\d+)*))? # an optional fractional part (?:E(?P<exp>[-+]?\d+(_\d+)*))? # and optional exponent ) \s*\Z # and optional whitespace to finish """, re.VERBOSE | re.IGNORECASE) class Fraction(numbers.Rational): """This class implements rational numbers. In the two-argument form of the constructor, Fraction(8, 6) will produce a rational number equivalent to 4/3. Both arguments must be Rational. The numerator defaults to 0 and the denominator defaults to 1 so that Fraction(3) == 3 and Fraction() == 0. Fractions can also be constructed from: - numeric strings similar to those accepted by the float constructor (for example, '-2.3' or '1e10') - strings of the form '123/456' - float and Decimal instances - other Rational instances (including integers) """ __slots__ = ('_numerator', '_denominator') # We're immutable, so use __new__ not __init__ def __new__(cls, numerator=0, denominator=None, *, _normalize=True): """Constructs a Rational. Takes a string like '3/2' or '1.5', another Rational instance, a numerator/denominator pair, or a float. Examples -------- >>> Fraction(10, -8) Fraction(-5, 4) >>> Fraction(Fraction(1, 7), 5) Fraction(1, 35) >>> Fraction(Fraction(1, 7), Fraction(2, 3)) Fraction(3, 14) >>> Fraction('314') Fraction(314, 1) >>> Fraction('-35/4') Fraction(-35, 4) >>> Fraction('3.1415') # conversion from numeric string Fraction(6283, 2000) >>> Fraction('-47e-2') # string may include a decimal exponent Fraction(-47, 100) >>> Fraction(1.47) # direct construction from float (exact conversion) Fraction(6620291452234629, 4503599627370496) >>> Fraction(2.25) Fraction(9, 4) >>> Fraction(Decimal('1.47')) Fraction(147, 100) """ self = super(Fraction, cls).__new__(cls) if denominator is None: if type(numerator) is int: self._numerator = numerator self._denominator = 1 return self elif isinstance(numerator, numbers.Rational): self._numerator = numerator.numerator self._denominator = numerator.denominator return self elif isinstance(numerator, (float, Decimal)): # Exact conversion self._numerator, self._denominator = numerator.as_integer_ratio() return self elif isinstance(numerator, str): # Handle construction from strings. m = _RATIONAL_FORMAT.match(numerator) if m is None: raise ValueError('Invalid literal for Fraction: %r' % numerator) numerator = int(m.group('num') or '0') denom = m.group('denom') if denom: denominator = int(denom) else: denominator = 1 decimal = m.group('decimal') if decimal: decimal = decimal.replace('_', '') scale = 10**len(decimal) numerator = numerator * scale + int(decimal) denominator *= scale exp = m.group('exp') if exp: exp = int(exp) if exp >= 0: numerator *= 10**exp else: denominator *= 10**-exp if m.group('sign') == '-': numerator = -numerator else: raise TypeError("argument should be a string " "or a Rational instance") elif type(numerator) is int is type(denominator): pass # *very* normal case elif (isinstance(numerator, numbers.Rational) and isinstance(denominator, numbers.Rational)): numerator, denominator = ( numerator.numerator * denominator.denominator, denominator.numerator * numerator.denominator ) else: raise TypeError("both arguments should be " "Rational instances") if denominator == 0: raise ZeroDivisionError('Fraction(%s, 0)' % numerator) if _normalize: g = math.gcd(numerator, denominator) if denominator < 0: g = -g numerator //= g denominator //= g self._numerator = numerator self._denominator = denominator return self @classmethod def from_float(cls, f): """Converts a finite float to a rational number, exactly. Beware that Fraction.from_float(0.3) != Fraction(3, 10). """ if isinstance(f, numbers.Integral): return cls(f) elif not isinstance(f, float): raise TypeError("%s.from_float() only takes floats, not %r (%s)" % (cls.__name__, f, type(f).__name__)) return cls(*f.as_integer_ratio()) @classmethod def from_decimal(cls, dec): """Converts a finite Decimal instance to a rational number, exactly.""" from decimal import Decimal if isinstance(dec, numbers.Integral): dec = Decimal(int(dec)) elif not isinstance(dec, Decimal): raise TypeError( "%s.from_decimal() only takes Decimals, not %r (%s)" % (cls.__name__, dec, type(dec).__name__)) return cls(*dec.as_integer_ratio()) def as_integer_ratio(self): """Return the integer ratio as a tuple. Return a tuple of two integers, whose ratio is equal to the Fraction and with a positive denominator. """ return (self._numerator, self._denominator) def limit_denominator(self, max_denominator=1000000): """Closest Fraction to self with denominator at most max_denominator. >>> Fraction('3.141592653589793').limit_denominator(10) Fraction(22, 7) >>> Fraction('3.141592653589793').limit_denominator(100) Fraction(311, 99) >>> Fraction(4321, 8765).limit_denominator(10000) Fraction(4321, 8765) """ # Algorithm notes: For any real number x, define a *best upper # approximation* to x to be a rational number p/q such that: # # (1) p/q >= x, and # (2) if p/q > r/s >= x then s > q, for any rational r/s. # # Define *best lower approximation* similarly. Then it can be # proved that a rational number is a best upper or lower # approximation to x if, and only if, it is a convergent or # semiconvergent of the (unique shortest) continued fraction # associated to x. # # To find a best rational approximation with denominator <= M, # we find the best upper and lower approximations with # denominator <= M and take whichever of these is closer to x. # In the event of a tie, the bound with smaller denominator is # chosen. If both denominators are equal (which can happen # only when max_denominator == 1 and self is midway between # two integers) the lower bound---i.e., the floor of self, is # taken. if max_denominator < 1: raise ValueError("max_denominator should be at least 1") if self._denominator <= max_denominator: return Fraction(self) p0, q0, p1, q1 = 0, 1, 1, 0 n, d = self._numerator, self._denominator while True: a = n//d q2 = q0+a*q1 if q2 > max_denominator: break p0, q0, p1, q1 = p1, q1, p0+a*p1, q2 n, d = d, n-a*d k = (max_denominator-q0)//q1 bound1 = Fraction(p0+k*p1, q0+k*q1) bound2 = Fraction(p1, q1) if abs(bound2 - self) <= abs(bound1-self): return bound2 else: return bound1 @property def numerator(a): return a._numerator @property def denominator(a): return a._denominator def __repr__(self): """repr(self)""" return '%s(%s, %s)' % (self.__class__.__name__, self._numerator, self._denominator) def __str__(self): """str(self)""" if self._denominator == 1: return str(self._numerator) else: return '%s/%s' % (self._numerator, self._denominator) def _operator_fallbacks(monomorphic_operator, fallback_operator): """Generates forward and reverse operators given a purely-rational operator and a function from the operator module. Use this like: __op__, __rop__ = _operator_fallbacks(just_rational_op, operator.op) In general, we want to implement the arithmetic operations so that mixed-mode operations either call an implementation whose author knew about the types of both arguments, or convert both to the nearest built in type and do the operation there. In Fraction, that means that we define __add__ and __radd__ as: def __add__(self, other): # Both types have numerators/denominator attributes, # so do the operation directly if isinstance(other, (int, Fraction)): return Fraction(self.numerator * other.denominator + other.numerator * self.denominator, self.denominator * other.denominator) # float and complex don't have those operations, but we # know about those types, so special case them. elif isinstance(other, float): return float(self) + other elif isinstance(other, complex): return complex(self) + other # Let the other type take over. return NotImplemented def __radd__(self, other): # radd handles more types than add because there's # nothing left to fall back to. if isinstance(other, numbers.Rational): return Fraction(self.numerator * other.denominator + other.numerator * self.denominator, self.denominator * other.denominator) elif isinstance(other, Real): return float(other) + float(self) elif isinstance(other, Complex): return complex(other) + complex(self) return NotImplemented There are 5 different cases for a mixed-type addition on Fraction. I'll refer to all of the above code that doesn't refer to Fraction, float, or complex as "boilerplate". 'r' will be an instance of Fraction, which is a subtype of Rational (r : Fraction <: Rational), and b : B <: Complex. The first three involve 'r + b': 1. If B <: Fraction, int, float, or complex, we handle that specially, and all is well. 2. If Fraction falls back to the boilerplate code, and it were to return a value from __add__, we'd miss the possibility that B defines a more intelligent __radd__, so the boilerplate should return NotImplemented from __add__. In particular, we don't handle Rational here, even though we could get an exact answer, in case the other type wants to do something special. 3. If B <: Fraction, Python tries B.__radd__ before Fraction.__add__. This is ok, because it was implemented with knowledge of Fraction, so it can handle those instances before delegating to Real or Complex. The next two situations describe 'b + r'. We assume that b didn't know about Fraction in its implementation, and that it uses similar boilerplate code: 4. If B <: Rational, then __radd_ converts both to the builtin rational type (hey look, that's us) and proceeds. 5. Otherwise, __radd__ tries to find the nearest common base ABC, and fall back to its builtin type. Since this class doesn't subclass a concrete type, there's no implementation to fall back to, so we need to try as hard as possible to return an actual value, or the user will get a TypeError. """ def forward(a, b): if isinstance(b, (int, Fraction)): return monomorphic_operator(a, b) elif isinstance(b, float): return fallback_operator(float(a), b) elif isinstance(b, complex): return fallback_operator(complex(a), b) else: return NotImplemented forward.__name__ = '__' + fallback_operator.__name__ + '__' forward.__doc__ = monomorphic_operator.__doc__ def reverse(b, a): if isinstance(a, numbers.Rational): # Includes ints. return monomorphic_operator(a, b) elif isinstance(a, numbers.Real): return fallback_operator(float(a), float(b)) elif isinstance(a, numbers.Complex): return fallback_operator(complex(a), complex(b)) else: return NotImplemented reverse.__name__ = '__r' + fallback_operator.__name__ + '__' reverse.__doc__ = monomorphic_operator.__doc__ return forward, reverse # Rational arithmetic algorithms: Knuth, TAOCP, Volume 2, 4.5.1. # # Assume input fractions a and b are normalized. # # 1) Consider addition/subtraction. # # Let g = gcd(da, db). Then # # na nb na*db ± nb*da # a ± b == -- ± -- == ------------- == # da db da*db # # na*(db//g) ± nb*(da//g) t # == ----------------------- == - # (da*db)//g d # # Now, if g > 1, we're working with smaller integers. # # Note, that t, (da//g) and (db//g) are pairwise coprime. # # Indeed, (da//g) and (db//g) share no common factors (they were # removed) and da is coprime with na (since input fractions are # normalized), hence (da//g) and na are coprime. By symmetry, # (db//g) and nb are coprime too. Then, # # gcd(t, da//g) == gcd(na*(db//g), da//g) == 1 # gcd(t, db//g) == gcd(nb*(da//g), db//g) == 1 # # Above allows us optimize reduction of the result to lowest # terms. Indeed, # # g2 = gcd(t, d) == gcd(t, (da//g)*(db//g)*g) == gcd(t, g) # # t//g2 t//g2 # a ± b == ----------------------- == ---------------- # (da//g)*(db//g)*(g//g2) (da//g)*(db//g2) # # is a normalized fraction. This is useful because the unnormalized # denominator d could be much larger than g. # # We should special-case g == 1 (and g2 == 1), since 60.8% of # randomly-chosen integers are coprime: # https://en.wikipedia.org/wiki/Coprime_integers#Probability_of_coprimality # Note, that g2 == 1 always for fractions, obtained from floats: here # g is a power of 2 and the unnormalized numerator t is an odd integer. # # 2) Consider multiplication # # Let g1 = gcd(na, db) and g2 = gcd(nb, da), then # # na*nb na*nb (na//g1)*(nb//g2) # a*b == ----- == ----- == ----------------- # da*db db*da (db//g1)*(da//g2) # # Note, that after divisions we're multiplying smaller integers. # # Also, the resulting fraction is normalized, because each of # two factors in the numerator is coprime to each of the two factors # in the denominator. # # Indeed, pick (na//g1). It's coprime with (da//g2), because input # fractions are normalized. It's also coprime with (db//g1), because # common factors are removed by g1 == gcd(na, db). # # As for addition/subtraction, we should special-case g1 == 1 # and g2 == 1 for same reason. That happens also for multiplying # rationals, obtained from floats. def _add(a, b): """a + b""" na, da = a.numerator, a.denominator nb, db = b.numerator, b.denominator g = math.gcd(da, db) if g == 1: return Fraction(na * db + da * nb, da * db, _normalize=False) s = da // g t = na * (db // g) + nb * s g2 = math.gcd(t, g) if g2 == 1: return Fraction(t, s * db, _normalize=False) return Fraction(t // g2, s * (db // g2), _normalize=False) __add__, __radd__ = _operator_fallbacks(_add, operator.add) def _sub(a, b): """a - b""" na, da = a.numerator, a.denominator nb, db = b.numerator, b.denominator g = math.gcd(da, db) if g == 1: return Fraction(na * db - da * nb, da * db, _normalize=False) s = da // g t = na * (db // g) - nb * s g2 = math.gcd(t, g) if g2 == 1: return Fraction(t, s * db, _normalize=False) return Fraction(t // g2, s * (db // g2), _normalize=False) __sub__, __rsub__ = _operator_fallbacks(_sub, operator.sub) def _mul(a, b): """a * b""" na, da = a.numerator, a.denominator nb, db = b.numerator, b.denominator g1 = math.gcd(na, db) if g1 > 1: na //= g1 db //= g1 g2 = math.gcd(nb, da) if g2 > 1: nb //= g2 da //= g2 return Fraction(na * nb, db * da, _normalize=False) __mul__, __rmul__ = _operator_fallbacks(_mul, operator.mul) def _div(a, b): """a / b""" # Same as _mul(), with inversed b. na, da = a.numerator, a.denominator nb, db = b.numerator, b.denominator g1 = math.gcd(na, nb) if g1 > 1: na //= g1 nb //= g1 g2 = math.gcd(db, da) if g2 > 1: da //= g2 db //= g2 n, d = na * db, nb * da if d < 0: n, d = -n, -d return Fraction(n, d, _normalize=False) __truediv__, __rtruediv__ = _operator_fallbacks(_div, operator.truediv) def _floordiv(a, b): """a // b""" return (a.numerator * b.denominator) // (a.denominator * b.numerator) __floordiv__, __rfloordiv__ = _operator_fallbacks(_floordiv, operator.floordiv) def _divmod(a, b): """(a // b, a % b)""" da, db = a.denominator, b.denominator div, n_mod = divmod(a.numerator * db, da * b.numerator) return div, Fraction(n_mod, da * db) __divmod__, __rdivmod__ = _operator_fallbacks(_divmod, divmod) def _mod(a, b): """a % b""" da, db = a.denominator, b.denominator return Fraction((a.numerator * db) % (b.numerator * da), da * db) __mod__, __rmod__ = _operator_fallbacks(_mod, operator.mod) def __pow__(a, b): """a ** b If b is not an integer, the result will be a float or complex since roots are generally irrational. If b is an integer, the result will be rational. """ if isinstance(b, numbers.Rational): if b.denominator == 1: power = b.numerator if power >= 0: return Fraction(a._numerator ** power, a._denominator ** power, _normalize=False) elif a._numerator >= 0: return Fraction(a._denominator ** -power, a._numerator ** -power, _normalize=False) else: return Fraction((-a._denominator) ** -power, (-a._numerator) ** -power, _normalize=False) else: # A fractional power will generally produce an # irrational number. return float(a) ** float(b) else: return float(a) ** b def __rpow__(b, a): """a ** b""" if b._denominator == 1 and b._numerator >= 0: # If a is an int, keep it that way if possible. return a ** b._numerator if isinstance(a, numbers.Rational): return Fraction(a.numerator, a.denominator) ** b if b._denominator == 1: return a ** b._numerator return a ** float(b) def __pos__(a): """+a: Coerces a subclass instance to Fraction""" return Fraction(a._numerator, a._denominator, _normalize=False) def __neg__(a): """-a""" return Fraction(-a._numerator, a._denominator, _normalize=False) def __abs__(a): """abs(a)""" return Fraction(abs(a._numerator), a._denominator, _normalize=False) def __int__(a, _index=operator.index): """int(a)""" if a._numerator < 0: return _index(-(-a._numerator // a._denominator)) else: return _index(a._numerator // a._denominator) def __trunc__(a): """math.trunc(a)""" if a._numerator < 0: return -(-a._numerator // a._denominator) else: return a._numerator // a._denominator def __floor__(a): """math.floor(a)""" return a.numerator // a.denominator def __ceil__(a): """math.ceil(a)""" # The negations cleverly convince floordiv to return the ceiling. return -(-a.numerator // a.denominator) def __round__(self, ndigits=None): """round(self, ndigits) Rounds half toward even. """ if ndigits is None: floor, remainder = divmod(self.numerator, self.denominator) if remainder * 2 < self.denominator: return floor elif remainder * 2 > self.denominator: return floor + 1 # Deal with the half case: elif floor % 2 == 0: return floor else: return floor + 1 shift = 10**abs(ndigits) # See _operator_fallbacks.forward to check that the results of # these operations will always be Fraction and therefore have # round(). if ndigits > 0: return Fraction(round(self * shift), shift) else: return Fraction(round(self / shift) * shift) def __hash__(self): """hash(self)""" # To make sure that the hash of a Fraction agrees with the hash # of a numerically equal integer, float or Decimal instance, we # follow the rules for numeric hashes outlined in the # documentation. (See library docs, 'Built-in Types'). try: dinv = pow(self._denominator, -1, _PyHASH_MODULUS) except ValueError: # ValueError means there is no modular inverse. hash_ = _PyHASH_INF else: # The general algorithm now specifies that the absolute value of # the hash is # (|N| * dinv) % P # where N is self._numerator and P is _PyHASH_MODULUS. That's # optimized here in two ways: first, for a non-negative int i, # hash(i) == i % P, but the int hash implementation doesn't need # to divide, and is faster than doing % P explicitly. So we do # hash(|N| * dinv) # instead. Second, N is unbounded, so its product with dinv may # be arbitrarily expensive to compute. The final answer is the # same if we use the bounded |N| % P instead, which can again # be done with an int hash() call. If 0 <= i < P, hash(i) == i, # so this nested hash() call wastes a bit of time making a # redundant copy when |N| < P, but can save an arbitrarily large # amount of computation for large |N|. hash_ = hash(hash(abs(self._numerator)) * dinv) result = hash_ if self._numerator >= 0 else -hash_ return -2 if result == -1 else result def __eq__(a, b): """a == b""" if type(b) is int: return a._numerator == b and a._denominator == 1 if isinstance(b, numbers.Rational): return (a._numerator == b.numerator and a._denominator == b.denominator) if isinstance(b, numbers.Complex) and b.imag == 0: b = b.real if isinstance(b, float): if math.isnan(b) or math.isinf(b): # comparisons with an infinity or nan should behave in # the same way for any finite a, so treat a as zero. return 0.0 == b else: return a == a.from_float(b) else: # Since a doesn't know how to compare with b, let's give b # a chance to compare itself with a. return NotImplemented def _richcmp(self, other, op): """Helper for comparison operators, for internal use only. Implement comparison between a Rational instance `self`, and either another Rational instance or a float `other`. If `other` is not a Rational instance or a float, return NotImplemented. `op` should be one of the six standard comparison operators. """ # convert other to a Rational instance where reasonable. if isinstance(other, numbers.Rational): return op(self._numerator * other.denominator, self._denominator * other.numerator) if isinstance(other, float): if math.isnan(other) or math.isinf(other): return op(0.0, other) else: return op(self, self.from_float(other)) else: return NotImplemented def __lt__(a, b): """a < b""" return a._richcmp(b, operator.lt) def __gt__(a, b): """a > b""" return a._richcmp(b, operator.gt) def __le__(a, b): """a <= b""" return a._richcmp(b, operator.le) def __ge__(a, b): """a >= b""" return a._richcmp(b, operator.ge) def __bool__(a): """a != 0""" # bpo-39274: Use bool() because (a._numerator != 0) can return an # object which is not a bool. return bool(a._numerator) # support for pickling, copy, and deepcopy def __reduce__(self): return (self.__class__, (self._numerator, self._denominator)) def __copy__(self): if type(self) == Fraction: return self # I'm immutable; therefore I am my own clone return self.__class__(self._numerator, self._denominator) def __deepcopy__(self, memo): if type(self) == Fraction: return self # My components are also immutable return self.__class__(self._numerator, self._denominator)
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