-- The Computer Language Benchmarks Game
-- http://benchmarksgame.alioth.debian.org/
-- contributed by Mike Pall

local function fannkuch(n)
  local p, q, s, sign, maxflips, sum = {}, {}, {}, 1, 0, 0
  for i=1,n do 
    p[i] = i; 
    q[i] = i; 
    s[i] = i 
  end
  repeat
    -- Copy and flip.
    local q1 = p[1]				-- Cache 1st element.
    if q1 ~= 1 then
      for i=2,n do 
        q[i] = p[i] 
      end		-- Work on a copy.
      local flips = 1
      repeat
	      local qq = q[q1]
	      if qq == 1 then				-- ... until 1st element is 1.
	        sum = sum + sign*flips
	        if flips > maxflips then 
            maxflips = flips 
          end -- New maximum?
	        break
	      end
	      q[q1] = q1
	      if q1 >= 4 then
	        local i, j = 2, q1 - 1
	        repeat 
            q[i], q[j] = q[j], q[i]; 
            i = i + 1; 
            j = j - 1; 
          until i >= j
	      end
	      q1 = qq; 
        flips = flips + 1
      until false
    end
    -- Permute.
    if sign == 1 then
      p[2], p[1] = p[1], p[2]; sign = -1	-- Rotate 1<-2.
    else
      p[2], p[3] = p[3], p[2]; sign = 1		-- Rotate 1<-2 and 1<-2<-3.
      for i=3,n do
	      local sx = s[i]
	      if sx ~= 1 then 
          s[i] = sx-1; 
          break 
        end
	      if i == n then 
          return sum, maxflips 
        end	-- Out of permutations.
	      s[i] = i
	      -- Rotate 1<-...<-i+1.
	      local t = p[1]; 
        for j=1,i do 
          p[j] = p[j+1] 
        end; 
        p[i+1] = t
      end
    end
  until false
end

local n = tonumber(arg and arg[1]) or 1
if ravi then
  ravi.compile(fannkuch)
end
if jit then
  -- luajit - warmup 
  fannkuch(n)
end
local t1 = os.clock()
local sum, flips = fannkuch(n)
local t2 = os.clock()
io.write(sum, "\nPfannkuchen(", n, ") = ", flips, "\n")
io.write("elapsed time in secs ", (t2-t1), "\n")