Generating permutations with n consecutive repeated elements

Think of it as a tree traversal problem. The nodes are initial segments of the permutations, each of which can be extended in either p ways or p-1 ways (depending on whether or not the last n characters have been the same).

Some non-optimized code:

from string import ascii_uppercase

def children(s,p,r,n):
    chars = ascii_uppercase[:p]
    if len(s)==r:
        return []
    elif len(s) >= n and s[-n:] == s[-1]*n:
        return [s+c for c in chars if c != s[-1]]
    else:
        return [s+c for c in chars]

def extensions(s,p,r,n):
    kids = children(s,p,r,n)
    if kids == []:
        return [s]
    else:
        perms = []
        for kid in kids:
            perms.extend(extensions(kid,p,r,n))
        return perms

def perms(p,r,n):
    return extensions('',p,r,n)

For example,

>>> perms(2,4,2)
['AABA', 'AABB', 'ABAA', 'ABAB', 'ABBA', 'BAAB', 'BABA', 'BABB', 'BBAA', 'BBAB']

It takes perms(6,10,3) around a minute to generate the 58792500 permutations of ABCDEF of length 10 with no repeated block longer than 3. It is recursive, but the depth of the recursion is bounded by `r, so the recursion shouldn't crash (unless the list itself is too large to hold in memory). You could look into non-recursive list traversal algorithms and write it as a generator rather than something which returns a list.

On Edit The following is still recursive, but it uses generators hence is more memory-efficient:

def yield_leaves(s,chars,r,n):
    if len(s) == r:
        yield s
    else:
        avoid = s[-1] if len(s) >= n and s[-n:] == s[-1]*n else ''
        for c in chars:
            if c != avoid:
                yield from yield_leaves(s+c,chars,r,n)

def yield_perms(chars,r,n):
    yield from yield_leaves('',chars,r,n)

I changed the interface to make it closer to your code. For example,

p = list(yield_perms('ABCDEF',10,3))

is comparable to p = perms(6,10,3). It actually seems to be slightly faster.

You should not exclude recursion for this kind of problem (although iterative approaches are generally faster). The memory problem is one of implementation. It is not inherent to recursion vs iteration.

Here's an example using recursion to define a generator function:

def permute(letters,size,maxConsec):
    if size == 1: yield from letters; return
    for s in permute(letters,size-1,maxConsec):
        for c in letters:
           if size<=maxConsec or s[-1]!=c or not s.endswith(c*maxConsec):
               yield s+c

for p in permute("AB",4,2): print(p)
    
AABA
AABB
ABAA
ABAB
ABBA
BAAB
BABA
BABB
BBAA
BBAB

It is always going to take a long time for larger strings given the sheer number of permutations to be produced. One of the advantages of using a generator function is that it doesn't need to store all the intermediate results in a list. So you can use it as an iterator to count the values for example, or efficiently make a list at the end:

print( sum(1 for _ in permute("ABCDEF",10,3)) ) # 15.0 seconds

print( len(list(permute("ABCDEF",10,3))) )      # 16.2 seconds

58,792,500 permutations (out of 60,466,176 possible without constraint)

An iterative solution (without recursion) that needs to store intermediate result will add memory considerations but can provide some gain in performance:

def permute(letters,size,maxConsec):
    letterMax = [(c,c*maxConsec) for c in letters]
    result    = [""]
    for sz in range(size):
        canRepeat = sz < maxConsec
        result = [ s+c for s in result for c,m in letterMax
                       if canRepeat or s[-1]!=c or not s.endswith(m) ]
    return result

print(len(permute("ABCDEF",10,3))) # 12.9 seconds

in any case, it's not going to work under a second to generate a list with 58 million values.

list(range(58792500)) # 1.9 seconds

[EDIT] here is a further optimized version of the iterative function as discussed in comments and other improvements

from itertools import product
def permute2(letters,size,maxConsec):
    result   = map("".join,product(*[letters]*maxConsec))
    if size == maxConsec: return list(result)
    xLetters = { x:[c for c in letters if c != x] for x in letters }
    xSuffix  = { c*maxConsec for c in letters }
    for _ in range(size-maxConsec):
        result = [ r+c for r in result
                       for c in (xLetters[r[-1]] if r[-maxConsec:] in xSuffix else letters) ]
    return result

print(len(permute2("ABCDEF",10,3))) # 8.9 seconds