I'd like to extend my instrumental Profiler in order to avoid it affect too much performances.
Im my current implementation, I'm using a ProfilerHelper taking one string, which is put whereever you want in the profiling f().
The ctor is starting the measurement and the dector is closing it, logging the Delta in an unordered_map entry, which is key is the string.
Now, I'd like to turn all of that into a faster stuff.
First of all, I'd like to create a string LUT (Look Up Table) contaning the f()s names at compile time, and turn the unordered_map to a plain vector which is paired by the string function LUT.
Now the question is: I've managed to create a LUT but std::string_view, but I cannot find a way to extend it at compile time.
A first rought trial sounds like this:
template<unsigned N>
constexpr auto LUT() {
std::array<std::string_view, N> Strs{};
for (unsigned n = 0; n < N; n++) {
Strs[n] = "";
}
return Strs;
};
constexpr std::array<std::string_view, 0> StringsLUT { LUT<0>() };
constexpr auto AddString(std::string_view const& Str)
{
constexpr auto Size = StringsLUT.size();
std::array<std::string_view, Size + 1> Copy{};
for (auto i = 0; i < Size; ++i)
Copy[i] = StringsLUT[i];
Copy[Size] = Str;
return Copy;
};
int main()
{
constexpr auto Strs = AddString(__builtin_FUNCTION());
//for (auto const Str : Strs)
std::cout << Strs[0] << std::endl;
}
So my idea should be to recall the AddString whenever needed in my f()s to be profiled, extending this list at compile time.
But of course I should take the returned Copy and replace the StringsLUT everytime, to land to a final StringsLUT with all the f() names inside it.
Is there a way to do that at compile time?
Sorry, but I'm just entering the magic "new" world of constexpr applied to LUT right in these days.
Tx for your support in advance.
I was writing a lexer file that matches simple custom delimited strings of the form xyz$this is stringxyz. This is nearly how I did it:
%{
char delim[16];
uint8_t dlen;
%}
%%
.*$ {
dlen = yyleng-1;
strncpy(delim, yytext, dlen);
BEGIN(STRING);
}
<STRING>. {
if(yyleng >= dlen) {
if(strncmp(delim, yytext[yyleng-dlen], dlen) == 0) {
BEGIN(INITIAL);
return STR;
}
}
yymore();
}
%%
Now I wanted to convert this to reentrant lexer. But I don't know how to make delim and dlen as local variables inside yylex apart from modifying generated lexer. Someone please help me how should I do this.
I don't recommend to store these in yyextra because, these variables need not persist across multiple calls to yylex. Hence I would prefer an answer that guides me towards declaring these as local variables.
In the (f)lex file, any indented lines between the %% and the first rule are copied verbatim into yylex() prior to the first statement, precisely to allow you to declare and initialize local variables.
This behaviour is guaranteed by the Posix specification; it is not a flex extension: (emphasis added)
Any such input (beginning with a <blank>or within "%{" and "%}" delimiter lines) appearing at the beginning of the Rules section before any rules are specified shall be written to lex.yy.c after the declarations of variables for the yylex() function and before the first line of code in yylex(). Thus, user variables local to yylex() can be declared here, as well as application code to execute upon entry to yylex().
A similar statement is in the Flex manual section 5.2, Format of the Rules Section
The strategy you propose will work, certainly, but it's not very efficient. You might want to consider using input() to read characters one at a time, although that's not terribly efficient either. In any event, delim is unnecessary:
%%
int dlen;
[^$\n]{1,16}\$ {
dlen = yyleng-1;
yymore();
BEGIN(STRING);
}
<STRING>. {
if(yyleng > dlen * 2) {
if(memcmp(yytext, yytext + yyleng - dlen, dlen) == 0) {
/* Remove the delimiter from the reported value of yytext. */
yytext += dlen + 1;
yyleng -= 2 * dlen + 1;
yytext[yyleng] = 0;
return STR;
}
}
yymore();
}
%%
I am trying to learn the Dart language, by transposing the exercices given by my school for C programming.
The very first exercice in our C pool is to write a function print_alphabet() that prints the alphabet in lowercase; it is forbidden to print the alphabet directly.
In POSIX C, the straightforward solution would be:
#include <unistd.h>
void print_alphabet(void)
{
char c;
c = 'a';
while (c <= 'z')
{
write(STDOUT_FILENO, &c, 1);
c++;
}
}
int main(void)
{
print_alphabet();
return (0);
}
However, as far as I know, the current version of Dart (1.1.1) does not have an easy way of dealing with characters. The farthest I came up with (for my very first version) is this:
void print_alphabet()
{
var c = "a".codeUnits.first;
var i = 0;
while (++i <= 26)
{
print(c.toString());
c++;
}
}
void main() {
print_alphabet();
}
Which prints the ASCII value of each character, one per line, as a string ("97" ... "122"). Not really what I intended…
I am trying to search for a proper way of doing this. But the lack of a char type like the one in C is giving me a bit of a hard time, as a beginner!
Dart does not have character types.
To convert a code point to a string, you use the String constructor String.fromCharCode:
int c = "a".codeUnitAt(0);
int end = "z".codeUnitAt(0);
while (c <= end) {
print(String.fromCharCode(c));
c++;
}
For simple stuff like this, I'd use "print" instead of "stdout", if you don't mind the newlines.
There is also:
int char_a = 'a'.codeUnitAt(0);
print(String.fromCharCodes(new Iterable.generate(26, (x) => char_a + x)));
or, using newer list literal syntax:
int char_a = 'a'.codeUnitAt(0);
int char_z = 'z'.codeUnitAt(0);
print(String.fromCharCodes([for (var i = char_a; i <= char_z; i++) i]));
As I was finalizing my post and rephrasing my question’s title, I am no longer barking up the wrong tree thanks to this question about stdout.
It seems that one proper way of writing characters is to use stdout.writeCharCode from the dart:io library.
import 'dart:io';
void ft_print_alphabet()
{
var c = "a".codeUnits.first;
while (c <= "z".codeUnits.first)
stdout.writeCharCode(c++);
}
void main() {
ft_print_alphabet();
}
I still have no clue about how to manipulate character types, but at least I can print them.
I am trying to print cv::Mat which contains my image. However whenever I print the Mat using cout, a 2D array printed into my text file. I want to print one one pixel in one line only. How can i print line wise pixels from cv::Mat.
A generic for_each loop, you could use it to print your data
/**
*#brief implement details of for_each_channel, user should not use this function
*/
template<typename T, typename UnaryFunc>
UnaryFunc for_each_channel_impl(cv::Mat &input, int channel, UnaryFunc func)
{
int const rows = input.rows;
int const cols = input.cols;
int const channels = input.channels();
for(int row = 0; row != rows; ++row){
auto *input_ptr = input.ptr<T>(row) + channel;
for(int col = 0; col != cols; ++col){
func(*input_ptr);
input_ptr += channels;
}
}
return func;
}
use it like
for_each_channel_impl<uchar>(input, 0, [](uchar a){ std::cout<<(size_t)a<<", "; });
you could do some optimization to continuous channel, then it may looks like
/**
*#brief apply stl like for_each algorithm on a channel
*
* #param
* T : the type of the channel(ex, uchar, float, double and so on)
* #param
* channel : the channel need to apply for_each algorithm
* #param
* func : Unary function that accepts an element in the range as argument
*
*#return :
* return func
*/
template<typename T, typename UnaryFunc>
inline UnaryFunc for_each_channel(cv::Mat &input, int channel, UnaryFunc func)
{
if(input.channels() == 1 && input.isContinuous()){
return for_each_continuous_channels<T>(input, func);
}else{
return for_each_channel_impl<T>(input, channel, func);
}
}
This kind of generic loopsave me a lot of times, I hope you find it helpful.If there are
any bugs, or you have better idea, please tell me.
I would like to design some generic algorithms for opencl too, sadly it do not support
template, I hope one day CUDA will become an open standard, or opencl will support template.
This works for any number of channels as long as the channels type are base on byte, non-byte
channel may not work.
I've been trying to implement a BASIC language interpreter (in C/C++) but I haven't found any book or (thorough) article which explains the process of parsing the language constructs. Some commands are rather complex and hard to parse, especially conditionals and loops, such as IF-THEN-ELSE and FOR-STEP-NEXT, because they can mix variables with constants and entire expressions and code and everything else, for example:
10 IF X = Y + Z THEN GOTO 20 ELSE GOSUB P
20 FOR A = 10 TO B STEP -C : PRINT C$ : PRINT WHATEVER
30 NEXT A
It seems like a nightmare to be able to parse something like that and make it work. And to make things worse, programs written in BASIC can easily be a tangled mess. That's why I need some advice, read some book or whatever to make my mind clear about this subject. What can you suggest?
You've picked a great project - writing interpreters can be lots of fun!
But first, what do we even mean by an interpreter? There are different types of interpreters.
There is the pure interpreter, where you simply interpret each language element as you find it. These are the easiest to write, and the slowest.
A step up, would be to convert each language element into some sort of internal form, and then interpret that. Still pretty easy to write.
The next step, would be to actually parse the language, and generate a syntax tree, and then interpret that. This is somewhat harder to write, but once you've done it a few times, it becomes pretty easy.
Once you have a syntax tree, you can fairly easily generate code for a custom stack virtual machine. A much harder project is to generate code for an existing virtual machine, such as the JVM or CLR.
In programming, like most engineering endeavors, careful planning greatly helps, especially with complicated projects.
So the first step is to decide which type of interpreter you wish to write. If you have not read any of a number of compiler books (e.g., I always recommend Niklaus Wirth's "Compiler Construction" as one of the best introductions to the subject, and is now freely available on the web in PDF form), I would recommend that you go with the pure interpreter.
But you still need to do some additional planning. You need to rigorously define what it is you are going to be interpreting. EBNF is great for this. For a gentile introduction EBNF, read the first three parts of a Simple Compiler at http://www.semware.com/html/compiler.html It is written at the high school level, and should be easy to digest. Yes, I tried it on my kids first :-)
Once you have defined what it is you want to be interpreting, you are ready to write your interpreter.
Abstractly, you're simple interpreter will be divided into a scanner (technically, a lexical analyzer), a parser, and an evaluator. In the simple pure interpolator case, the parser and evaluator will be combined.
Scanners are easy to write, and easy to test, so we won't spend any time on them. See the aforementioned link for info on crafting a simple scanner.
Lets (for example) define your goto statement:
gotostmt -> 'goto' integer
integer -> [0-9]+
This tells us that when we see the token 'goto' (as delivered by the scanner), the only thing that can follow is an integer. And an integer is simply a string a digits.
In pseudo code, we might handle this as so:
(token - is the current token, which is the current element just returned via the scanner)
loop
if token == "goto"
goto_stmt()
elseif token == "gosub"
gosub_stmt()
elseif token == .....
endloop
proc goto_stmt()
expect("goto") -- redundant, but used to skip over goto
if is_numeric(token)
--now, somehow set the instruction pointer at the requested line
else
error("expecting a line number, found '%s'\n", token)
end
end
proc expect(s)
if s == token
getsym()
return true
end
error("Expecting '%s', found: '%s'\n", curr_token, s)
end
See how simple it is? Really, the only hard thing to figure out in a simple interpreter is the handling of expressions. A good recipe for handling those is at: http://www.engr.mun.ca/~theo/Misc/exp_parsing.htm Combined with the aforementioned references, you should have enough to handle the sort of expressions you would encounter in BASIC.
Ok, time for a concrete example. This is from a larger 'pure interpreter', that handles a enhanced version of Tiny BASIC (but big enough to run Tiny Star Trek :-) )
/*------------------------------------------------------------------------
Simple example, pure interpreter, only supports 'goto'
------------------------------------------------------------------------*/
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <setjmp.h>
#include <ctype.h>
enum {False=0, True=1, Max_Lines=300, Max_Len=130};
char *text[Max_Lines+1]; /* array of program lines */
int textp; /* used by scanner - ptr in current line */
char tok[Max_Len+1]; /* the current token */
int cur_line; /* the current line number */
int ch; /* current character */
int num; /* populated if token is an integer */
jmp_buf restart;
int error(const char *fmt, ...) {
va_list ap;
char buf[200];
va_start(ap, fmt);
vsprintf(buf, fmt, ap);
va_end(ap);
printf("%s\n", buf);
longjmp(restart, 1);
return 0;
}
int is_eol(void) {
return ch == '\0' || ch == '\n';
}
void get_ch(void) {
ch = text[cur_line][textp];
if (!is_eol())
textp++;
}
void getsym(void) {
char *cp = tok;
while (ch <= ' ') {
if (is_eol()) {
*cp = '\0';
return;
}
get_ch();
}
if (isalpha(ch)) {
for (; !is_eol() && isalpha(ch); get_ch()) {
*cp++ = (char)ch;
}
*cp = '\0';
} else if (isdigit(ch)) {
for (; !is_eol() && isdigit(ch); get_ch()) {
*cp++ = (char)ch;
}
*cp = '\0';
num = atoi(tok);
} else
error("What? '%c'", ch);
}
void init_getsym(const int n) {
cur_line = n;
textp = 0;
ch = ' ';
getsym();
}
void skip_to_eol(void) {
tok[0] = '\0';
while (!is_eol())
get_ch();
}
int accept(const char s[]) {
if (strcmp(tok, s) == 0) {
getsym();
return True;
}
return False;
}
int expect(const char s[]) {
return accept(s) ? True : error("Expecting '%s', found: %s", s, tok);
}
int valid_line_num(void) {
if (num > 0 && num <= Max_Lines)
return True;
return error("Line number must be between 1 and %d", Max_Lines);
}
void goto_line(void) {
if (valid_line_num())
init_getsym(num);
}
void goto_stmt(void) {
if (isdigit(tok[0]))
goto_line();
else
error("Expecting line number, found: '%s'", tok);
}
void do_cmd(void) {
for (;;) {
while (tok[0] == '\0') {
if (cur_line == 0 || cur_line >= Max_Lines)
return;
init_getsym(cur_line + 1);
}
if (accept("bye")) {
printf("That's all folks!\n");
exit(0);
} else if (accept("run")) {
init_getsym(1);
} else if (accept("goto")) {
goto_stmt();
} else {
error("Unknown token '%s' at line %d", tok, cur_line); return;
}
}
}
int main() {
int i;
for (i = 0; i <= Max_Lines; i++) {
text[i] = calloc(sizeof(char), (Max_Len + 1));
}
setjmp(restart);
for (;;) {
printf("> ");
while (fgets(text[0], Max_Len, stdin) == NULL)
;
if (text[0][0] != '\0') {
init_getsym(0);
if (isdigit(tok[0])) {
if (valid_line_num())
strcpy(text[num], &text[0][textp]);
} else
do_cmd();
}
}
}
Hopefully, that will be enough to get you started. Have fun!
I will certainly get beaten by telling this ...but...:
First, I am actually working on a standalone library ( as a hobby ) that is made of:
a tokenizer, building linear (flat list) of tokens from the source text and following the same sequence as the text ( lexems created from the text flow ).
A parser by hands (syntax analyse; pseudo-compiler )
There is no "pseudo-code" nor "virtual CPU/machine".
Instructions(such as 'return', 'if' 'for' 'while'... then arithemtic expressions ) are represented by a base c++-struct/class and is the object itself. The base object, I name it atom, have a virtual method called "eval", among other common members, that is the "execution/branch" also by itself. So no matter I have an 'if' statement with its possible branchings ( single statement or bloc of statements/instructions ) as true or false condition, it will be called from the base virtual atom::eval() ... and so on for everything that is an atom.
Even 'objects' such as variables are 'atom'. 'eval()' will simply return its value from a variant container held by the atom itself ( pointer, refering to the 'local' variant instance (the instance variant iself) held the 'atom' or to another variant held by an atom that is created in a given 'bloc/stack'. So 'atom' are 'inplace' instructions/objects.
As of now, as an example, chunk of not really meaningful 'code' as below just works:
r = 5!; // 5! : (factorial of 5 )
Response = 1 + 4 - 6 * --r * ((3+5)*(3-4) * 78);
if (Response != 1){ /* '<>' also is not equal op. */
return r^3;
}
else{
return 0;
}
Expressions ( arithemtics ) are built into binary tree expression:
A = b+c; =>
=
/ \
A +
/ \
b c
So the 'instruction'/statement for expression like above is the tree-entry atom that in the above case, is the '=' (binary) operator.
The tree is built with atom::r0,r1,r2 :
atom 'A' :
r0
|
A
/ \
r1 r2
Regarding 'full-duplex' mecanism between c++ runtime and the 'script' library, I've made class_adaptor and adaptor<> :
ex.:
template<typename R, typename ...Args> adaptor_t<T,R, Args...>& import_method(const lstring& mname, R (T::*prop)(Args...)) { ... }
template<typename R, typename ...Args> adaptor_t<T,R, Args...>& import_property(const lstring& mname, R (T::*prop)(Args...)) { ... }
Second: I know there are plenty of tools and libs out there such as lua, boost::bind<*>, QML, JSON, etc... But in my situation, I need to create my very own [edit] 'independant' [/edit] lib for "live scripting". I was scared that my 'interpreter' could take a huge amount of RAM, but I am surprised that it is not as big as using QML,jscript or even lua :-)
Thank you :-)
Don't bother with hacking a parser together by hand. Use a parser generator. lex + yacc is the classic lexer/parser generator combination, but a Google search will reveal plenty of others.