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-- 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 7
local n = 10
local sum, flips = fannkuch(n)
io.write(sum, "\nPfannkuchen(", n, ") = ", flips, "\n")
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