PHP: Efficiently Search through Collections - php

Question

I have collections of numbers (arbitrary order) to store.
psuedocode:
id_a:[3,5,7,11]
id_x:[3,5,10,21]
id_b:[12,24,25,26]
etc.
I need to be able to search through all the collections and return the group_IDs.
For example, if I look up 5, I should get back ['id_a','id_x']. I want to do this efficiently with some sort of mapping, not by looping through all numbers of all collections. I also want to be able to map directly to each key and get back the collection (e.g., 'id_x' returns [3,5,10,21]) ; again I prefer this be done efficiently without looping through the keys.
edit:
I could use the numbers as the keys and efficiently get back 'id_'. Or, I could go the other way and use 'id_' as keys and efficiently get back the array of numbers. However, I want to be able to go efficiently in both directions. I guess I could maintain two arrays, but that seems messy.

Answer 1

Your examples all show the array values in sorted order. If they are always in sorted order, then you can use a binary search to find known values. This code:
function binarySearch($needle, array $haystack) {
$high = count($haystack) - 1;
$low = 0;
$mid = false;
while ($high >= $low) {
$mid = ($high + $low) >> 1;
$t = $needle - $haystack[$mid];
if ($t < 0) {
$high = $mid - 1;
} elseif ($t > 0) {
$low = $mid + 1;
} else {
return $mid;
}
}
return $mid;
}
function searchArrays($needle) {
static $id_a = array(3,5,7,11);
static $id_x = array(3,5,10,21);
static $id_b = array(12,24,25,26);
static $arrayNames = array('id_a', 'id_x', 'id_b');
$rv = array();
foreach ($arrayNames as $arrayName) {
$array = $$arrayName;
$index = binarySearch($needle, $array);
if ($array[$index] == $needle) {
$rv[] = $arrayName;
}
}
return $rv;
}
$needles = range(3,8);
foreach ($needles as $needle) {
$result = searchArrays($needle);
printf("searchArrays(%s)=%s\n", $needle, join(', ', $result));
}
will output the following:
searchArrays(3)=id_a, id_x
searchArrays(4)=
searchArrays(5)=id_a, id_x
searchArrays(6)=
searchArrays(7)=id_a
searchArrays(8)=