I am making a chess game and I'm looking to return a mutable null character from an array of pieces when the index of the array (a Vec2 is out of bounds), the reason I need to do this is that my function for moving the piece needs a mutable reference to the Indexed piece, long story short I ended up needing to create a static NULL_PIECE that I could reference within the function but this is potentially quite dangerous as you'll see from my code
impl Index<IVec2> for Board {
type Output = Piece;
fn index(&self, index : IVec2) -> &Self::Output{
if (index.abs() != index) || (index.max_element() > WIDTH-1) {
&Piece('\0') // this works
} else {
let i : usize = (index.x + WIDTH* index.y).try_into().unwrap();
&self.pieces[i]
}
}
}
impl IndexMut<IVec2> for Board {
fn index_mut(&mut self, index: IVec2) -> &mut Self::Output{
if (index.abs() != index) || (index.max_element() > WIDTH-1) {
// &mut Piece('\0') // this does not work
unsafe {&mut NULL_PIECE} // this works but I don't like it
} else {
let i : usize = (index.x + WIDTH * index.y).try_into().unwrap();
&mut self.pieces[i]
}
}
}
There is a lot of potential for this to cause an error in the event that this mutates to be a piece because of the recursion I've implemented on the piece movement.
You can find the GitHub link here:
https://github.com/LyndonAlcock/chess_test/tree/main/src
Instead of implementing Index you could write it as:
impl Board {
fn get(&self, index: IVec2) -> Option<&Piece> {
if (index.abs() != index) || (index.max_element() > WIDTH-1) {
None
} else {
let i = (index.x + WIDTH* index.y).try_into().ok()?;
Some(&self.pieces[i])
}
}
fn get_mut(&mut self, index: IVec2) -> Option<&mut Piece> {
if (index.abs() != index) || (index.max_element() > WIDTH-1) {
None
} else {
let i = (index.x + WIDTH * index.y).try_into().ok()?;
Some(&mut self.pieces[i])
}
}
}
Index implementations should panic when the index is out of bounds.
Related
I have some tests which have some variables that hold some important data and I'd like to print their data when an assertion fails. Getting the data I need consumes the variables, so the printing code must own the variables. In this example, I'd want to call dump_foo_data once an assertion fails:
struct Foo();
fn dump_foo_data(f: Foo) {
eprintln!("Behold, Foo data: ");
}
#[test]
fn my_test() {
let f = Foo();
eprintln!("begin");
// do a test
&f;
let success = true;
assert!(success);
// do another test
&f;
let success = false;
assert!(success);
}
I can make a very bad solution by making dump_foo_data non-returning and panic:
fn dump_foo_data(f: Foo) -> ! {
eprintln!("Behold, Foo data: ");
panic!();
}
Then instead of using assert!, I check the failure with an if and maybe call dump_foo_data:
let success = true;
if !success {
dump_foo_data(f);
}
This is too many lines of code, and I need to specify f. In reality, I have more than one variable like f that I need to dump data from, so it's not very nice to list out single relevant local variable in every check.
I couldn't figure out how to write a macro to make this better because I'd still need to pass every relevant local variable to the macro.
I couldn't think of a way to use std::panic either. update_hook would need to take ownership of f, then I couldn't use it in tests.
Is there any good way to do this in Rust?
Edit: I've thought of another approach: put each relevant local in an Rc then pass each of those to std::panic::update_hook. I've not confirmed whether this'll work yet.
Edit 2: Maybe I could abuse break to do what I explained with goto in a comment.
One way that doesn't use any macro or shared-interior-mutability-reference magic might be to repossess f:
fn check_or_dump(success: bool, f: Foo) -> Foo {
match success {
true => f,
false => panic!("Behold foo data: {:?}", dump_foo_data(f)),
}
}
You use it like this:
let f = Foo();
let success = true;
let f = check_or_dump(success, f);
let success = false;
let f = check_or_dump(success, f);
// and so on.
Here's a solution without macro or interior mutability and that doesn't require you to list all the variables on each check. It is inspired by this answer:
struct Foo();
fn dump_foo_data(_f: Foo) {
eprintln!("Behold, Foo data: ");
}
#[test]
fn my_test() {
let f = Foo();
let doit = || -> Option<()> {
eprintln!("begin");
// do a test
&f;
let success = true;
success.then_some(())?;
// do another test
&f;
let success = false;
success.then_some(())?;
Some(())
};
if let None = doit() {
dump_foo_data (f);
panic!("Test failure");
}
}
Playground
I've worked out a solution using the panic handler:
use std::rc::Rc;
use std::cell::{Cell, RefCell};
use std::panic::PanicInfo;
thread_local! {
static TL_PANIC_TARGETS: RefCell<Vec<Rc<dyn PanicTrigger>>> = RefCell::new(vec![]);
}
pub trait PanicTrigger {
fn panic_trigger(self: Rc<Self>);
}
pub fn register_panic_trigger<P: PanicTrigger + 'static>(p: Rc<P>) {
TL_PANIC_TARGETS.with(|v: _| {
v.borrow_mut().push(p.clone());
});
}
#[ctor::ctor]
fn set_panic_hook() {
let old_hook = std::panic::take_hook();
std::panic::set_hook(Box::new(move |pi: &PanicInfo| {
run_panic_triggers(pi);
old_hook(pi);
}));
}
fn run_panic_triggers(_: &PanicInfo) {
TL_PANIC_TARGETS.with(|v: _| {
for pt in v.take() {
pt.panic_trigger();
}
});
}
struct Foo();
fn dump_foo_data(_f: Foo) {
eprintln!("Behold, Foo data: ");
}
impl PanicTrigger for Cell<Option<Foo>> {
fn panic_trigger(self: Rc<Self>) {
if let Some(f) = self.take() {
dump_foo_data(f);
}
}
}
#[test]
fn my_test() {
let f = Rc::new(Cell::new(Some(Foo())));
register_panic_trigger(f.clone());
let success = true;
assert!(success);
let success = false;
assert!(success);
}
fn main() { }
Basically, you put the relevant data in an Rc and keep a local reference and put one in TLS for the panic handler. You need to put it in an Option in a Cell so that you can move out of it.
Types that don't need to be owned to print relevant data can be registered too, and you don't need to implement PanicTrigger on a Cell<Option<T>>, just T.
This is thread-safe.
Because the data is so wrapped up, it's harder to manipulate in the test body. But now you can use normal assert!. It's a trade-off.
I have this snippet:
let mut animation_index = 0 as usize;
let mut ptr : *mut usize = &mut animation_index as _;
{
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
unsafe {
*ptr += 1;
println!("{}", animation_index);
}
}
})
);
}
Basically it adds a callback such that if and when I press space, the integer variable animation_index goes up by 1. This works, but requires the use of mutable pointers and unsafe, which is very ugly.
I'd like to have the same logic but ideally do it with pure safe rust isntead.
It looks like you are trying to share a mutable value across threads.
Typically, this is done with atomics, Arc<Mutex<T>> or Arc<RwLock<T>>.
use std::synce::{Arc, RwLock};
let mut animation_index = Arc::new(RwLock::new(0usize));
{
// a clone of the counter that can be moved into the callback
let animation_index = animation_index.clone();
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
let index = animation_index.write().unwrap();
*index += 1;
println!("{}", index);
}
})
);
}
With atomics it would look something like this:
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
let mut animation_index = Arc::new(AtomicUsize::new(0));
{
// a clone of the counter that can be moved into the callback
let animation_index = animation_index.clone();
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
let index = animation_index.fetch_add(1, Ordering::SeqCst);
println!("{}", index);
}
})
);
}
Need to create a function that implements the attached algorithm, to which all words are passed in the function arguments.
For example:
f ("dfd" dd "ddd");
My code:
fun main() {
var s = readLine();
var w = Array(128){0} //To mark characters from a word 1
var g = Array(128){0}//When we encounter a space, we add units from the first array to the corresponding elements of the second, zeroing them in the first.
if(s!=null)
{
for(c in s)
{
if(c.toInt() > 127 || c.toInt()<0) {
println("Input error, try again");
return;
}
//Checking for space.
if(c.toInt() != 32) w[c.toInt()] = 1;
else
for(k in 0..127)
{
if(w[k] == 1)
{
g[k] += 1;
w[k] = 0;
}
}
}
//For the last word, if there was no space after it.
for(k in 0..127)
{
if(w[k] == 1)
{
g[k] += 1;
w[k] = 0;
}
}
}
//Displaying matched characters to the screen
for(k in 0..127)
{
if(g[k]>1)
{
println(k.toChar());
}
}
}
This program searches for characters that match at least two words in a string
Example
input: hello world
output: lo
There's already utilities for these in Kotlin, I highly recommend you to read the docs before asking these type of questions.
The groupingBy should do what you want:
readLine()?.let { input ->
input.groupingBy { it }.eachCount()
.forEach { if (it.value > 1 && it.key != ' ') println(it.key) }
}
The program aims to use a loop to check if the index of a iterator variable meets certain criteria (i.g., index == 3). If find the desired index, return Some(123), else return None.
fn main() {
fn foo() -> Option<i32> {
let mut x = 5;
let mut done = false;
while !done {
x += x - 3;
if x % 5 == 0 {
done = true;
}
for (index, value) in (5..10).enumerate() {
println!("index = {} and value = {}", index, value);
if index == 3 {
return Some(123);
}
}
return None; //capture all other other possibility. So the while loop would surely return either a Some or a None
}
}
}
The compiler gives this error:
error[E0308]: mismatched types
--> <anon>:7:9
|
7 | while !done {
| ^ expected enum `std::option::Option`, found ()
|
= note: expected type `std::option::Option<i32>`
= note: found type `()`
I think the error source might be that a while loop evaluates to a (), thus it would return a () instead of Some(123). I don't know how to return a valid Some type inside a loop.
The value of any while true { ... } expression is always (). So the compiler expects your foo to return an Option<i32> but finds the last value in your foo body is ().
To fix this, you can add a return None outside the original while loop. You can also use the loop construct like this:
fn main() {
// run the code
foo();
fn foo() -> Option<i32> {
let mut x = 5;
loop {
x += x - 3;
for (index, value) in (5..10).enumerate() {
println!("index = {} and value = {}", index, value);
if index == 3 {
return Some(123);
}
}
if x % 5 == 0 {
return None;
}
}
}
}
The behaviour of while true { ... } statements is maybe a bit quirky and there have been a few requests to change it.
Here is two pretty similar Levenshtein Distance algorithms.
Swift implementation:
https://gist.github.com/bgreenlee/52d93a1d8fa1b8c1f38b
And Objective-C implementation:
https://gist.github.com/boratlibre/1593632
The swift one is dramatically slower then ObjC implementation
I've send couple of hours to make it faster but... It seems like Swift arrays and Strings manipulation are not as fast as objC.
On 2000 random Strings calculations Swift implementation is about 100(!!!) times slower then ObjC.
Honestly speaking, I've got no idea what could be wrong, coz even this part of swift
func levenshtein(aStr: String, bStr: String) -> Int {
// create character arrays
let a = Array(aStr)
let b = Array(bStr)
...
is few times slower then whole algorithm in Objective C
Is anyone knows how to speedup swift calculations?
Thank you in advance!
Append
After all suggested improvements swift code looks like this.
And it is 4 times slower then ObjC in release configuration.
import Foundation
class Array2D {
var cols:Int, rows:Int
var matrix:UnsafeMutablePointer<Int>
init(cols:Int, rows:Int) {
self.cols = cols
self.rows = rows
matrix = UnsafeMutablePointer<Int>(malloc(UInt(cols * rows) * UInt(sizeof(Int))))
for i in 0...cols*rows {
matrix[i] = 0
}
}
subscript(col:Int, row:Int) -> Int {
get {
return matrix[cols * row + col] as Int
}
set {
matrix[cols*row+col] = newValue
}
}
func colCount() -> Int {
return self.cols
}
func rowCount() -> Int {
return self.rows
}
}
extension String {
func levenshteinDistanceFromStringSwift(comparingString: NSString) -> Int {
let aStr = self
let bStr = comparingString
// let a = Array(aStr.unicodeScalars)
// let b = Array(bStr.unicodeScalars)
let a:NSString = aStr
let b:NSString = bStr
var dist = Array2D(cols: a.length + 1, rows: b.length + 1)
for i in 1...a.length {
dist[i, 0] = i
}
for j in 1...b.length {
dist[0, j] = j
}
for i in 1...a.length {
for j in 1...b.length {
if a.characterAtIndex(i-1) == b.characterAtIndex(j-1) {
dist[i, j] = dist[i-1, j-1] // noop
} else {
dist[i, j] = min(
dist[i-1, j] + 1, // deletion
dist[i, j-1] + 1, // insertion
dist[i-1, j-1] + 1 // substitution
)
}
}
}
return dist[a.length, b.length]
}
func levenshteinDistanceFromStringObjC(comparingString: String) -> Int {
let aStr = self
let bStr = comparingString
//It is really strange, but I should link Objective-C coz dramatic slow swift performance
return aStr.compareWithWord(bStr, matchGain: 0, missingCost: 1)
}
}
malloc?? NSString?? and at the end 4 times speed decrease? Is anybody needs swift anymore?
There are multiple reasons why the Swift code is slower than the Objective-C code.
I made a very simple test case by comparing two fixed strings 100 times.
Objective-C code: 0.026 seconds
Swift code: 3.14 seconds
The first reason is that a Swift Character represents an "extended grapheme cluster",
which can contain several Unicode code points (e.g. "flags"). This makes the
decomposition of a string into characters slow. On the other hand, Objective-C
NSString stores the strings as a sequence of UTF-16 code points.
If you replace
let a = Array(aStr)
let b = Array(bStr)
by
let a = Array(aStr.utf16)
let b = Array(bStr.utf16)
so that the Swift code works on UTF-16 sequences as well then the time goes down
to 1.88 seconds.
The allocation of the 2-dimensional array is also slow. It is faster to allocate
a single one-dimensional array. I found a simple Array2D class here:
http://blog.trolieb.com/trouble-multidimensional-arrays-swift/
class Array2D {
var cols:Int, rows:Int
var matrix: [Int]
init(cols:Int, rows:Int) {
self.cols = cols
self.rows = rows
matrix = Array(count:cols*rows, repeatedValue:0)
}
subscript(col:Int, row:Int) -> Int {
get {
return matrix[cols * row + col]
}
set {
matrix[cols*row+col] = newValue
}
}
func colCount() -> Int {
return self.cols
}
func rowCount() -> Int {
return self.rows
}
}
Using that class in your code
func levenshtein(aStr: String, bStr: String) -> Int {
let a = Array(aStr.utf16)
let b = Array(bStr.utf16)
var dist = Array2D(cols: a.count + 1, rows: b.count + 1)
for i in 1...a.count {
dist[i, 0] = i
}
for j in 1...b.count {
dist[0, j] = j
}
for i in 1...a.count {
for j in 1...b.count {
if a[i-1] == b[j-1] {
dist[i, j] = dist[i-1, j-1] // noop
} else {
dist[i, j] = min(
dist[i-1, j] + 1, // deletion
dist[i, j-1] + 1, // insertion
dist[i-1, j-1] + 1 // substitution
)
}
}
}
return dist[a.count, b.count]
}
the time in the test case goes down to 0.84 seconds.
The last bottleneck that I found in the Swift code is the min() function.
The Swift library has a built-in min() function which is faster. So just removing
the custom function from the Swift code reduces the time for the test case to
0.04 seconds, which is almost as good as the Objective-C version.
Addendum: Using Unicode scalars seems to be even slightly faster:
let a = Array(aStr.unicodeScalars)
let b = Array(bStr.unicodeScalars)
and has the advantage that it works correctly with surrogate pairs such
as Emojis.