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# only a small file, but there's some hairy stuff in here!
"""
>>> f = Field('test', 16, 31)
>>> f
<Field 'test'>
>>> f.encode(65535)
65535
>>> f.encode(65536)
Traceback (most recent call last):
File \"<stdin>\", line 1, in ?
File \"field.py\", line 25, in encode
raise ValueError(\"field '%s' can't accept value %s\"
ValueError: field 'test' can't accept value 65536
>>>
"""
class Field(object):
def __init__(self, name, left, right, signedness=False, valclass=int, overlap=False):
self.name = name
self.left = left
self.right = right
width = self.right - self.left + 1
# mask applies before shift!
self.mask = 2**width - 1
self.signed = signedness == 'signed'
self.valclass = valclass
self.overlap = overlap == 'overlap'
def __repr__(self):
return '<Field %r>'%(self.name,)
def encode(self, value):
if not issubclass(self.valclass, type(value)):
raise ValueError("field '%s' takes '%s's, not '%s's"
%(self.name, self.valclass.__name__, type(value).__name__))
if not self.signed and value < 0:
raise ValueError("field '%s' is unsigned and can't accept value %d"
%(self.name, value))
# that this does the right thing is /not/ obvious (but true!)
if ((value >> 31) ^ value) & ~(self.mask >> self.signed):
raise ValueError("field '%s' can't accept value %s"
%(self.name, value))
value &= self.mask
value = long(value)
value <<= (32 - self.right - 1)
if value & 0x80000000L:
# yuck:
return ~int((~value)&0xFFFFFFFFL)
else:
return int(value)
def decode(self, inst):
mask = self.mask
v = (inst >> 32 - self.right - 1) & mask
if self.signed and (~mask >> 1) & mask & v:
v = ~(~v&mask)
return self.valclass(v)
def r(self, v, labels, pc):
return self.decode(v)
if __name__=='__main__':
import doctest
doctest.testmod()
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