---
--- Lua Fun - a high-performance functional programming library for LuaJIT
---
--- Copyright (c) 2013-2016 Roman Tsisyk <roman@tsisyk.com>
---
--- Distributed under the MIT/X11 License. See COPYING.md for more details.
---

local exports = {}
local methods = {}

-- compatibility with Lua 5.1/5.2
local unpack = rawget(table, "unpack") or unpack

--------------------------------------------------------------------------------
-- Tools
--------------------------------------------------------------------------------

local return_if_not_empty = function(state_x, ...)
	if state_x == nil then
		return nil
	end
	return ...
end

local call_if_not_empty = function(fun, state_x, ...)
	if state_x == nil then
		return nil
	end
	return state_x, fun(...)
end

local function deepcopy(orig) -- used by cycle()
	local orig_type = type(orig)
	local copy
	if orig_type == 'table' then
		copy = {}
		for orig_key, orig_value in next, orig, nil do
			copy[deepcopy(orig_key)] = deepcopy(orig_value)
		end
	else
		copy = orig
	end
	return copy
end

local iterator_mt = {
	-- usually called by for-in loop
	__call = function(self, param, state)
		return self.gen(param, state)
	end,
	__tostring = function(self)
		return '<generator>'
	end,
	-- add all exported methods
	__index = methods
}

local wrap = function(gen, param, state)
	return setmetatable({
		gen = gen,
		param = param,
		state = state
	}, iterator_mt), param, state
end
exports.wrap = wrap

local unwrap = function(self)
	return self.gen, self.param, self.state
end
methods.unwrap = unwrap

--------------------------------------------------------------------------------
-- Basic Functions
--------------------------------------------------------------------------------

local nil_gen = function(_param, _state)
	return nil
end

local string_gen = function(param, state)
	local state = state + 1
	if state > #param then
		return nil
	end
	local r = string.sub(param, state, state)
	return state, r
end

local ipairs_gen = ipairs({}) -- get the generating function from ipairs
local pairs_gen = pairs({ a = 0 }) -- get the generating function from pairs
local map_gen = function(tab, key)
	local value
	local key, value = pairs_gen(tab, key)
	return key, key, value
end

local rawiter = function(obj, param, state)
	assert(obj ~= nil, "invalid iterator")
	if type(obj) == "table" then
		local mt = getmetatable(obj)
		if mt ~= nil then
			if mt == iterator_mt then
				return obj.gen, obj.param, obj.state
			elseif mt.__ipairs ~= nil then
				return mt.__ipairs(obj)
			elseif mt.__pairs ~= nil then
				return mt.__pairs(obj)
			end
		end
		if #obj > 0 then
			-- array
			return ipairs(obj)
		else
			-- hash
			return map_gen, obj, nil
		end
	elseif (type(obj) == "function") then
		return obj, param, state
	elseif (type(obj) == "string") then
		if #obj == 0 then
			return nil_gen, nil, nil
		end
		return string_gen, obj, 0
	end
	error(string.format('object %s of type "%s" is not iterable',
			obj, type(obj)))
end

local iter = function(obj, param, state)
	return wrap(rawiter(obj, param, state))
end
exports.iter = iter

local method0 = function(fun)
	return function(self)
		return fun(self.gen, self.param, self.state)
	end
end

local method1 = function(fun)
	return function(self, arg1)
		return fun(arg1, self.gen, self.param, self.state)
	end
end

local method2 = function(fun)
	return function(self, arg1, arg2)
		return fun(arg1, arg2, self.gen, self.param, self.state)
	end
end

local export0 = function(fun)
	return function(gen, param, state)
		return fun(rawiter(gen, param, state))
	end
end

local export1 = function(fun)
	return function(arg1, gen, param, state)
		return fun(arg1, rawiter(gen, param, state))
	end
end

local export2 = function(fun)
	return function(arg1, arg2, gen, param, state)
		return fun(arg1, arg2, rawiter(gen, param, state))
	end
end

local each = function(fun, gen, param, state)
	repeat
		state = call_if_not_empty(fun, gen(param, state))
	until state == nil
end
methods.each = method1(each)
exports.each = export1(each)
methods.for_each = methods.each
exports.for_each = exports.each
methods.foreach = methods.each
exports.foreach = exports.each

--------------------------------------------------------------------------------
-- Generators
--------------------------------------------------------------------------------

local range_gen = function(param, state)
	local stop, step = param[1], param[2]
	local state = state + step
	if state > stop then
		return nil
	end
	return state, state
end

local range_rev_gen = function(param, state)
	local stop, step = param[1], param[2]
	local state = state + step
	if state < stop then
		return nil
	end
	return state, state
end

local range = function(start, stop, step)
	if step == nil then
		if stop == nil then
			if start == 0 then
				return nil_gen, nil, nil
			end
			stop = start
			start = stop > 0 and 1 or -1
		end
		step = start <= stop and 1 or -1
	end

	assert(type(start) == "number", "start must be a number")
	assert(type(stop) == "number", "stop must be a number")
	assert(type(step) == "number", "step must be a number")
	assert(step ~= 0, "step must not be zero")

	if (step > 0) then
		return wrap(range_gen, {stop, step}, start - step)
	elseif (step < 0) then
		return wrap(range_rev_gen, {stop, step}, start - step)
	end
end
exports.range = range

local duplicate_table_gen = function(param_x, state_x)
	return state_x + 1, unpack(param_x)
end

local duplicate_fun_gen = function(param_x, state_x)
	return state_x + 1, param_x(state_x)
end

local duplicate_gen = function(param_x, state_x)
	return state_x + 1, param_x
end

local duplicate = function(...)
	if select('#', ...) <= 1 then
		return wrap(duplicate_gen, select(1, ...), 0)
	else
		return wrap(duplicate_table_gen, {...}, 0)
	end
end
exports.duplicate = duplicate
exports.replicate = duplicate
exports.xrepeat = duplicate

local tabulate = function(fun)
	assert(type(fun) == "function")
	return wrap(duplicate_fun_gen, fun, 0)
end
exports.tabulate = tabulate

local zeros = function()
	return wrap(duplicate_gen, 0, 0)
end
exports.zeros = zeros

local ones = function()
	return wrap(duplicate_gen, 1, 0)
end
exports.ones = ones

local rands_gen = function(param_x, _state_x)
	return 0, RandomInt(param_x[1], param_x[2])
end

local rands_nil_gen = function(_param_x, _state_x)
	return 0, RandomFloat(0, 1)
end

local rands = function(n, m)
	if n == nil and m == nil then
		return wrap(rands_nil_gen, 0, 0)
	end
	assert(type(n) == "number", "invalid first arg to rands")
	if m == nil then
		m = n
		n = 0
	else
		assert(type(m) == "number", "invalid second arg to rands")
	end
	assert(n < m, "empty interval")
	return wrap(rands_gen, {n, m - 1}, 0)
end
exports.rands = rands

--------------------------------------------------------------------------------
-- Slicing
--------------------------------------------------------------------------------

local nth = function(n, gen_x, param_x, state_x)
	assert(n > 0, "invalid first argument to nth")
	-- An optimization for arrays and strings
	if gen_x == ipairs_gen then
		return param_x[n]
	elseif gen_x == string_gen then
		if n <= #param_x then
			return string.sub(param_x, n, n)
		else
			return nil
		end
	end
	for i=1,n-1,1 do
		state_x = gen_x(param_x, state_x)
		if state_x == nil then
			return nil
		end
	end
	return return_if_not_empty(gen_x(param_x, state_x))
end
methods.nth = method1(nth)
exports.nth = export1(nth)

local head_call = function(state, ...)
	if state == nil then
		error("head: iterator is empty")
	end
	return ...
end

local head = function(gen, param, state)
	return head_call(gen(param, state))
end
methods.head = method0(head)
exports.head = export0(head)
exports.car = exports.head
methods.car = methods.head

local tail = function(gen, param, state)
	state = gen(param, state)
	if state == nil then
		return wrap(nil_gen, nil, nil)
	end
	return wrap(gen, param, state)
end
methods.tail = method0(tail)
exports.tail = export0(tail)
exports.cdr = exports.tail
methods.cdr = methods.tail

local take_n_gen_x = function(i, state_x, ...)
	if state_x == nil then
		return nil
	end
	return {i, state_x}, ...
end

local take_n_gen = function(param, state)
	local n, gen_x, param_x = param[1], param[2], param[3]
	local i, state_x = state[1], state[2]
	if i >= n then
		return nil
	end
	return take_n_gen_x(i + 1, gen_x(param_x, state_x))
end

local take_n = function(n, gen, param, state)
	assert(n >= 0, "invalid first argument to take_n")
	return wrap(take_n_gen, {n, gen, param}, {0, state})
end
methods.take_n = method1(take_n)
exports.take_n = export1(take_n)

local take_while_gen_x = function(fun, state_x, ...)
	if state_x == nil or not fun(...) then
		return nil
	end
	return state_x, ...
end

local take_while_gen = function(param, state_x)
	local fun, gen_x, param_x = param[1], param[2], param[3]
	return take_while_gen_x(fun, gen_x(param_x, state_x))
end

local take_while = function(fun, gen, param, state)
	assert(type(fun) == "function", "invalid first argument to take_while")
	return wrap(take_while_gen, {fun, gen, param}, state)
end
methods.take_while = method1(take_while)
exports.take_while = export1(take_while)

local take = function(n_or_fun, gen, param, state)
	if type(n_or_fun) == "number" then
		return take_n(n_or_fun, gen, param, state)
	else
		return take_while(n_or_fun, gen, param, state)
	end
end
methods.take = method1(take)
exports.take = export1(take)

local drop_n = function(n, gen, param, state)
	assert(n >= 0, "invalid first argument to drop_n")
	local i
	for i=1,n,1 do
		state = gen(param, state)
		if state == nil then
			return wrap(nil_gen, nil, nil)
		end
	end
	return wrap(gen, param, state)
end
methods.drop_n = method1(drop_n)
exports.drop_n = export1(drop_n)

local drop_while_x = function(fun, state_x, ...)
	if state_x == nil or not fun(...) then
		return state_x, false
	end
	return state_x, true, ...
end

local drop_while = function(fun, gen_x, param_x, state_x)
	assert(type(fun) == "function", "invalid first argument to drop_while")
	local cont, state_x_prev
	repeat
		state_x_prev = deepcopy(state_x)
		state_x, cont = drop_while_x(fun, gen_x(param_x, state_x))
	until not cont
	if state_x == nil then
		return wrap(nil_gen, nil, nil)
	end
	return wrap(gen_x, param_x, state_x_prev)
end
methods.drop_while = method1(drop_while)
exports.drop_while = export1(drop_while)

local drop = function(n_or_fun, gen_x, param_x, state_x)
	if type(n_or_fun) == "number" then
		return drop_n(n_or_fun, gen_x, param_x, state_x)
	else
		return drop_while(n_or_fun, gen_x, param_x, state_x)
	end
end
methods.drop = method1(drop)
exports.drop = export1(drop)

local split = function(n_or_fun, gen_x, param_x, state_x)
	return take(n_or_fun, gen_x, param_x, state_x),
			 drop(n_or_fun, gen_x, param_x, state_x)
end
methods.split = method1(split)
exports.split = export1(split)
methods.split_at = methods.split
exports.split_at = exports.split
methods.span = methods.split
exports.span = exports.split

--------------------------------------------------------------------------------
-- Indexing
--------------------------------------------------------------------------------

local index = function(x, gen, param, state)
	local i = 1
	for _k, r in gen, param, state do
		if r == x then
			return i
		end
		i = i + 1
	end
	return nil
end
methods.index = method1(index)
exports.index = export1(index)
methods.index_of = methods.index
exports.index_of = exports.index
methods.elem_index = methods.index
exports.elem_index = exports.index
methods.contains = methods.index
exports.contains = exports.index

local indexes_gen = function(param, state)
	local x, gen_x, param_x = param[1], param[2], param[3]
	local i, state_x = state[1], state[2]
	local r
	while true do
		state_x, r = gen_x(param_x, state_x)
		if state_x == nil then
			return nil
		end
		i = i + 1
		if r == x then
			return {i, state_x}, i
		end
	end
end

local indexes = function(x, gen, param, state)
	return wrap(indexes_gen, {x, gen, param}, {0, state})
end
methods.indexes = method1(indexes)
exports.indexes = export1(indexes)
methods.elem_indexes = methods.indexes
exports.elem_indexes = exports.indexes
methods.indices = methods.indexes
exports.indices = exports.indexes
methods.elem_indices = methods.indexes
exports.elem_indices = exports.indexes

--------------------------------------------------------------------------------
-- Filtering
--------------------------------------------------------------------------------

local filter1_gen = function(fun, gen_x, param_x, state_x, a)
	while true do
		if state_x == nil or fun(a) then break end
		state_x, a = gen_x(param_x, state_x)
	end
	return state_x, a
end

-- call each other
local filterm_gen
local filterm_gen_shrink = function(fun, gen_x, param_x, state_x)
	return filterm_gen(fun, gen_x, param_x, gen_x(param_x, state_x))
end

filterm_gen = function(fun, gen_x, param_x, state_x, ...)
	if state_x == nil then
		return nil
	end
	if fun(...) then
		return state_x, ...
	end
	return filterm_gen_shrink(fun, gen_x, param_x, state_x)
end

local filter_detect = function(fun, gen_x, param_x, state_x, ...)
	if select('#', ...) < 2 then
		return filter1_gen(fun, gen_x, param_x, state_x, ...)
	else
		return filterm_gen(fun, gen_x, param_x, state_x, ...)
	end
end

local filter_gen = function(param, state_x)
	local fun, gen_x, param_x = param[1], param[2], param[3]
	return filter_detect(fun, gen_x, param_x, gen_x(param_x, state_x))
end

local filter = function(fun, gen, param, state)
	return wrap(filter_gen, {fun, gen, param}, state)
end
methods.filter = method1(filter)
exports.filter = export1(filter)
methods.remove_if = methods.filter
exports.remove_if = exports.filter

local grep = function(fun_or_regexp, gen, param, state)
	local fun = fun_or_regexp
	if type(fun_or_regexp) == "string" then
		fun = function(x) return string.find(x, fun_or_regexp) ~= nil end
	end
	return filter(fun, gen, param, state)
end
methods.grep = method1(grep)
exports.grep = export1(grep)

local partition = function(fun, gen, param, state)
	local neg_fun = function(...)
		return not fun(...)
	end
	return filter(fun, gen, param, state),
			 filter(neg_fun, gen, param, state)
end
methods.partition = method1(partition)
exports.partition = export1(partition)

--------------------------------------------------------------------------------
-- Reducing
--------------------------------------------------------------------------------

local foldl_call = function(fun, start, state, ...)
	if state == nil then
		return nil, start
	end
	return state, fun(start, ...)
end

local foldl = function(fun, start, gen_x, param_x, state_x)
	while true do
		state_x, start = foldl_call(fun, start, gen_x(param_x, state_x))
		if state_x == nil then
			break;
		end
	end
	return start
end
methods.foldl = method2(foldl)
exports.foldl = export2(foldl)
methods.reduce = methods.foldl
exports.reduce = exports.foldl

local length = function(gen, param, state)
	if gen == ipairs_gen or gen == string_gen then
		return #param
	end
	local len = 0
	repeat
		state = gen(param, state)
		len = len + 1
	until state == nil
	return len - 1
end
methods.length = method0(length)
exports.length = export0(length)

local is_null = function(gen, param, state)
	return gen(param, deepcopy(state)) == nil
end
methods.is_null = method0(is_null)
exports.is_null = export0(is_null)

local is_prefix_of = function(iter_x, iter_y)
	local gen_x, param_x, state_x = iter(iter_x)
	local gen_y, param_y, state_y = iter(iter_y)

	local r_x, r_y
	for i=1,10,1 do
		state_x, r_x = gen_x(param_x, state_x)
		state_y, r_y = gen_y(param_y, state_y)
		if state_x == nil then
			return true
		end
		if state_y == nil or r_x ~= r_y then
			return false
		end
	end
end
methods.is_prefix_of = is_prefix_of
exports.is_prefix_of = is_prefix_of

local all = function(fun, gen_x, param_x, state_x)
	local r
	repeat
		state_x, r = call_if_not_empty(fun, gen_x(param_x, state_x))
	until state_x == nil or not r
	return state_x == nil
end
methods.all = method1(all)
exports.all = export1(all)
methods.every = methods.all
exports.every = exports.all

local any = function(fun, gen_x, param_x, state_x)
	local r
	repeat
		state_x, r = call_if_not_empty(fun, gen_x(param_x, state_x))
	until state_x == nil or r
	return not not r
end
methods.any = method1(any)
exports.any = export1(any)
methods.some = methods.any
exports.some = exports.any

local sum = function(gen, param, state)
	local s = 0
	local r = 0
	repeat
		s = s + r
		state, r = gen(param, state)
	until state == nil
	return s
end
methods.sum = method0(sum)
exports.sum = export0(sum)

local product = function(gen, param, state)
	local p = 1
	local r = 1
	repeat
		p = p * r
		state, r = gen(param, state)
	until state == nil
	return p
end
methods.product = method0(product)
exports.product = export0(product)

local min_cmp = function(m, n)
	if n < m then return n else return m end
end

local max_cmp = function(m, n)
	if n > m then return n else return m end
end

local min = function(gen, param, state)
	local state, m = gen(param, state)
	if state == nil then
		error("min: iterator is empty")
	end

	local cmp
	if type(m) == "number" then
		-- An optimization: use math.min for numbers
		cmp = math.min
	else
		cmp = min_cmp
	end

	for _, r in gen, param, state do
		m = cmp(m, r)
	end
	return m
end
methods.min = method0(min)
exports.min = export0(min)
methods.minimum = methods.min
exports.minimum = exports.min

local min_by = function(cmp, gen_x, param_x, state_x)
	local state_x, m = gen_x(param_x, state_x)
	if state_x == nil then
		error("min: iterator is empty")
	end

	for _, r in gen_x, param_x, state_x do
		m = cmp(m, r)
	end
	return m
end
methods.min_by = method1(min_by)
exports.min_by = export1(min_by)
methods.minimum_by = methods.min_by
exports.minimum_by = exports.min_by

local max = function(gen_x, param_x, state_x)
	local state_x, m = gen_x(param_x, state_x)
	if state_x == nil then
		error("max: iterator is empty")
	end

	local cmp
	if type(m) == "number" then
		-- An optimization: use math.max for numbers
		cmp = math.max
	else
		cmp = max_cmp
	end

	for _, r in gen_x, param_x, state_x do
		m = cmp(m, r)
	end
	return m
end
methods.max = method0(max)
exports.max = export0(max)
methods.maximum = methods.max
exports.maximum = exports.max

local max_by = function(cmp, gen_x, param_x, state_x)
	local state_x, m = gen_x(param_x, state_x)
	if state_x == nil then
		error("max: iterator is empty")
	end

	for _, r in gen_x, param_x, state_x do
		m = cmp(m, r)
	end
	return m
end
methods.max_by = method1(max_by)
exports.max_by = export1(max_by)
methods.maximum_by = methods.maximum_by
exports.maximum_by = exports.maximum_by

local totable = function(gen_x, param_x, state_x)
	local tab, key, val = {}
	while true do
		state_x, val = gen_x(param_x, state_x)
		if state_x == nil then
			break
		end
		table.insert(tab, val)
	end
	return tab
end
methods.totable = method0(totable)
exports.totable = export0(totable)

local tomap = function(gen_x, param_x, state_x)
	local tab, key, val = {}
	while true do
		state_x, key, val = gen_x(param_x, state_x)
		if state_x == nil then
			break
		end
		tab[key] = val
	end
	return tab
end
methods.tomap = method0(tomap)
exports.tomap = export0(tomap)

--------------------------------------------------------------------------------
-- Transformations
--------------------------------------------------------------------------------

local map_gen = function(param, state)
	local gen_x, param_x, fun = param[1], param[2], param[3]
	return call_if_not_empty(fun, gen_x(param_x, state))
end

local map = function(fun, gen, param, state)
	return wrap(map_gen, {gen, param, fun}, state)
end
methods.map = method1(map)
exports.map = export1(map)

local enumerate_gen_call = function(state, i, state_x, ...)
	if state_x == nil then
		return nil
	end
	return {i + 1, state_x}, i, ...
end

local enumerate_gen = function(param, state)
	local gen_x, param_x = param[1], param[2]
	local i, state_x = state[1], state[2]
	return enumerate_gen_call(state, i, gen_x(param_x, state_x))
end

local enumerate = function(gen, param, state)
	return wrap(enumerate_gen, {gen, param}, {1, state})
end
methods.enumerate = method0(enumerate)
exports.enumerate = export0(enumerate)

local intersperse_call = function(i, state_x, ...)
	if state_x == nil then
		return nil
	end
	return {i + 1, state_x}, ...
end

local intersperse_gen = function(param, state)
	local x, gen_x, param_x = param[1], param[2], param[3]
	local i, state_x = state[1], state[2]
	if i % 2 == 1 then
		return {i + 1, state_x}, x
	else
		return intersperse_call(i, gen_x(param_x, state_x))
	end
end

-- TODO: interperse must not add x to the tail
local intersperse = function(x, gen, param, state)
	return wrap(intersperse_gen, {x, gen, param}, {0, state})
end
methods.intersperse = method1(intersperse)
exports.intersperse = export1(intersperse)

--------------------------------------------------------------------------------
-- Compositions
--------------------------------------------------------------------------------

local function zip_gen_r(param, state, state_new, ...)
	if #state_new == #param / 2 then
		return state_new, ...
	end

	local i = #state_new + 1
	local gen_x, param_x = param[2 * i - 1], param[2 * i]
	local state_x, r = gen_x(param_x, state[i])
	if state_x == nil then
		return nil
	end
	table.insert(state_new, state_x)
	return zip_gen_r(param, state, state_new, r, ...)
end

local zip_gen = function(param, state)
	return zip_gen_r(param, state, {})
end

-- A special hack for zip/chain to skip last two state, if a wrapped iterator
-- has been passed
local numargs = function(...)
	local n = select('#', ...)
	if n >= 3 then
		-- Fix last argument
		local it = select(n - 2, ...)
		if type(it) == 'table' and getmetatable(it) == iterator_mt and
			 it.param == select(n - 1, ...) and it.state == select(n, ...) then
			return n - 2
		end
	end
	return n
end

local zip = function(...)
	local n = numargs(...)
	if n == 0 then
		return wrap(nil_gen, nil, nil)
	end
	local param = { [2 * n] = 0 }
	local state = { [n] = 0 }

	local i, gen_x, param_x, state_x
	for i=1,n,1 do
		local it = select(n - i + 1, ...)
		gen_x, param_x, state_x = rawiter(it)
		param[2 * i - 1] = gen_x
		param[2 * i] = param_x
		state[i] = state_x
	end

	return wrap(zip_gen, param, state)
end
methods.zip = zip
exports.zip = zip

local cycle_gen_call = function(param, state_x, ...)
	if state_x == nil then
		local gen_x, param_x, state_x0 = param[1], param[2], param[3]
		return gen_x(param_x, deepcopy(state_x0))
	end
	return state_x, ...
end

local cycle_gen = function(param, state_x)
	local gen_x, param_x, state_x0 = param[1], param[2], param[3]
	return cycle_gen_call(param, gen_x(param_x, state_x))
end

local cycle = function(gen, param, state)
	return wrap(cycle_gen, {gen, param, state}, deepcopy(state))
end
methods.cycle = method0(cycle)
exports.cycle = export0(cycle)

-- call each other
local chain_gen_r1
local chain_gen_r2 = function(param, state, state_x, ...)
	if state_x == nil then
		local i = state[1]
		i = i + 1
		if i > #param / 3 then
			return nil
		end
		local state_x = param[3 * i]
		return chain_gen_r1(param, {i, state_x})
	end
	return {state[1], state_x}, ...
end

chain_gen_r1 = function(param, state)
	local i, state_x = state[1], state[2]
	local gen_x, param_x = param[3 * i - 2], param[3 * i - 1]
	return chain_gen_r2(param, state, gen_x(param_x, state[2]))
end

local chain = function(...)
	local n = numargs(...)
	if n == 0 then
		return wrap(nil_gen, nil, nil)
	end

	local param = { [3 * n] = 0 }
	local i, gen_x, param_x, state_x
	for i=1,n,1 do
		local elem = select(i, ...)
		gen_x, param_x, state_x = iter(elem)
		param[3 * i - 2] = gen_x
		param[3 * i - 1] = param_x
		param[3 * i] = state_x
	end

	return wrap(chain_gen_r1, param, {1, param[3]})
end
methods.chain = chain
exports.chain = chain

--------------------------------------------------------------------------------
-- Operators
--------------------------------------------------------------------------------

local operator = {
	----------------------------------------------------------------------------
	-- Comparison operators
	----------------------------------------------------------------------------
	lt  = function(a, b) return a < b end,
	le  = function(a, b) return a <= b end,
	eq  = function(a, b) return a == b end,
	ne  = function(a, b) return a ~= b end,
	ge  = function(a, b) return a >= b end,
	gt  = function(a, b) return a > b end,

	----------------------------------------------------------------------------
	-- Arithmetic operators
	----------------------------------------------------------------------------
	add = function(a, b) return a + b end,
	div = function(a, b) return a / b end,
	floordiv = function(a, b) return math.floor(a/b) end,
	intdiv = function(a, b)
		local q = a / b
		if a >= 0 then return math.floor(q) else return math.ceil(q) end
	end,
	mod = function(a, b) return a % b end,
	mul = function(a, b) return a * b end,
	neq = function(a) return -a end,
	unm = function(a) return -a end, -- an alias
	pow = function(a, b) return a ^ b end,
	sub = function(a, b) return a - b end,
	truediv = function(a, b) return a / b end,

	----------------------------------------------------------------------------
	-- String operators
	----------------------------------------------------------------------------
	concat = function(a, b) return a..b end,
	len = function(a) return #a end,
	length = function(a) return #a end, -- an alias

	----------------------------------------------------------------------------
	-- Logical operators
	----------------------------------------------------------------------------
	land = function(a, b) return a and b end,
	lor = function(a, b) return a or b end,
	lnot = function(a) return not a end,
	truth = function(a) return not not a end,
}
exports.operator = operator
methods.operator = operator
exports.op = operator
methods.op = operator

--------------------------------------------------------------------------------
-- module definitions
--------------------------------------------------------------------------------

-- a special syntax sugar to export all functions to the global table
setmetatable(exports, {
	__call = function(t, override)
		for k, v in pairs(t) do
			if _G[k] ~= nil then
				local msg = 'function ' .. k .. ' already exists in global scope.'
				if override then
					_G[k] = v
					print(msg .. ' Overwritten.')
				else
					print(msg .. ' Skipped.')
				end
			else
				_G[k] = v
			end
		end
	end,
})

return exports
