wren/test/benchmark/fannkuch.lua
-- 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 = 9
local sum, flips = fannkuch(n)
io.write(sum, "\nPfannkuchen(", n, ") = ", flips, "\n")