I have a recursive function that seems to be reusing memory it shouldn't.
I'm developing in Goland 2021.1.1 with go version 1.16.2.
I've written a function to generate the partitions of an integer. The
partitions of n are all the sets of integers that add up to n, so the
partitions of 3 are {3}, {2,1} and {1,1,1}. As n grows, the number of
partitions grows exponentially (to be precise, as e^(n^½)). I wrote a
recursive function, which works great up to n=7. At n=8, one of the
partitions is {2,2,2,1}, which is obviously wrong; and since the function
is recursive, every subsequent n is wrong, too.
I just spent quite a long time stepping through the code, and I found that
what seems to be happening is that a slice declared in my recursive
function is being reused in two different recursions. Specifically, my
function declares
var temp [][]int
to build up the list of partitions. When I compute Partitions(8), I have to
compute Partitions(4), 5, 6 and 7. I memoize each set of partitions so I
only have to compute them once.
It all goes wrong when I'm working on Partitions(7). At this point,
Partitions(8) has already added 6 cases, the last of which is {2,2,2,2}.
This is stored in temp[5], the temp corresponding to n=8, of course. Then I
compute Partitions(7), which should create a new temp [][]int; I'll call
this temp//7. temp[6]//7 gets {2,2,2,1}, and at that point temp[5]//8
changes from {2,2,2,2} to {2,2,2,1}. The addresses of temp[6]//7 and
temp[5]//8 are different, but the addresses of the *elements* of these
slices are the same.
This has to be wrong.
I suspect a bug in go, but I suppose it's possible there's a bug in goland.
I'm running on macOS 11.3.1, just in case that's relevant.
I'm happy to share the source code if anyone is interested.
Thanks,
J
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