Php randomized with an % chance - mysql

Question

I need something where I really dont know how to make it.
Here is an example:
user 1 = 3
user 2 = 1
user 3 = 6
What I want with this, is that user 1, has 30% chance, user 2 has 10% and user 3 60%. But those numbers can also be 0.01 instead of 6. What now want, is that it gets randomised but also have the % of chances. I dont really know how to explain it. That it then draws an number with 60% chance for user 3, 10 chance for user 2 and 30% chance for user 1. But, this last can be as long as possible. How to do this? Sorry, I am really bad in explaining.
Thanks!

Answer 1

What you want can be described as a "weighted probability distribution" or to be technical a "discrete distribution" and a "Categorical distribution":
A categorical distribution is a discrete probability distribution that describes the possible results of a random variable that can take on one of K possible elementary events, with the probability of each elementary event separately specified.
-- Wikipedia
Given you have a random variable with uniform distribution in the range 0 to 1 you can build any distribution you want by using the Inverse Method.
Your first step is to normalize the distribution. That means to make sure that the area of below the curve equals one (that is, that your weight do not sum more than 100%). For a discrete distribution, it means to make sure that sum of the weights is equal to one. [Which is the same as taking the values as a vector and calculate the unit vector in the same direction], just take the each value and divide it by the sum of the values.
Therefore, you go from this:
(original)
user 1 = 3
user 2 = 1
user 3 = 6
To this:
sum = 3 + 1 + 6 = 10
(normalized)
user 1 = 3 / 10 = 0.3
user 2 = 1 / 10 = 0.1
user 3 = 6 / 10 = 0.6
Next, get the cumulative distribution. That is, for each value you do not want its (normalized) weight but the weight of it plus all previous ones.
Therefore, you go from this
(normalized)
user 1 = 0.3
user 2 = 0.1
user 3 = 0.6
To this:
(cumulative)
user 1 = 0.3
user 2 = 0.1 + 0.3 = 0.4
user 3 = 0.6 + 0.3 + 0.1 = 1
Finally, you get your random variable with uniform distribution in the range 0 to 1 and check below which value it falls:
$r = (float)rand()/(float)getrandmax();
if ($r <= 0.3) return "user 1"; // user 1 = 0.3
else if ($r <= 0.4) return "user 2"; // user 2 = 0.4
else return "user 3"; // user 3 = 1
Note: The range is inclusive because PHP is weird.
Ok, all in one go, in (ugly) PHP:
$p = ['user 1' => 3, 'user 2' => 1, 'user 3' => 6];
$s = array_sum($p);
$n = array_map(function($i) use ($p, $s){return $i/$s;},$p);
$a = []; $t = 0;
foreach($n as $k => $i) {$t += $i; $a[$k] = $t;}
$r = (float)rand()/(float)getrandmax();
foreach($a as $k => $i) { if ($r <= $i) return $k; }
Try online.
Let us reimplement in MySQL because reasons.
First we need a table with the input, for example:
SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`
Then we sum the values
SELECT sum(chance) FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input
Then we normalize
SELECT id, chance / sum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input CROSS JOIN (SELECT sum(chance) as sum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input) s
Then we cumulate
SELECT id, chance / sum as sum, (#tmp := #tmp + chance / sum) as csum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input CROSS JOIN (SELECT sum(chance) as sum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input) s CROSS JOIN (SELECT #tmp := 0) cheat
Then we pick
SELECT id from (
SELECT id, chance / sum as sum, (#tmp := #tmp + chance / sum) as csum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input CROSS JOIN (SELECT sum(chance) as sum FROM (SELECT 'user 1' AS `id`, 3 AS `chance`
UNION
SELECT 'user 2' AS `id`, 1 AS `chance`
UNION
SELECT 'user 3' AS `id`, 6 AS `chance`) input) s CROSS JOIN (SELECT #tmp := 0) cheat) a
CROSS JOIN (SELECT RAND() as r) random
WHERE csum > r
LIMIT 1
Try online.

Answer 2

Edit: This answer is based on the title "PHP...." not the tag "mysql"
Each user has n tickets. In your case you have 10 tickets and therefore need a random number between 0 and 9. An unelegant solution could be:
<?php
$tickets = array();
$number_of_tickets = 0;
foreach($users as $user) {
for($i = 0; i < $user->tickets; $i++) {
$tickets[] = $user->id;
$tickets++;
}
}
$lucky_draw = rand(0, $number_of_tickets);
$winner = tickets[$lucky_draw] //ID of the user
print("And the winner is...." . $winner);