I'm porting an Objective-C iOS application to CoronaSDK which uses Lua. I have a signed 16-bit integer and I want to isolate the high and low bytes in it. In C you can simply do:
uint8_t a = (dx >> 8) & 0xff;
uint8_t b = dx & 0xff;
Lua has no bitwise operators. How can I do this in Lua? This is for a mobile application and external binary libraries to extend Lua would most likely not work in all devices, if any. Any pure Lua solutions to this problem that you could suggest?
I don't know Lua but can give you a general idea. Right shift by n is equivalent to divide by 2^n. You can first divide by 2^8 = 256 to get the MSBs and then multiply and divide by 2^8 to get LSBs.
MSBs = dx / 256
LSBleftsifted = (ds * 256)
LSBs = LSBleftsifted / 256
This is considering that all this is an unsigned integer operation.
Based on #ash answer, here are a couple of Lua functions. I'm sure they can be generalized, but this does what I needed:
local function getLowByte16( value )
local high_value = math.floor(value/256)
high_value = high_value*256
local low_value = value - high_value
return low_value
end
local function getHighByte16( value )
local high_value = math.floor(value/256)
return high_value
end
Related
Assuming I have a declaration like this: final int input = 0xA55AA9D2;, I'd like to get a list of [0xA5, 0x5A, 0xA9, 0xD2]. It is easily achievable in Java by just right shifting the input by 24, 16, 8 and 0 respectively with subsequent cast to byte in order to cut precision to 8-bit value.
But how to do the same with Dart? I can't find sufficient information about numbers encoding (e.g. in Java front 1 means minus, but how is minus encoded here?) and transformations (e.g. how to cut precision) in order to solve this task.
P.S.: I solved this for 32-bit numbers using out.add([value >> 24, (value & 0x00FFFFFF) >> 16, (value & 0x0000FFFF) >> 8, value & 0X000000FF]); but it feels incredibly ugly, I feel that SDK provides more convenient means to split an arbitrarily precised number into bytes
The biggest issue here is that a Dart int is not the same type on the VM and in a browser.
On the native VM, an int is a 64-bit two's complement number.
In a browser, when compiled to JavaScript, an int is just a non-fractional double because JavaScript only has doubles as numbers.
If your code is only running on the VM, then getting the bytes is as simple as:
int number;
List<int> bytes = List.generate(8, (n) => (number >> (8 * n)) & 0xFF);
In JavaScript, bitwise operations only work on 32-bit integers, so you could do:
List<int> bytes = List.generate(4, (n) => (number >> (8 * n)) & 0xFF);
and get the byte representation of number.toSigned(32).
If you want a number larger than that, I'd probably use BigInt:
var bigNumber = BigInt.from(number).toSigned(64);
var b255 = BigInt.from(255);
List<int> bytes = List.generate(8, (n) => ((bigNumber >> (8 * n)) & b255).toInt());
From the documentation to the int class:
The default implementation of int is 64-bit two's complement integers with operations that wrap to that range on overflow.
Note: When compiling to JavaScript, integers are restricted to values that can be represented exactly by double-precision floating point values. The available integer values include all integers between -2^53 and 2^53 ...
(Most modern systems use two's complement for signed integers.)
If you need your Dart code to work portably for both web and for VMs, you can use package:fixnum to use fixed-width 32- or 64-bit integers.
I want to apply a bitwise AND operation in 64 bits in Lua 5.1. Is there an algorithm for it? (I have no idea on how to do it.)
Note: I only need to operate on 48 bits at total, and I am not having trouble with them.
In the game's Lua I'm scripting there's the bit32 library only.
local function band48 (x, y)
local xl = x % 4294967296
local yl = y % 4294967296
local xh = (x - xl) / 4294967296
local yh = (y - yl) / 4294967296
return bit32.band(xh, yh) * 4294967296 + bit32.band(xl, yl)
end
print(band48(7 * 2^33 + 3, 5*2^33 + 5)) --> 5*2^33+1 = 42949672961
Lua is using double floating numbers internally by default. It has only 52 bits for mantissa, so you can't safely store 64-bit integers without risking to get invalid floating point values. With 32 bits it's quite safe. Lua 5.2 manuals describe what happens in bit32 lib with the numbers:
Unless otherwise stated, all functions accept numeric arguments in the
range (-2^51,+2^51); each argument is normalized to the remainder of its
division by 2^32 and truncated to an integer (in some unspecified way),
so that its final value falls in the range [0,2^32 - 1]. Similarly, all
results are in the range [0,2^32 - 1].
You'll have to work in 32-bit chunks.Or maybe introduce your own 64-bits type, probably hosted with userdata, and define 64-bit actions for that type.
I have a lua function that converts ip addresses to 32 bit int
local str = "127.0.0.1"
local o1,o2,o3,o4 = str:match("(%d%d?%d?)%.(%d%d?%d?)%.(%d%d?%d?)%.(%d%d?%d?)" )
local num = 2^24*o1 + 2^16*o2 + 2^8*o3 + o4
I would like to have the inverse function, i.e. get the 4 bytes from the int
You can use bit or bit32 libraries (included in Lua 5.2+ and LuaJIT and available as modules for 5.1). You can also use the reverse operations to what you already have:
print(math.floor(num / 2^24), math.floor((num % 2^24) / 2^16),
math.floor((num % 2^16) / 2^8), num % 2^8)
Use string.unpack/pack to convert most primitive types to or from byte array (string in Lua).
Even though Lua does not differentiate between floating point numbers and integers, there are some cases when you want to use integers. What is the best way to covert a number to an integer if you cannot do a C-like cast or without something like Python's int?
For example when calculating an index for an array in
idx = position / width
how can you ensure idx is a valid array index? I have come up with a solution that uses string.find, but maybe there is a method that uses arithmetic that would obviously be much faster. My solution:
function toint(n)
local s = tostring(n)
local i, j = s:find('%.')
if i then
return tonumber(s:sub(1, i-1))
else
return n
end
end
You could use math.floor(x)
From the Lua Reference Manual:
Returns the largest integer smaller than or equal to x.
Lua 5.3 introduced a new operator, called floor division and denoted by //
Example below:
Lua 5.3.1 Copyright (C) 1994-2015 Lua.org, PUC-Rio
>12//5
2
More info can be found in the lua manual
#Hofstad is correct with the math.floor(Number x) suggestion to eliminate the bits right of the decimal, you might want to round instead. There is no math.round, but it is as simple as math.floor(x + 0.5). The reason you want to round is because floats are usually approximate. For example, 1 could be 0.999999996
12.4 + 0.5 = 12.9, floored 12
12.5 + 0.5 = 13, floored 13
12.6 + 0.5 = 13.1, floored 13
local round = function(a, prec)
return math.floor(a + 0.5*prec) -- where prec is 10^n, starting at 0
end
why not just use math.floor()? it would make the indices valid so long as the numerator and denominator are non-negative and in valid ranges.
I need a base converter function for Lua. I need to convert from base 10 to base 2,3,4,5,6,7,8,9,10,11...36 how can i to this?
In the string to number direction, the function tonumber() takes an optional second argument that specifies the base to use, which may range from 2 to 36 with the obvious meaning for digits in bases greater than 10.
In the number to string direction, this can be done slightly more efficiently than Nikolaus's answer by something like this:
local floor,insert = math.floor, table.insert
function basen(n,b)
n = floor(n)
if not b or b == 10 then return tostring(n) end
local digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
local t = {}
local sign = ""
if n < 0 then
sign = "-"
n = -n
end
repeat
local d = (n % b) + 1
n = floor(n / b)
insert(t, 1, digits:sub(d,d))
until n == 0
return sign .. table.concat(t,"")
end
This creates fewer garbage strings to collect by using table.concat() instead of repeated calls to the string concatenation operator ... Although it makes little practical difference for strings this small, this idiom should be learned because otherwise building a buffer in a loop with the concatenation operator will actually tend to O(n2) performance while table.concat() has been designed to do substantially better.
There is an unanswered question as to whether it is more efficient to push the digits on a stack in the table t with calls to table.insert(t,1,digit), or to append them to the end with t[#t+1]=digit, followed by a call to string.reverse() to put the digits in the right order. I'll leave the benchmarking to the student. Note that although the code I pasted here does run and appears to get correct answers, there may other opportunities to tune it further.
For example, the common case of base 10 is culled off and handled with the built in tostring() function. But similar culls can be done for bases 8 and 16 which have conversion specifiers for string.format() ("%o" and "%x", respectively).
Also, neither Nikolaus's solution nor mine handle non-integers particularly well. I emphasize that here by forcing the value n to an integer with math.floor() at the beginning.
Correctly converting a general floating point value to any base (even base 10) is fraught with subtleties, which I leave as an exercise to the reader.
you can use a loop to convert an integer into a string containting the required base. for bases below 10 use the following code, if you need a base larger than that you need to add a line that mapps the result of x % base to a character (usign an array for example)
x = 1234
r = ""
base = 8
while x > 0 do
r = "" .. (x % base ) .. r
x = math.floor(x / base)
end
print( r );