I have a string full of values that are separated by a comma. Needless to say this can be done with $strings = explode(',',$fullstring);.
Now, I need to compare a user-submitted-value to this value to see 'how equal' it is. Ideally I'd like to check that maximum one letter/symbol/number/... is not equal. For example, if the hardcoded string is 'table':
'tabl' would be true
'tablz' would be true
'tab' would be false
If you understand what I mean. Of course the strings can also contain spaces, apostrophes, numeric values. I really have no idea how I should get started on this. I've already set up a filter to check whether the value is equal (using if(in_array(strtolower($userValue),array_map('strtolower',$strings)))), but in the else clause I need some method to detect what I just explained.
It sounds like the functionality you're looking for is best served by levenshtein.
Have a look at http://php.net/manual/en/function.levenshtein.php for examples and a description of how to use it.
calculate Levenshtein distance between the two strings: http://php.net/manual/en/function.levenshtein.php and check if the distance is less than some threshold value.
I'm looking for a solution to convert all numbers in a given range to another number in the same range, and later convert that number back.
More concrete, let's say I have the numbers 1..100.
The easiest way to convert all numbers to another one in the same range is to use: b = 99 -a; later get the original with a = 99 - b;.
My problem is that I want to simulate some randomness.
I want to implement this in PHP, but the coding language doesn't matter.
WHY?
You maybe say why? Good question :)
I am generating some easy to read short code string based on id-s, and because the id's are incremented one by one, my consecutive short codes are too similar.
Later I need to "decode" the short codes, to get the id.
What my algorithm is doing now is:
0000001 -> ababac, 0000002 -> ababad, 0000003 -> ababaf, etc.
later
ababac -> 0000001, ababad -> 0000002, ababaf -> 0000003, etc.
So before I actually generate the short code I want to "randomize" the number as much as possible.
Option 1:
Why dont you just have a database of conversion? i.e each record has a "real" id, and a "random md5" string or something
Option 2:
Use a rainbow table - maybe even a MD5 lookup table for the range 0 - 10,000 or whatever. Then just do a hashtable lookup
Finally I found a solution based on module operator, on the math forum.
The solution can be found here:
https://math.stackexchange.com/questions/259891/function-to-convert-each-number-in-a-m-n-to-another-number-in-the-same-range
I have a tricky question that I've looked into a couple of times without figuring it out.
Some backstory: I am making a textbased RPG-game where players fight against animals/monsters etc. It works like any other game where you hit a number of hitpoints on each other every round.
The problem: I am using the random-function in php to generate the final value of the hit, depending on levels, armor and such. But I'd like the higher values (like the max hit) to appear less often than the lower values.
This is an example-graph:
How can I reproduce something like this using PHP and the rand-function? When typing rand(1,100) every number has an equal chance of being picked.
My idea is this: Make a 2nd degree (or quadratic function) and use the random number (x) to do the calculation.
Would this work like I want?
The question is a bit tricky, please let me know if you'd like more information and details.
Please, look at this beatiful article:
http://www.redblobgames.com/articles/probability/damage-rolls.html
There are interactive diagrams considering dice rolling and percentage of results.
This should be very usefull for you.
Pay attention to this kind of rolling random number:
roll1 = rollDice(2, 12);
roll2 = rollDice(2, 12);
damage = min(roll1, roll2);
This should give you what you look for.
OK, here's my idea :
Let's say you've got an array of elements (a,b,c,d) and you won't to randomly pick one of them. Doing a rand(1,4) to get the random element index, would mean that all elements have an equal chance to appear. (25%)
Now, let's say we take this array : (a,b,c,d,d).
Here we still have 4 elements, but not every one of them has equal chances to appear.
a,b,c : 20%
d : 40%
Or, let's take this array :
(1,2,3,...,97,97,97,98,98,98,99,99,99,100,100,100,100)
Hint : This way you won't only bias the random number generation algorithm, but you'll actually set the desired probability of apparition of each one (or of a range of numbers).
So, that's how I would go about that :
If you want numbers from 1 to 100 (with higher numbers appearing more frequently, get a random number from 1 to 1000 and associate it with a wider range. E.g.
rand = 800-1000 => rand/10 (80->100)
rand = 600-800 => rand/9 (66->88)
...
Or something like that. (You could use any math operation you imagine, modulo or whatever... and play with your algorithm). I hope you get my idea.
Good luck! :-)
Ok so lets say I have an array that can be 0 - X large x being in the tens of thousands, I know insane notion but none the less just for the sake of how vast the array can be. What I need to do is device a function that can take a couple parameters. One would be how many numbers am I looking to sum to make the number I want to check, the next would be the number I want to check for. The next would be the array itself.
At first I figured something like array_sum() would give me a little help but thats only for totaling the entire sum of the array. What I need to do is say I either want 1 - 10 different values again for sake of example to see if this will total what I am seeking. Now its possible I could think of something up on my own if I can only figure out how to check that 1-10 concept. Its most likely only going to be 2 at any given time but I want it dynamic for potential future needs. So anyone know of an algorithm concept I can come build up to check a given array like this? I know its possible, I just can't fully wrap my head around it at 3am in the morning.
EDIT
$test_case = array(0,1,2,3,4,5,6,7,8,9,10,11,12,13);
function find_sum($totalby = 2, $expected = 0, $updown = "subtract")
{
//something here to take the total by and then check to see if expected is found
}
now what I mean to try and figure out is if any 2 numbers equal 0 when subtracted (although $updown could be "add" also for later expansion of this function. Anyway in this case scenario I should have specified if anything equals zero. Example I want to find if a 2 numbers equal zero together. Grant it Not exactly sure if my array above will find a zero result with just subtracting any 2 of the given numbers, but it expresses what type of array I am working with to holdfully achieve my goal.
Sounds like all you need is to use array_slice before using array_sum.
Something like:
function subarr_sum($array,$offset,$length) {
$newarray = array_slice($array,$offset,$length);
return array_sum($newarray);
}
I have just found this great tutorial as it is something that I need.
However, after having a look, it seems that this might be inefficient. The way it works is, first generate a unique key then check if it exists in the database to make sure it really is unique. However, the larger the database gets the slower the function gets, right?
Instead, I was thinking, is there a way to add ordering to this function? So all that has to be done is check the previous entry in the DB and increment the key. So it will always be unique?
function generate_chars()
{
$num_chars = 4; //max length of random chars
$i = 0;
$my_keys = "123456789abcdefghijklmnopqrstuvwxyz"; //keys to be chosen from
$keys_length = strlen($my_keys);
$url = "";
while($i<$num_chars)
{
$rand_num = mt_rand(1, $keys_length-1);
$url .= $my_keys[$rand_num];
$i++;
}
return $url;
}
function isUnique($chars)
{
//check the uniqueness of the chars
global $link;
$q = "SELECT * FROM `urls` WHERE `unique_chars`='".$chars."'";
$r = mysql_query($q, $link);
//echo mysql_num_rows($r); die();
if( mysql_num_rows($r)>0 ):
return false;
else:
return true;
endif;
}
The tiny url people like to use random tokens because then you can't just troll the tiny url links. "Where does #2 go?" "Oh, cool!" "Where does #3 go?" "Even cooler!" You can type in random characters but it's unlikely you'll hit a valid value.
Since the key is rather sparse (4 values each having 36* possibilities gives you 1,679,616 unique values, 5 gives you 60,466,176) the chance of collisions is small (indeed, it's a desired part of the design) and a good SQL index will make the lookup be trivial (indeed, it's the primary lookup for the url so they optimize around it).
If you really want to avoid the lookup and just unse auto-increment you can create a function that turns an integer into a string of seemingly-random characters with the ability to convert back. So "1" becomes "54jcdn" and "2" becomes "pqmw21". Similar to Base64-encoding, but not using consecutive characters.
(*) I actually like using less than 36 characters -- single-cased, no vowels, and no similar characters (1, l, I). This prevents accidental swear words and also makes it easier for someone to speak the value to someone else. I even map similar charactes to each other, accepting "0" for "O". If you're entirely machine-based you could use upper and lower case and all digits for even greater possibilities.
In the database table, there is an index on the unique_chars field, so I don't see why that would be slow or inefficient.
UNIQUE KEY `unique_chars` (`unique_chars`)
Don't rush to do premature optimization on something that you think might be slow.
Also, there may be some benefit in a url shortening service that generates random urls instead of sequential urls.
I don't know why you'd bother. The premise of the tutorial is to create a "random" URL. If the random space is large enough, then you can simply rely on pure, dumb luck. If you random character space is 62 characters (A-Za-z0-9), the the 4 characters they use, given a reasonable random number generator, is 1 in 62^4, which is 1 in 14,776,336. Five characters is 1 in 916,132,832. So, a conflict is, literally, "1 in a billion".
Obviously, as the documents fill, your odds increase for the chance of a collision.
With 10,000 documents, it's 1 in 91,613, almost 1 in 100,000 (for round numbers).
That means, for every new document, you have a 1 in 91,613 chance of hitting the DB again for another pull on the slot machine.
It is not deterministic. It's random. It's luck. In theory, you can hit a string of really, really, bad luck and just get collision after collision after collision. Also, it WILL, eventually, fill up. How many URLs do you plan on hashing?
But if 1 in 91,613 odds isn't good enough, boosting it to 6 chars makes it more than 1 in 5M for 10,000 documents. We're talking almost LOTTO odds here.
Simply put, make the key big enough (7 characters? 8?) and the problem pretty much "wishes" itself out of existence.
Couldn't you encode the URL as Base36 when it's generated, and then decode it when visited - that would allow you to remove the database completely?
A snippet from Channel9:
The formula is simple, just turn the
Entry ID of our post, which is a long
into a short string by Base-36
encoding it and then stick
'http://ch9.ms/' onto the front of it.
This produces reasonably short URLs,
and can be computed at either end
without any need for a database look
up. The result, a URL like
http://ch9.ms/A49H is then used in
creating the twitter link.
I solved a similar problem by implementing an alogirthm that used to generate serial numbers one-by-one in base36. I had my own oredring of base36 characters all of which are unique. Since it was generating numbers serially I did not have to worry about duplication. Complexity and randomness of the number depends on the ordering of base36 numbers[characters]... that too for public only becuase to my application they are serial numbers :)
Check out this guys functions - http://www.pgregg.com/projects/php/base_conversion/base_conversion.php source - http://www.pgregg.com/projects/php/base_conversion/base_conversion.inc.phps
You can use any base you like, for example to convert 554512 to base 62, call
$tiny = base_base2base(554512, 10, 62); and that evaluates to $tiny = '2KFk'.
So, just pass in the unique id of the database record.
In a project I used this in a removed a few characters from the $sChars string, and am using base 58. You can also rearrange the characters in the string if you want the values to be less easy to guess.
You could of course add ordering by simply numbering the urls:
http://mytinyfier.com/1
http://mytinyfier.com/2
and so on. But if the hash key is indexed in the database (which it obviously should be), the performance boost would be minimal at best.
I wouldn't bother doing ordered enumeration for two reasons:
1) SQL servers are very effective at checking such hash collisions (given correct indexes)
2) That might hurt privacy, as users would be able to easily figure out what other users are tinyurl-ing.
Use autoincrement on the database, and get the latest id as described by http://www.acuras.co.uk/articles/24-php-use-mysqlinsertid-to-get-the-last-entered-auto-increment-value
Perhaps this is a bit off-answer, but, my general rule for creating always unique keys is simple md5( time() * 100 + rand( 0, 100 ) ); There is a one in 100,000 chance that if two people are using the same service at the same second they will get the same result (nie impossible).
That said, md5( rand( 0, n ) ) works too.
That might work, but the easiest way to accomplish the problem would probably be with hashing. Theoretically speaking, hashing runs in O(1) time, as in, it only has to perform the hash, and then does only one actual hit to the database to retrieve the value. Then, you would introduce complications for checking for hash collisions, but it seems like this is probably what most of the tinyurl providers do. And, a good hash function isn't terribly hard to write.
I have also created small tinyurl service.
I wrote a script in Python that was generating keys and store in MySQL table named tokens with status U(Unused).
But, I am doing it in offline mode. I have a corn job on my VPS. It runs a script every 10 minutes. The script check if there are less than 1000 keys in the table, it keep generating keys and inserting them if they are unique and not already exists in the table until the key's count up to 1000.
For my service, 1000 keys for 10 minutes are more than enough, you can set the timing or number of keys generated according to your need.
Now when any tiny url needs to be created on my website, my PHP script just fetch any key which is unused from the table and marked its status as T(taken). PHP script does not have to bother about its uniqueness as my python script already populated only unique keys.
Couldn't you just trim the hash to the length you wish?
$tinyURL = substr(md5($longURL . time()),0,4);
Granted, this may not provide as much pseudo randomness as using the entire string length. But, if you hash the long URL concatenated with the time(), wouldn't this be sufficient? Thoughts on using this method? Thanks!