iterators.lua

-- Prosody IM

-- Copyright (C) 2008-2010 Matthew Wild

-- Copyright (C) 2008-2010 Waqas Hussain

-- 

-- This project is MIT/X11 licensed. Please see the

-- COPYING file in the source package for more information.

--

--[[ Iterators ]]--

-- Reverse an iterator

function reverse(f, s, var)

	local results = {};

	-- First call the normal iterator

	while true do

		local ret = { f(s, var) };

		var = ret[1];

	        if var == nil then break; end

		table.insert(results, 1, ret);

	end

	-- Then return our reverse one

	local i,max = 0, #results;

	return function (results)

			if i<max then

				i = i + 1;

				return unpack(results[i]);

			end

		end, results;

end

-- Iterate only over keys in a table

local function _keys_it(t, key)

	return (next(t, key));

end

function keys(t)

	return _keys_it, t;

end

-- Iterate only over values in a table

function values(t)

	local key, val;

	return function (t)

		key, val = next(t, key);

		return val;

	end, t;

end

-- Given an iterator, iterate only over unique items

function unique(f, s, var)

	local set = {};

	return function ()

		while true do

			local ret = { f(s, var) };

			var = ret[1];

		        if var == nil then break; end

		        if not set[var] then

				set[var] = true;

				return var;

			end

		end

	end;

end

--[[ Return the number of items an iterator returns ]]--

function count(f, s, var)

	local x = 0;

	while true do

		local ret = { f(s, var) };

		var = ret[1];

	        if var == nil then break; end

		x = x + 1;

	end	

	return x;

end

-- Return the first n items an iterator returns

function head(n, f, s, var)

	local c = 0;

	return function (s, var)

		if c >= n then

			return nil;

		end

		c = c + 1;

		return f(s, var);

	end, s;

end

-- Skip the first n items an iterator returns

function skip(n, f, s, var)

	for i=1,n do

		var = f(s, var);

	end

	return f, s, var;

end

-- Return the last n items an iterator returns

function tail(n, f, s, var)

	local results, count = {}, 0;

	while true do

		local ret = { f(s, var) };

		var = ret[1];

	        if var == nil then break; end

		results[(count%n)+1] = ret;

		count = count + 1;

	end

	if n > count then n = count; end

	local pos = 0;

	return function ()

		pos = pos + 1;

		if pos > n then return nil; end

		return unpack(results[((count-1+pos)%n)+1]);

	end

	--return reverse(head(n, reverse(f, s, var)));

end

-- Convert the values returned by an iterator to an array

function it2array(f, s, var)

	local t, var = {};

	while true do

		var = f(s, var);

	        if var == nil then break; end

		table.insert(t, var);

	end

	return t;

end

-- Treat the return of an iterator as key,value pairs, 

-- and build a table

function it2table(f, s, var)

	local t, var = {};

	while true do

		var, var2 = f(s, var);

	        if var == nil then break; end

		t[var] = var2;

	end

	return t;

end