with javascript_semantics
function str_srb(string input, integer n)
    string res = input
    integer l = length(input),
            m = min(n,l),
            count = sum(sq_eq(input[-m..-1],'1')),
            k = l-n
    for i=l to 1 by -1 do
        integer bit = odd(input[i])
        count += iff(k>0?odd(input[k]):0)-bit
        if count and not bit then res[i] = '1' end if
        k -= 1
    end for
    assert(count=0)
    return res
end function

constant tests = {{"1000",2,2},{"0100",2,2},{"0010",2,2},{"0000",2,2},
                  {"010000000000100000000010000000010000000100000010000010000100010010",0,3}}

for i=1 to length(tests) do
    {string input, integer l, integer m} = tests[i]
    printf(1,"input: %s (width %d)\n",{input,length(input)})
    for n=l to m do
        printf(1,"n = %d: %s\n",{n,str_srb(input,n)})
    end for
end for