91 lines
1.9 KiB
Python
91 lines
1.9 KiB
Python
import ctypes
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def is_i32(n):
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return -2**31 <= n < 2**31
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class OberonInt:
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'''
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Let's model the Oberon RISC integers,
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32-bit, two's complement.
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'''
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def __init__(self, initial=0):
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assert is_i32(initial)
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self.value = ctypes.c_int32(initial)
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assert self.value.value == initial
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def __add__(self, other):
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'''
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Return carry bit and new value.
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'''
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if not isinstance(other, OberonInt):
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other = OberonInt(other)
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n = self.value.value + other.value.value
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carry = not is_i32(n)
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if carry:
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n &= (2**31-1)
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return int(carry), OberonInt(n)
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__radd__ = __add__
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def negate(self):
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# Instead of binary ops, just cheat:
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return OberonInt(-self.value.value)
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def __sub__(self, other):
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if not isinstance(other, OberonInt):
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other = OberonInt(other)
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return self + other.negate()
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__rsub__ = __sub__
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def __repr__(self):
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#b = bin(self.value.value & (2**32-1))
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return f'OberonInt({self.value.value})'
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def __eq__(self, other):
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assert isinstance(other, OberonInt)
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return self.value.value == other.value.value
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obmin, zero, one, obmax = map(OberonInt, (
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-(2**31),
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0,
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1,
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2**31-1,
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))
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# Addition
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carry, z = obmax + one
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assert carry
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assert z == zero
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# Negation
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negative_one = one.negate()
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carry, m = obmin + negative_one
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assert carry
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assert m == obmax
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# Ergo, subtraction.
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carry, m = obmin - one
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assert carry
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assert m == obmax
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carry, hmm = obmax + obmax
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assert carry
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assert hmm.value.value == 2**31 - 2
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carry, eh = obmax - hmm
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assert not carry
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assert eh == one
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assert (hmm + one)[1] == obmax
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## if initial >= 2**31:
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## raise ValueError(f'too big: {initial!r}')
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## if initial < -2**31:
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## raise ValueError(f'too small: {initial!r}')
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