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Add support for negative slice indices

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In python a negative indices usually mean start counting from the right side.
I.e. if the index is negative is acutal index used is len(l) + i. E.g. l[-2]
equals l[len(l)-2].

Being able to specify an index this way also comes in handy for migen slices in
some cases. E.g. the following snippet can be implement to shift an abitrary
length register n bits to the right:
	reg.eq(Cat(Replicate(0, n), reg[-n:])

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
This commit is contained in:
Lars-Peter Clausen 2013-03-12 21:34:36 +01:00
parent b75fb7f97c
commit 72579a6129
1 changed files with 6 additions and 0 deletions
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6
migen/fhdl/structure.py
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@ -89,10 +89,16 @@ class Value(HUID):
def __getitem__(self, key):
if isinstance(key, int):
if (key < 0):
key += len(self)
return _Slice(self, key, key+1)
elif isinstance(key, slice):
start = key.start or 0
stop = key.stop or len(self)
if (start < 0):
start += len(self)
if (stop < 0):
stop += len(self)
if stop > len(self):
stop = len(self)
if key.step != None: