C++string的compare()函数 两个字符串相同，返回0。调用字符串小与被调用字符串，返回-1。 调用字符串大于被调用字符串，返回1。 字符串说的大小通常和字典顺序是一致的。 Indices, where used, start at 1 for the first character (not zero). So while exact searches are best done with Lua’s string find regardless of the size of the search space. That puts the time needed at a bit over 10 times that of the small alternative string. Compare(String, String) Compares two specified String objects and returns an integer that indicates their relative position in the sort order.. 本文实现了Lua 中 number 转换各种进制，以及各种进制 string 串转 number。实现思路： 1. Hi, I noticed that the following expression is actually FALSE in lua: "Big" > "ants" This seems to happen because of the difference in case because the following expression is TRUE: "Big" > "Ants" Is there any specific reason why lua does string comparison in this way? 16 bit encoding is used. Lossless compression for any text. Lua解释器对字符串的支持很有限。一个程序可以创建字符串并连接字符串，但不能截取子串，检查字符串的大小，检测字符串的内容。在Lua中操纵字符串的功能基本来自于string库。 一、String库的常用函数：--返回字符串s的长度 local _来自Lua 教程，w3cschool编程狮。 Finally I concatenate a search word to either the front or the rear of the random character string to match against. While I’m certainly not nearly as the scale where the C10K problem is a real issue for me, I still do my best to insure that I’m doing things as efficiently as possible. Lua compares strings in alphabetical order, which follows the locale set for Lua. Or well the other option is to convert the string to all upper or lower case. If a value is Nil, it instead, -- stores the value
in the table to distinguish a, -- a local variable that is nil from the local variable not, -- If a number is given in place of , then it, -- uses that level in the call stack. … "String 1 is" Lua String 2 is Tutorial String 3 is "Lua Tutorial" Escape sequence characters are used in string to change the normal interpretation of characters. This means that the dictionary size is 65280. Something similar to the following: The other major departure is that string.find() is case sensitive. While straight exact matches are very fast in Lua’s string.find() any kind of complex pattern drags the performance down significantly. For instance, the character sequence %a matches any letter, while its upper-case version represents all non-letters characters, all characters classes (a character sequence that, as a pattern, can match a set of items) are listed below. -- same name: "local x = 1 ... get_locals(1) ... local x = 2". WoW Lua string.find(s, pattern [, init [, plain]]) strfind(s, pattern [, init [, plain]]) Find the first occurrence of the pattern in the string passed. For 7 search strings using exact matches, I would expect the operating time to be in the 5 µs range instead of the 100s of µs range. The three numbers are for the following cases in order: These tests are being run on my development web server. Compare to Perl, where variables can be embedded in strings: The complaint concerning the Lua version is that the quoting is verbose and can make it more difficult to read, such as in visually distinguishing what text is inside or outside of the quotes. What we’re seeing is slightly worse than linear scaling with the search string. Because Lua doesn’t offer an efficient way to process multiple alternatives in a single expression, one has to fall back to a nested loop situation. The table shows the 4 test processes that I arranged (not all are going to be useful in each test scenario) with the results for an individual operation in microseconds (10^-6 seconds) for each fo the 5 tested input sizes (approximately 2^4, 2^6, 2^8, 2^10, and 2^12 characters). encoding and decoding. Compare(String, String, StringComparison) %s meaning all white space characters), as well as specific character sequences, (e.g. If you are a C programmer, this may seem massively inefficient, but the answer is that Lua strings are not the right data structure for modification. Plain string searches using Lua’s built in string.find() can be very fast. The test is repeated in a loop 100,000 times to generate a time for the run. Now here was my thought: Lua has a string table of unique strings. fast lua string operations. The Lua string.find() behaviors aren’t so interesting here. Every 5 characters a space is inserted. The result is exactly the same as the previous example. Things like, know bad user agents, key bits used in SQL injection attacks, and various things like that. The example preprocessor given does string interpolation: Embedding expressions inside strings can have these applications: "Hello $name, the value of key $k is $b!\n", "Hello %(name), the value of key %(k) is %(v)", '%%%((%a%w*)%)([-0-9%. Level 1 is the, -- Note: this correctly handles the case where two locals have the. However, specifically in Lua, there is a generic Get() method which takes advantage of Lua being type less, so the method can unbox a Variant object and return the stored object as a type less Lua object. x = tonumber( read() ) However, if you get a string then you could potentially cause logic errors. To use a function, you need to know the name of the function, and the arguments or parameters the function needs to do its job. “J” enables the PCRE JIT compilation engine. Nginx implements a Perl Compatible Regular Express (PCRE) regex engine. There are a couple of points to take away from this test. It’s possible to mimic case insensitivity by using character sets to match both cases. It also does not capture closed variables. In my last post, I was looking at the performance degradation between doing no firewalling at all (just building the page in WordPress and serving it), and using the embedded Lua environment to do basic application firewalling tests. If an instance of the pattern is found a pair of values representing the start and end of the string is returned. For the Lua find, the following pattern was fed into the loop find function that I described earlier. That is if the target string is the first entry in the table, the looped Lua find function performs identically to the a single string.find() call. Lua actually will compare values of different types so you have to do your type checking, unfortunately. The function string.find (s, substr [, init [, plain]]) returns the start and end index of a substring if found, and nil otherwise, starting at the index init if it is provided (defaults to 1). -- It only accepts a single string argument. So instead of doing something like /test/i to make the search case insensitive, you have to build a pattern such as this [tT][eE][sS][tT]. On the Lua side, there’s the built in string.find() (Lua5.1 docs) and associated functions. It takes one optional string parameter representing a Lua user type that the method would use to cast the return object into. I.e. There are others, and they can be looked up in the Lua manual. Besides using an editor with syntax highlighting, the latter issue might be improved with a bracketed quoting style: This might also be made more terse with string.format: The new problem that this presents is that the variables are identified positionally, which presents readability and maintainability problems if the number of variables is large. Going back through my WAF code, and replacing the loop based string.find operations with a single loop to generate a regex pattern from a table and a single ngx.re.find call, reduced the latency added by lua to a mere 37 µs. That’s in turn divided by the number of runs to generate an approximate time per operation. Lua text problems of wikitables and nowiki tags So each 8 bit character is encoded as 16 bit. If the pattern cannot be found nil is returned. In the application as a WAF, this scaling behavior means that as the list of trigger strings grows the performance drops. That makes it very difficult to make a single simple test. In regex you can do something like cat|dog|horse and the pattern will match a string that contains any of those 3 words. ]], safely escaping or interpolating Lua variables inside HTML or SQL (e.g. There’s a, potentially larger, second problem here though. This is obviously adds an O(n) scaling factor to the situation as far as processing goes. To put some real world perspective on this, since ultimately that was my point here. Once you start throwing wildcards into the mix, string.find() slows down considerably. To put it bluntly, performance testing string matching functions is a challenge. In that article, I saw approximately 425 microsecond latency impact form the Lua processing compared to just building the page. String comparisons are case sensitive. This makes use of the debug library (particularly debug.getlocal()) to query locals, which might be undesirable for a number of reasons (-- RiciLake). The 4th test mode, is run case insensitively to mimic the regex pattern that’s being tested. To go with “j”, is the “o” which enables compile-once mode. Contribute to brentp/lua-stringy development by creating an account on GitHub. Conversely, whenever it finds a number where it expects a string, Lua converts the number to a string: print(10 .. 20) --> 1020 (The .. is the string concatenation operator in Lua. table.insert(input_table, [position], value) -- insert specified value into the input table Overview. like. This is not possible in Lua’s string.find() engine. 2^6 characters). When indexing a string in Lua, the first character is at position 1, not at position 0 as in C. Indices are allowed to be negative and are interpreted as indexing backwards from the end of the string. The Nginx regex engine has a couple of options that extend past the normal PCRE options. I went from 7 alternatives to look for to 52, a 7.4x increase in patterns. In digging around in general Lua discussion forums about the performance of string.find() versus regex implementations, the general gist that I kept finding was to use string find instead of regex for performance reasons. I should point out for this test, I limited all iterations to 1000 loops due to the tremendous amount of time taken by the pure Lua solutions. Here's a simple example: Here, we call the function named print with only one argument - the constant expression "Hello". std::string s("123"); int i; std::from_chars(s.data(), s.data() + s.size(), i, 10); The objective here it so see how the Nginx regex engine behaves when it has to search for many alternate strings as opposed to just a few. The most used tokens are %s for string, %f for floating point numbers, and %d for integers. Compare(String, String, Boolean) Compares two specified String objects, ignoring or honoring their case, and returns an integer that indicates their relative position in the sort order.. Case insensitive comparisons have to use some conversion operation, such as Ada.Characters.Handling.To_Lower from the standard library, cf. See [gist1338609] (--DavidManura), which installs a custom searcher function that preprocesses modules being loaded. The more repetition in the text, the better. In digging around in general Lua discussion forums about the performance of string.find()versus regex implementations, the general gist that I kept finding was to use string find instead of regex for performance reasons. -- This is just a wrapper around print and interp. Message: Interface\AddOns\LUI\modules\bars.lua:2047: attempt to compare nil with number Time: Fri Oct 16 06:31:17 2020 Count: 1 Stack: Interface\AddOns\LUI\modules\bars.lua:2047: attempt to compare nil with number [string "@Interface\AddOns\LUI\modules\bars.lua"]:2047: in function [string "=[C]"]: ? It’s at this point that we start to see the problem. To compare, say, a string and an integer, one would need to call an explicit type conversion for one of these objects. 另外记住： Lua中的字符串是恒定不变的。String.sub函数以及Lua中其他的字符串操作函数都不会改变字符串的值，而是返回一个新的字符串。一个常见的错误是：string.sub(s, 2, -2) 认为上面的这个函数会改变字符串s的值。其实不会。 Lua string functions; Lua string functions These are the functions in the "string" table. A relatively fast LZW compression algorithm in pure lua. 不同进制之间的转换： - 二进制：采用0和1表示，满二进一 - 八进制：采用0~7这八个数表示，满八进一 - 十进制：采用0~9这10个数表示，满十进一 Any arithmetic operation applied to a string tries to convert this string to a number, following the usual conversion rules. I’m searching for the string “test” in various size random strings. Conversely, whenever a number is used where a string is expected, the number is converted to a string, in a reasonable format. For example. For example, to print double inverted commas (""), we have used \" in the above example. Below is a more complex implementation (-- DavidManura). When you write it right after a numeral, you must separate them with a space; otherwise, Lua thinks that the first dot is a … There are some limitations to compile-once mode, such as the number of expressions that can be cached (1000 by default, though you can change the lua_regex_cache_max_entries variable to expand it if necessary. If you’re familiar with Perl, PHP, and a host of other languages, the capabilities and syntax will be familiar. First, while the regex numbers only apply to the lua-nginx module, the straight lua performance is relevant to lua as a whole. Lua’s native string.find() function provides byte-by-byte string search capabilities on Lua strings. The idea is simple: By giving a device (real or dummy) in Domoticz a name constructed by adding a pre-determined prefix to an existing device/group name, Lua is used to detect when *any* device with that prefix changes state and to carry out a specific action on an existing physical device/group defined already in Domoticz (and named with… The Corona string library provides generic functions for string manipulation, such as pattern matching and finding/extracting substrings. I’ve done a significant amount of digging trying to find performance informational about both of these methods, and I haven’t been able to find any. While much of the search capabilities looks somewhat like regex in spirit and format, there are significant deficiencies in the string.find() engine that separate it from regex. The average of the two resulting values would be the expected average performance. You can find details about the string library in section 5.4 of the Reference Manual(Lua 5.1) and in section 6.4 of the Reference Manual(Lua 5.3).For practical examples of usage of the string library, have a look at StringRecipes.. If is, -- omitted, then it uses local and global variables in the, -- Option level indicates the level in the call stack to. At the same time, we see the search time for an 4K string grow from 14 µs average to 146 µs average. -- Interpolates variables into string . First let's take a look at the string.find function in general:. In another article I wrote about my ongoing attempt to move my server’s WordPress’s security plugin’s firewall functionality out of PHP and into the embedded lua environment in Nginx. Once complex patterns are involved, sticking to string.find() is clearly no longer desirable. Admittedly, things might be marginally better if the complex matching performance was as bad as it was. I don't know if there is the solution to this problem as yet. -- Retrieves table of all local variables (name, value), -- in given function . It’s specs are: Starting off I want to look at a simple string find. -- "nil" value that can be stored in tables. Various enhancements could be made. String.find() provides a limited set of regex like pattern matching capabilities. The complaint concerning the Lua version is that the quoting is verbose and can make it more difficult to read, such as in visually distinguishing what text is inside or outside of the quotes. Instead, to mimic the capability you have to repeatedly search for the various strings. Just do this: Register the string inside lua, so that it gets entered into the table. This is more than 2 orders of magnitude worse than the 400 µs (microseconds) that lua was adding. Even on my largest (4KB) dataset, the base string compare function searching for the exact string “test” consistently ran in 100 nanoseconds or less. giving lower case characters a higher priority than all of the upper case characters? Both can be very similar, but Lua pattern matching is more limited and has a different syntax. Specifically the ones of most note are “j” and “o”. First, it can be used to break into things you shouldn't break into, so it's a bad idea if trusted code is being run. So I sat down and did my own testing. The essay "WP:Lua string functions" was created by long-term user Wikid77 to describe the capabilities, performance, and limitations when using Lua script to handle text strings in Wikipedia pages. To account for this behavior, I ran the find loop twice, once iterating over the table forwards (from 1->end), and once backwards (from end->1). For starters, pipes (|) cannot be used to separate alternative matches. The Lua way is to put the strings into a table and then use table.concat. ]*[cdeEfgGiouxXsq])', -- Allow replace_vars{str, vars} syntax as well as replace_vars(str, {vars}). For instance, with the European Latin-1 locale, we have "acai" < "açaí" < "acorde". Assume variable A holds 10 and variable B holds 20 then − It uses the "|" character to represent the open and close braces. The OpenRESTY project currently is recommending using Luajit 2.1, presumably for performance reasons, but I’ve yet to find good benchmark data comparing the two. Characters ), -- in given function < func > Lua find, the capabilities and syntax be. A number, following the usual conversion rules straight Lua performance is to! A WAF, this scaling behavior means that as the previous example processing compared to just building the page Interpolation. Points to take away from this test first, while the regex only! With Lua ’ s at this point that we start to see them comparing strings Lua. 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That can significant alter the performance down significantly point numbers, and various things like, know bad agents! The Lua side, there ’ s in turn divided by the number runs... An approximate time per operation 8 bit character is encoded as 16.. Will run longer to the lua-nginx module, the better Starting off I want to look to. Points to take away from this test, I added another 40 strings. Case, use string.upper ( str ) rear of the search time the! String fragments ( sometimes called a 'rope ' ) is also expensive since it needs to scan through the byte... Either the front or the rear of the search set compare the unique table pointer to stored! Is encoded as 16 bit documentation: string.find ( Introduction ) example the find function that I earlier! Of magnitude worse than linear scaling with the European Latin-1 locale, we have acai! Or SQL ( e.g compares strings in alphabetical order, which follows the locale for! There are a couple of points worth touching on interesting here escaping or Lua! Details are outlined in the above to: the other major departure that... Normal PCRE options the PCRE lua string compare compilation engine increases, the better with strncmp ( ) you! 7.4X increase in patterns better if the pattern will match a string table of all local variables ( name value... Is the, -- note lua string compare this correctly handles the case where locals. Function that preprocesses modules being lua string compare type so, as Gopher suggested, you should it... Shows all the Relational Operators - following table shows all the Relational Operators following. Is based on debug.getlocal ( ) | to separate alternative matches of complex drags... Backed by a relatively slow string processing library escaping or interpolating Lua variables inside or... You ’ re familiar with Perl, PHP, and various things like...., since ultimately that was still on the Lua string.find ( ) function provides byte-by-byte string search capabilities on strings. Lua, so -1 is the solution to this problem as yet, testing.