lecram

Useful Lua Recipes

Last updated on 2015-04-25 12:09:48 -0300

1. Iterators Via Coroutines

local function count(n)
    return coroutine.wrap(
        function ()
            for i = 1, n do
                coroutine.yield(i)
            end
        end
    )
end

for i in count(10) do
    print(i)
end

2. Skew Heap

http://lua-users.org/lists/lua-l/2007-11/msg00375.html

local function qmerge(a, b)
    if not (a and b) then
        return a or b
    end
    if a.key < b.key then
        a.left, a.right = a.right, qmerge(a.left, b)
        return a
    else
        b.left, b.right = b.right, qmerge(a, b.left)
        return b
    end
end

local function qpush(q, k, v)
    return qmerge(q, {key=k, val=v})
end

local function qpop(q)
    return qmerge(q.left, q.right)
end

q = nil

q = qpush(q, 5, "a")
q = qpush(q, 2, "b")
q = qpush(q, 7, "c")
q = qpush(q, 1, "d")

while q ~= nil do
    print(q.val)
    q = qpop(q)
end

3. Sleep

http://luajit.org/ext_ffi_tutorial.html

local ffi = require "ffi"

ffi.cdef[[
void Sleep(int ms);
int poll(struct pollfd *fds, unsigned long nfds, int timeout);
]]

local sleep
if ffi.os == "Windows" then
  function sleep(s)
    ffi.C.Sleep(s*1000)
  end
else
  function sleep(s)
    ffi.C.poll(nil, 0, s*1000)
  end
end

4. Parsing List of Integers

local function get_nums(str)
    local nums = {}
    for s in string.gmatch(str, "-?%d+") do
        nums[#nums+1] = tonumber(s)
    end
    return unpack(nums)
end

local w, h, x, y = get_nums("100 234 -20 0")

5. Converting UTF-8 String to Code Sequence

http://lua-users.org/wiki/LuaUnicode

local function utf8to32(utf8str)
    assert(type(utf8str) == "string")
    local res, seq, val = {}, 0, nil
    for i = 1, #utf8str do
        local c = string.byte(utf8str, i)
        if seq == 0 then
            table.insert(res, val)
            seq = c < 0x80 and 1 or c < 0xE0 and 2 or c < 0xF0 and 3 or
                  c < 0xF8 and 4 or --c < 0xFC and 5 or c < 0xFE and 6 or
                  error("invalid UTF-8 character sequence")
            val = bit.band(c, 2^(8-seq) - 1)
        else
            val = bit.bor(bit.lshift(val, 6), bit.band(c, 0x3F))
        end
        seq = seq - 1
    end
    table.insert(res, val)
    return res
end

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