Calculate set partitions with specific subset sizes

Question

Given a set with n elements, I need to find all the partitions of that set where there are k subsets of almost equal size.

For example, for a set with 7 elements and 3 subsets I only want partitions where there are two subsets with 2 elements each and one subset with 3 elements. I do not want a partition that has subsets with 1, 2, and 4 elements.

Put another way, there are 877 possible partitions for a set of 7 elements, but I'm only interested in the 105 (?) partitions that are made up subsets with 2/2/3 elements:

                               

In reality n is around 35, which means that there are approximately 2.81 * 10²⁷ partitions, "only" 8,338,573,669,964,101 partitions with three subsets. As such, I can't possibly calculate them all and wade through to find the ones that I want.

What's an algorithm for calculating just the partitions of interest? (Not the number of partitions, but that actual members in each sub-set for each partition.)

Answer 1

I thought it might be interesting to benchmark the three solutions that have been been contributed to date. If anyone wants to see additional tests, please let me know and I'll add them.

Niklas1

def enumerate_part(i, j, lo, y)
  # i = index of the current part
  # j = index of the current number in the current part
  # lo = lower bound for numbers to chose
  # y = output yielder
  if i == @parts.size
    y << @parts.map(&:dup)
    return
  end
  if j == @sizes[i]
    enumerate_part(i + 1, 0, @sizes[i + 1] == @sizes[i] ? @parts[i][0] : 1, y)
    return
  end
  (lo..@n).each do |x|
    next if @used[x]
    @used[x] = true
    @parts[i][j] = x
    enumerate_part(i, j + 1, x + 1, y)
    @used[x] = false
  end
end

def niklas1(n, k)
  @n, @k = n, k
  @used = n.times.map { false }
  #  @sizes = [2,2,3]  for n = 7, k = 3
  @sizes = (k - n % k).times.map { n / k } + (n % k).times.map { (n + k - 1) / k }
  @parts = @sizes.map { |i| [0]*i }
  results = Enumerator.new { |y| enumerate_part(0,0,1,y) }
  cnt = results.count
  puts "count = #{cnt} != #{@sz}" unless cnt == @sz   
end

Niklas2

def dfs(i, j, lo, eq, sz, state)
  parts, y, n, k, left = state
  if i == k
    y << parts
    return
  end
  if j == sz
    mid = i+1 == n % k
    neweq, newsz = mid ? [k - i - 1, sz-1] : [eq - 1, sz]
    dfs(i+1, 0, mid ? 1 : parts[i][0], neweq, newsz, state)
    return
  end
  higher = ((j == 0) ? (eq * sz - j - 1) : (sz - j - 1))
  left.drop(higher).each_with_index do |x,idx|
    break if x < lo
    parts[i][j] = x
    left.delete_at(idx + higher)
    dfs(i, j + 1, x + 1, eq, sz, state)
    left.insert(idx + higher, x)
  end
end

def niklas2(n, k)
  parts = k.times.map{|i| [0]*(i < n%k ? (n+k-1)/k : n/k) }
  left = n.downto(1).to_a
  results = Enumerator.new { |y|
    dfs(0, 0, 1, (n % k == 0) ? k : n % k, (n + k - 1) / k, [parts, y, n, k, left])
  }
  cnt = results.count
  puts "count = #{cnt} != #{@sz}" unless cnt == @sz
end

Marc-André

def enum(n, k)
  # Pick smaller_size items from the list, repeat smaller_n times
  # then pick larger_size items from the list, repeat larger_n times.
  smaller_n = n.div k
  larger_times = n % k
  smaller_times = k - larger_times
  larger_n = smaller_n + 1

  return to_enum(:enum, n, k) { calc_size(n, smaller_n, smaller_times, larger_n, larger_times) } unless block_given?

  all = [*1..n]
  # split all into one subset to group with the smaller_n and another
  # to group with the larger_n.
  all.combination(smaller_n * smaller_times).each do |smaller|
    larger = all - smaller
    subdivide(smaller, smaller_n) do |small|
      subdivide(larger, larger_n) do |large|
        yield [*small, *large]
      end
    end
  end
end

# Subdivides elems into groups of n, keeping the elements sorted
# and generating only the sorted such combinations.
def subdivide(elems, n)
  return yield [] if elems.empty?
  # No choice for the first element, because we want to keep things sorted.
  first, *rest = elems
  rest.combination(n - 1).each do |comb|
    remain = rest - comb
    subdivide(remain, n) do |sub|
      yield [[first, *comb], *sub]
    end
  end
end

def calc_size(n, smaller_n, smaller_times, larger_n, larger_times)
  all = [
    smaller_times.times.map do |i|
      Array.new(n - i*smaller_n).combination(smaller_n)
    end,
    larger_times.times.map do |i|
      Array.new(n - smaller_times*smaller_n - i*larger_n).combination(larger_n)
    end
  ]
  # Multiply everything, but divide by the number of symmetries, because
  # we don't want to distinguish (1,2), (3,4), ... from (3,4), (1,2), ...
  all.map do |enums|
    enums.map(&:size).inject(1, :*) / enums.permutation.size
  end.inject(:*)
end

def marc_andre(n, k)
  a = enum(n, k).to_a
  a.size
end

Cary1

require 'set'

def cary1(n, k)
  m, r = n.divmod(k)
  cnt = doit([*1..n], Array.new(k-r, m) + Array.new(r, m+1)).to_a.map(&:to_a).size
  puts "count = #{cnt} != #{@sz}" unless cnt == @sz 
end

def doit(items, subs, so_far=[], combos=Set.new)
  return combos << (so_far+[items]).to_set if subs.size == 1
  items.combination(subs.first){|c|doit(items-c,subs[1..-1],so_far+[c],combos)}
  combos
end

Cary2

def cary2(n, k)
  m, r = n.divmod(k)
  cnt = doit2([*1..n], Array.new(k-r, m) + Array.new(r, m+1)).to_a.map(&:to_a).size
  puts "count = #{cnt} != #{@sz}" unless cnt == @sz 
end

def doit2(list, group_sizes)
  @group_sizes = group_sizes
  @combos = []
  @empty_group_filled_by_size = group_sizes.uniq.product([false]).to_h
  recurse(list, group_sizes.map { |sub| [] })
  @combos
end

def recurse(remaining_items_to_assign, so_far)
  empty_group_filled_by_size = @empty_group_filled_by_size.dup
  so_far.each_with_index do |group, ndx|
    next if group.size == @group_sizes[ndx] # all positions in group allocated
    if group.empty?
      group_size = @group_sizes[ndx]
      empty_group_filled_by_size[group_size] ? next :
        empty_group_filled_by_size[group_size] = true
    end
    so_far[ndx] << remaining_items_to_assign.first
    if remaining_items_to_assign.size == 1
      @combos << so_far.map { |group| group.dup } # deep copy
    else
      recurse(remaining_items_to_assign[1..-1], so_far)
    end
    so_far[ndx].pop
  end
end  

Benchmark code

require 'benchmark'

def bench_it(n, k, iterations)
  puts "\nn = #{n}, k = #{k}, size = #{@sz = marc_andre(n,k)}\n" 
  Benchmark.bm(%w[Niklas Marc-André Cary].map(&:size).max) do |bm|

    bm.report('Niklas1') do
      iterations.times do
        niklas1(n, k)
      end
    end

    bm.report('Niklas2') do
     iterations.times do
       niklas2(n, k)
     end
    end

    bm.report('Marc-André') do
      iterations.times do
        marc_andre(n, k)
      end
    end

    bm.report('Cary1') do
      iterations.times do
        cary1(n, k)
      end
    end

    bm.report('Cary2') do
      iterations.times do
        cary2(n, k)
      end
    end
  end
end

iterations = 1
bench_it( 7, 3, iterations)
bench_it(10, 2, iterations)
bench_it(10, 3, iterations)
bench_it(10, 4, iterations)
bench_it(13, 2, iterations)
bench_it(13, 3, iterations)
bench_it(13, 4, iterations)
bench_it(13, 5, iterations)
bench_it(16, 2, iterations)
bench_it(16, 3, iterations)
bench_it(16, 4, iterations)
bench_it(18, 2, iterations)
bench_it(18, 3, iterations)
bench_it(20, 2, iterations)

Results

The results are best read with this clip playing in the background.

n = 7, k = 3, size = 105
                 user     system      total        real
Niklas1      0.000000   0.000000   0.000000 (  0.001131)
Niklas2      0.000000   0.000000   0.000000 (  0.000595)
Marc-André   0.000000   0.000000   0.000000 (  0.000911)
Cary1        0.000000   0.000000   0.000000 (  0.003241)
Cary2        0.000000   0.000000   0.000000 (  0.000922)

n = 10, k = 2, size = 126
                 user     system      total        real
Niklas1      0.010000   0.000000   0.010000 (  0.004986)
Niklas2      0.000000   0.000000   0.000000 (  0.001031)
Marc-André   0.000000   0.000000   0.000000 (  0.000880)
Cary1        0.000000   0.000000   0.000000 (  0.003850)
Cary2        0.000000   0.000000   0.000000 (  0.001607)

n = 10, k = 3, size = 2100
                 user     system      total        real
Niklas1      0.040000   0.000000   0.040000 (  0.042930)
Niklas2      0.010000   0.000000   0.010000 (  0.012296)
Marc-André   0.020000   0.000000   0.020000 (  0.017632)
Cary1        0.080000   0.000000   0.080000 (  0.081082)
Cary2        0.020000   0.000000   0.020000 (  0.018739)

n = 10, k = 4, size = 6300
                 user     system      total        real
Niklas1      0.090000   0.000000   0.090000 (  0.094029)
Niklas2      0.030000   0.000000   0.030000 (  0.030908)
Marc-André   0.040000   0.000000   0.040000 (  0.038247)
Cary1        0.510000   0.000000   0.510000 (  0.512819)
Cary2        0.060000   0.000000   0.060000 (  0.059061)

n = 13, k = 2, size = 1716
                 user     system      total        real
Niklas1      0.210000   0.000000   0.210000 (  0.219898)
Niklas2      0.020000   0.000000   0.020000 (  0.017828)
Marc-André   0.020000   0.000000   0.020000 (  0.015917)
Cary1        0.030000   0.000000   0.030000 (  0.029272)
Cary2        0.020000   0.000000   0.020000 (  0.022058)

n = 13, k = 3, size = 45045
                 user     system      total        real
Niklas1      1.480000   0.010000   1.490000 (  1.484895)
Niklas2      0.350000   0.000000   0.350000 (  0.343872)
Marc-André   0.470000   0.010000   0.480000 (  0.493646)
Cary1        1.890000   0.010000   1.900000 (  1.895684)
Cary2        0.520000   0.010000   0.530000 (  0.531843)

n = 13, k = 4, size = 200200
                 user     system      total        real
Niklas1      4.160000   0.070000   4.230000 (  4.225072)
Niklas2      1.210000   0.000000   1.210000 (  1.216814)
Marc-André   2.170000   0.030000   2.200000 (  2.203629)
Cary1       29.930000   0.580000  30.510000 ( 30.545276)
Cary2        1.720000   0.040000   1.760000 (  1.757895)

n = 13, k = 5, size = 600600
                 user     system      total        real
Niklas1     12.750000   0.040000  12.790000 ( 12.800083)
Niklas2      3.070000   0.010000   3.080000 (  3.085927)
Marc-André   3.630000   0.010000   3.640000 (  3.637411)
Cary1      191.200000   0.890000 192.090000 (192.319270)
Cary2        5.290000   0.300000   5.590000 (  5.625138)

n = 16, k = 2, size = 6435
                 user     system      total        real
Niklas1      2.120000   0.010000   2.130000 (  2.131065)
Niklas2      0.090000   0.010000   0.100000 (  0.092635)
Marc-André   0.080000   0.000000   0.080000 (  0.076312)
Cary1        0.290000   0.000000   0.290000 (  0.295062)
Cary2        0.070000   0.000000   0.070000 (  0.071995)

n = 16, k = 3, size = 1009008
                 user     system      total        real
Niklas1     62.370000   0.940000  63.310000 ( 63.404404)
Niklas2      8.070000   0.020000   8.090000 (  8.099837)
Marc-André  14.080000   0.330000  14.410000 ( 14.424437)
Cary1       48.930000   0.220000  49.150000 ( 49.227445)
Cary2       13.540000   0.280000  13.820000 ( 13.856880)

n = 16, k = 4, size = 2627625
                 user     system      total        real
Niklas2     22.530000   0.290000  22.820000 ( 23.252738)
Marc-André  35.850000   1.160000  37.010000 ( 37.159520)
Cary2       39.850000   0.860000  40.710000 ( 40.780489)

n = 18, k = 2, size = 24310
                 user     system      total        real
Niklas2      0.280000   0.000000   0.280000 (  0.288286)
Marc-André   0.240000   0.020000   0.260000 (  0.262669)
Cary2        0.240000   0.010000   0.250000 (  0.245008)

n = 18, k = 3, size = 2858856
                 user     system      total        real
Niklas2     37.150000   2.670000  39.820000 ( 48.484321)
Marc-André  68.010000   1.350000  69.360000 ( 70.249403)
Cary2       48.300000   0.840000  49.140000 ( 49.591279)

n = 20, k = 2, size = 92378
                 user     system      total        real
Niklas2      1.260000   0.000000   1.260000 (  1.271094)
Marc-André   1.180000   0.040000   1.220000 (  1.205478)
Cary2        1.210000   0.010000   1.220000 (  1.232024)

Interpretation

We appear to have a clear winner. Congratulations, Marc-André! Niklas, no gold medal this time, but silver's not bad.

I will speak to my code and let @Niklas and @Marc-André speak to theirs, as they better understand how their code works. My code appears to work reasonable well when the value of k is small (2 or 3). As that parameter increases, however, my code spends an increasing percentage of time throwing out duplicates (e.g., dismissing [[2,3],[3,4,5],[6,7] when I already "have" [[6,7,[3,4,5][2,3]]. In particular, see the results for n = 13, k = 5. That's in part because I designed the code for arbitrary partitions, not just for those that look like [m, m,...,m, m+1, m+1,...m+1]. I will have a look at whether there is anything I can do to exploit that structure (without drastically changing my code), but I'm not optimistic about the outcome.

Edit

I've updated the benchmarks to include the results for my second solution ("cary2"). It posts solution times that are roughly comparable to @Marc-André's, which is not surprising considering that both algorithms avoid unnecessary enumerations of combinations.

Edit 2

I've updated the benchmarks to include @Niklas' second solution. I've labelled the new one "Niklas2" and the original one "Nicklas1". I also added two tests for n = 18. I tried running n = 20, k = 3 (for "Niklas1", "Marc-André" and "Cary2"), but cancelled it after 5 minutes or so, as it had not obtained an initial solution.

I believe the results speak for themselves.

Answer 2

Important observation: Under the condition that the partition sizes must not differ by more than 1, the multiset of partition sizes is uniquely defined. We have (n % k) parts of size ceil(n / k) and (k - n % k) partitions of size floor(n / k).

So let's fix the partition sizes and just enumerate all possible subsets for all partitions:

@n = n = 7
@k = k = 3
@used = n.times.map { false }
#  @sizes = [2,2,3]  for n = 7, k = 3
@sizes = (k - n % k).times.map { n / k } + (n % k).times.map { (n + k - 1) / k }
@parts = @sizes.map { |i| [0]*i }
def enumerate_part(i, j, lo, y)
  # i = index of the current part
  # j = index of the current number in the current part
  # lo = lower bound for numbers to chose
  # y = output yielder
  if i == @parts.size
    y << @parts.map(&:dup)
    return
  end
  if j == @sizes[i]
    enumerate_part(i + 1, 0, @sizes[i + 1] == @sizes[i] ? @parts[i][0] : 1, y)
    return
  end
  (lo..@n).each do |x|
    next if @used[x]
    @used[x] = true
    @parts[i][j] = x
    enumerate_part(i, j + 1, x + 1, y)
    @used[x] = false
  end
end

results = Enumerator.new { |y| enumerate_part(0,0,1,y) }
puts results.count

Note that for the partitions of equal size, I assign them increasing minima to achieve uniqueness. This can cause a bit of backtracking (at most 2 levels), so it's not optimal. We might want to avoid assigning numbers where we already know that the lower bound will be too high for the next, equally sized partition(s). It gets a bit complex at that point, so I kept it simple. It is definitely possible to improve this.

It uses only O(n) space because I mutate global state instead of copying stuff around (yes, I yield a defensive copy to the enumerator, but that is not necessary if you process the results immediately in every iteration). Maybe not the most elegant solution, but the goal is to achieve good speed.

Output:

105

It is no fun running this with larger n (beyond 20). I blame Ruby for that, but fortunately this particular implementation will look very similar in almost any other language.

UPDATE: Just for fun, I improved the algorithm to be completely linear in the output size (apart from the potential n factor due to maintaining the set of leftover numbers). It should be several times faster that way:

def dfs(i, j, lo, eq, sz, state)
  parts, y, n, k, left = state
  if i == k
    y << parts
    return
  end
  if j == sz
    mid = i+1 == n % k
    neweq, newsz = mid ? [k - i - 1, sz-1] : [eq - 1, sz]
    dfs(i+1, 0, mid ? 1 : parts[i][0], neweq, newsz, state)
    return
  end
  higher = ((j == 0) ? (eq * sz - j - 1) : (sz - j - 1))
  left.drop(higher).each_with_index do |x,idx|
    break if x < lo
    parts[i][j] = x
    left.delete_at(idx + higher)
    dfs(i, j + 1, x + 1, eq, sz, state)
    left.insert(idx + higher, x)
  end
end

def niklas(n, k)
  parts = k.times.map{|i| [0]*(i < n%k ? (n+k-1)/k : n/k) }
  left = n.downto(1).to_a
  results = Enumerator.new { |y|
    dfs(0, 0, 1, (n % k == 0) ? k : n % k, (n + k - 1) / k, [parts, y, n, k, left])
  }
  cnt = results.count
  puts "count = #{cnt} != #{@sz}" unless cnt == @sz
end

Answer 3

Here's an approach that avoids the unnecessary enumerations that were performed by my first solution. This solution is referred to as 'cary2' in the benchmarks.

Code

def doit(list, group_sizes)
  @group_sizes = group_sizes
  @combos = []
  @empty_group_filled_by_size = group_sizes.uniq.product([false]).to_h
  recurse(list, group_sizes.map { |sub| [] })
  @combos
end

def recurse(remaining_items_to_assign, assigned_so_far)
  empty_group_filled_by_size = @empty_group_filled_by_size.dup
  assigned_so_far.each_with_index do |group, ndx|
    next if group.size == @group_sizes[ndx] # all positions in group allocated
    if group.empty?
      group_size = @group_sizes[ndx]
      empty_group_filled_by_size[group_size] ? next :
        empty_group_filled_by_size[group_size] = true
    end
    assigned_so_far[ndx] << remaining_items_to_assign.first
    if remaining_items_to_assign.size == 1
      @combos << assigned_so_far.map { |group| group.dup } # deep copy
    else
      recurse(remaining_items_to_assign[1..-1], assigned_so_far)
    end
    assigned_so_far[ndx].pop
  end
end

Explanation

• Suppose we have 12 items to allocate to three bins of size 2 (B2a, B2b and B2c) and two bins of size 3 (B3a and B3b). Items are sequentially assigned to bins.

• The first item can be assigned to one of the size 2 bins (say B2a) or to one of the size 3 bins (say B3a). It cannot be sequentially assigned to each empty bin of a given size, as that would result in double-counting. For each given item being assigned to bins, the array empty_group_filled_by_size is used to keep track of whether the item has already been assigned to the first empty bin of a particular size.

• The allocation possibilities for the second item depend on which bin item 1 has assigned to. If item 1 has been assigned to B2a, item 2 can be assigned to B2a, to one of the other two (still empty) size 2 bins (say B2b) or to to one of the two (empty) size 3 bins (say B3a). If item 1 has been assigned to B3a, item 2 can be assigned to any one of the three size 2 bins (say B2a), to B3a or to B3b .

• These assignments continue until there remains only one item to assign, at which time there is only one bin that is not full, and it has space for just one more item. After saving that assignment of items to bins in an array (@combos), the recursion backtracks to consider other possibilities.

Answer 4

I believe this solution (referred to as 'cary1' in the benchmarks) will do the job:

Code

require 'set'

def doit(items, subs, so_far=[], combos=Set.new)
  return combos << (so_far+[items]).to_set if subs.size == 1
  items.combination(subs.first){|c|doit(items-c,subs[1..-1],so_far+[c],combos)}
  combos
end

Demo

doit([1,2,3,4,5], [2,3]).to_a.map(&:to_a) # 10 combinations
  #=> [[[1, 2], [3, 4, 5]],   [[1, 3], [2, 4, 5]],   [[1, 4], [2, 3, 5]],
  #    [[1, 5], [2, 3, 4]],   [[2, 3], [1, 4, 5]],   [[2, 4], [1, 3, 5]],
  #    [[2, 5], [1, 3, 4]],   [[3, 4], [1, 2, 5]],   [[3, 5], [1, 2, 4]],
  #    [[4, 5], [1, 2, 3]]] 

doit([1,2,3,4,5], [2,1,2]).to_a.map(&:to_a) # 15 combinations
  #=> [[[1, 2], [3], [4, 5]],   [[1, 2], [4], [3, 5]],   [[1, 2], [5], [3, 4]],
  #    [[1, 3], [2], [4, 5]],   [[1, 3], [4], [2, 5]],   [[1, 3], [5], [2, 4]],
  #    [[1, 4], [2], [3, 5]],   [[1, 4], [3], [2, 5]],   [[1, 4], [5], [2, 3]],
  #    [[1, 5], [2], [3, 4]],   [[1, 5], [3], [2, 4]],   [[1, 5], [4], [2, 3]],
  #    [[2, 3], [1], [4, 5]],   [[2, 4], [1], [3, 5]],   [[2, 5], [1], [3, 4]]] 

doit([1,2,3,4,5,6,7], [2,2,3]).to_a.map(&:to_a) # 105 combinations
  # => [[[1, 2], [3, 4], [5, 6, 7]],   [[1, 2], [3, 5], [4, 6, 7]],
  #     [[1, 2], [3, 6], [4, 5, 7]],   [[1, 2], [3, 7], [4, 5, 6]],
  #     [[1, 2], [4, 5], [3, 6, 7]],   [[1, 2], [4, 6], [3, 5, 7]],
  #     ...
  #     [[4, 5], [6, 7], [1, 2, 3]],   [[4, 6], [5, 7], [1, 2, 3]],
  #     [[4, 7], [5, 6], [1, 2, 3]]] 

Explanaton

• I took the array subs as a given, as constructing subs seems like a separate and not very difficult problem.
• To avoid [[1],[2,3],[4]] and [[4],[2,3],[1]] from both being included in the result, I converted both of these to sets. When attempting to add these to a set of results, only the first would be added. I used sets because nothing was said of the elements of the given set, specifically whether each pair could be compared with <=>. Depending on how the results are to be used, it may be sufficient to leave the result as a set of sets. Alternatively, the set of sets can easily be converted to an array of arrays, as I have done in the "Demo" section.

If subs in doit() is not given, it can be be created like so, before calling doit:

def divide_up(items, k)
  m, r = items.size.divmod(k)
  puts "m = #{m}, r = #{r}"
  doit(items, Array.new(k-r, m) + Array.new(r, m+1))
end  

Edit: I made a small change to have the method return a set of sets, thinking these probably could be used directly. If not, they can easily be converted to an array of arrays, as I have done in the demo section.

Answer 5

Here's a good way to do it, generating all possibilities only once by taking care of keeping them sorted, as well as a quick way to calculate lazily the number of answers:

def enum(n, k)
  # Pick smaller_size items from the list, repeat smaller_n times
  # then pick larger_size items from the list, repeat larger_n times.
  smaller_n = n.div k
  larger_times = n % k
  smaller_times = k - larger_times
  larger_n = smaller_n + 1

  return to_enum(:enum, n, k) { calc_size(n, smaller_n, smaller_times, larger_n, larger_times) } unless block_given?

  all = [*1..n]
  # split all into one subset to group with the smaller_n and another
  # to group with the larger_n.
  all.combination(smaller_n * smaller_times).each do |smaller|
    larger = all - smaller
    subdivide(smaller, smaller_n) do |small|
      subdivide(larger, larger_n) do |large|
        yield [*small, *large]
      end
    end
  end
end

# Subdivides elems into groups of n, keeping the elements sorted
# and generating only the sorted such combinations.
def subdivide(elems, n)
  return yield [] if elems.empty?
  # No choice for the first element, because we want to keep things sorted.
  first, *rest = elems
  rest.combination(n - 1).each do |comb|
    remain = rest - comb
    subdivide(remain, n) do |sub|
      yield [[first, *comb], *sub]
    end
  end
end

def calc_size(n, smaller_n, smaller_times, larger_n, larger_times)
  all = [
    smaller_times.times.map do |i|
      Array.new(n - i*smaller_n).combination(smaller_n)
    end,
    larger_times.times.map do |i|
      Array.new(n - smaller_times*smaller_n - i*larger_n).combination(larger_n)
    end
  ]
  # Multiply everything, but divide by the number of symmetries, because
  # we don't want to distinguish (1,2), (3,4), ... from (3,4), (1,2), ...
  all.map do |enums|
    enums.map(&:size).inject(1, :*) / enums.permutation.size
  end.inject(:*)
end

p enum(7, 3).size      # => 105 (instant)
p enum(7, 3).first(5)  # => [[[1, 2], [3, 4], [5, 6, 7]],
                       #     [[1, 3], [2, 4], [5, 6, 7]],
                       #     [[1, 4], [2, 3], [5, 6, 7]],
                       #     [[1, 2], [3, 5], [4, 6, 7]],
                       #     [[1, 3], [2, 5], [4, 6, 7]]]
p enum(7, 3).count     # => 105 (quick)
p enum(35, 3).size     # => 564121960420200 (instant)
p enum(35, 3).first(2) # => [[[1..11], [12..23], [24..35]], 
                       #     [[1..11], [12..22, 24], [23, 25..35]]]
p enum(35, 3).count    # => will take forever, should return 564121960420200

Note: Just for kicks, this can also calculate the size lazily by building some enumerators and using size, without iterating through them. This will work in Ruby 2.0+ only though, as it require Enumerator#size.

For added fun:

require 'with_progress'
enum(16, 3).with_progress.count # => enjoy!