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-- Example 18. Generate list permutations

-- Permutes a list by taking each element and

-- recursively re-ordering the remaining elements.

-- For instance, given {1, 2, 3}:

-- Takes 1 and re-orders 2 and 3 (1, 2, 3 and 1, 3, 2).

-- Takes 2 and re-orders 1 and 3 (2, 1, 3 and 2, 3, 1).

-- Takes 3 and re-orders 1 and 2 (3, 1, 2 and 3, 2, 1).

local function permute(list, i)

i = i or 1

if i > #list then

coroutine.yield(list)

else

for j = i, #list do

list[i], list[j] = list[j], list[i]

permute(list, i + 1)

list[i], list[j] = list[j], list[i]

end

-- Iterator.

local function permutations(list)

return coroutine.wrap(function() permute(list) end)

end

for permutation in permutations{1, 2, 3} do

--[[ process permutation ]]

--8<--------------------------------------------------------------------------

print(table.unpack(permutation))

--8<--------------------------------------------------------------------------

end