quick sort


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by Yonaba

in lua

Source Code

-- (In-place) Quicksort implementation
-- See : http://en.wikipedia.org/wiki/Quicksort#In-place_version

-- Partitions a portion of a list
local function partition(list, comp, left, right, pivot_index)
	local pivot_value = list[pivot_index]
	list[pivot_index], list[right] = list[right], list[pivot_index]
	local store_index = left
	for i = left, right-1 do
		if comp(list[i], pivot_value) then
			list[i], list[store_index] = list[store_index], list[i]
			store_index = store_index + 1
		end
	end
	list[store_index], list[right] = list[right], list[store_index]
	return store_index
end

-- Performs in-place Quicksort
local function quicksort(list, comp, left, right)
	left, right = left or 1, right or #list
	if left < right then
		local pivot_index = math.random(left, right)
		local new_pivot_index = partition(list, comp, left, right, pivot_index)
		quicksort(list, comp, left, new_pivot_index - 1)
		quicksort(list, comp, new_pivot_index + 1, right)
	end
	return list
end

-- Quicksort function wrapper, to shadow access 
-- to left and right bounds
-- list: a list to be ordered, in-place
-- comp: (optional) a comparison function
--  defaults to function(a, b) return a < b end
-- returns: the passed-in list, sorted
return function (list, comp)
	comp = comp or function(a, b) return a < b end
	return quicksort(list, comp)
end
-- Tests for quick_sort.lua
local quicksort = require 'quick_sort'

local total, pass = 0, 0

local function dec(str, len)
	return #str < len
	   and str .. (('.'):rep(len-#str))
		  or str:sub(1,len)
end

local function run(message, f)
	total = total + 1
	local ok, err = pcall(f)
	if ok then pass = pass + 1 end
	local status = ok and 'PASSED' or 'FAILED'
	print(('%02d. %68s: %s'):format(total, dec(message,68), status))
end

-- Comparison functions
local function le(a,b) return a <= b end
local function ge(a,b) return a >= b end

-- Checks if list is sorted
function is_sorted(list, comp)
	comp = comp or le
	for i = 2, #list do
		if not comp(list[i-1],list[i]) then return false end
	end
	return true
end

-- Generates a table of n random values
local function gen(n)
	local t = {}
	for i = 1, n do t[i] = math.random(n) end
	return t
end

math.randomseed(os.time())


run('Empty arrays', function()
	local t = {}
	assert(is_sorted(quicksort({})))
end)

run('Already sorted array', function()
	local t = {1, 2, 3, 4, 5}
	assert(is_sorted(quicksort(t)))
end)

run('Sorting a large array (1e4 values)', function()
	local t = gen(1e4)
	assert(is_sorted(quicksort(t)))
	assert(is_sorted(quicksort(t, ge), ge))
end)

print(('-'):rep(80))
print(('Total : %02d: Pass: %02d - Failed : %02d - Success: %.2f %%')
	:format(total, pass, total-pass, (pass*100/total)))