-- two implementations of a sort function
-- this is an example only. Lua has now a built-in function "sort"

-- extracted from Programming Pearls, page 110
local function qsort(x,l,u,f)
 if l<u then
  local m=math.random(u-(l-1))+l-1	-- choose a random pivot in range l..u
  x[l],x[m]=x[m],x[l]			-- swap pivot to first position
  local t=x[l]				-- pivot value
  m=l
  local i=l+1
  while i<=u do
    -- invariant: x[l+1..m] < t <= x[m+1..i-1]
    if f(x[i],t) then
      m=m+1
      x[m],x[i]=x[i],x[m]		-- swap x[i] and x[m]
    end
    i=i+1
  end
  x[l],x[m]=x[m],x[l]			-- swap pivot to a valid place
  -- x[l+1..m-1] < x[m] <= x[m+1..u]
  qsort(x,l,m-1,f)
  qsort(x,m+1,u,f)
 end
end

local function selectionsort(x,n,f)
 local i=1
 while i<=n do
  local m,j=i,i+1
  while j<=n do
   if f(x[j],x[m]) then m=j end
   j=j+1
  end
 x[i],x[m]=x[m],x[i]			-- swap x[i] and x[m]
 i=i+1
 end
end

local function show(m,x)
 io.write(m,"\n\t")
 local i=1
 while x[i] do
  io.write(x[i])
  i=i+1
  if x[i] then io.write(",") end
 end
 io.write("\n")
end

local function testsorts(x)
 local n=1
 while x[n] do n=n+1 end; n=n-1		-- count elements
 show("original",x)
 qsort(x,1,n,function (x,y) return x<y end)
 show("after quicksort",x)
 selectionsort(x,n,function (x,y) return x>y end)
 show("after reverse selection sort",x)
 qsort(x,1,n,function (x,y) return x<y end)
 show("after quicksort again",x)
end

-- array to be sorted
x={"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"}

testsorts(x)