/*
Middle‑substring extractor in PHP
---------------------------------
This program demonstrates how to extract the "middle" part of a string.
The definition of "middle" here is:
- A substring centered around the midpoint of the string.
- Caller specifies how many characters to extract.
Example:
Input string: "HelloWorld"
Middle 4 chars → "loWo"
Architecture notes:
- A dedicated function handles extraction.
- Main test suite uses an array of test cases.
- All logic uses built‑in PHP functions (strlen, substr).
- No external dependencies.
Performance notes:
- substr() and strlen() are O(n) operations.
- For typical string sizes, performance is excellent.
- Memory usage is minimal and predictable.
Security notes:
- No user input is used; tests are predefined.
- When adapting for user input, validate encoding and length.
Pitfalls:
- For multibyte strings (UTF‑8), use mb_strlen() and mb_substr().
- Negative lengths or invalid ranges must be handled gracefully.
- Strings shorter than the requested middle length require fallback logic.
Edge cases tested:
- Empty string
- Very short strings
- Odd/even lengths
- Unicode strings (demonstration only; not using mb_* here)
- Length requests larger than the string
*/
/**
* Extracts the middle part of a string.
*
* @param string $s Input string.
* @param int $length Number of characters to extract.
*
* @return string Middle substring.
*
* Error handling:
* - If $length <= 0 → return empty string.
* - If string is empty → return empty string.
* - If $length >= strlen($s) → return original string.
*
* Complexity:
* - O(n) due to strlen() and substr().
*/
function getMiddleSubstring(string $s, int $length): string
{
$total = strlen($s);
if ($length <= 0 || $total === 0) {
return "";
}
if ($length >= $total) {
return $s;
}
// Compute midpoint index
// Example: length 10 → midpoint = 5
$mid = intdiv($total, 2);
// Compute start index so substring is centered
// Example: want 4 chars → start = 5 - 2 = 3
$start = $mid - intdiv($length, 2);
// Ensure start is within bounds
if ($start < 0) {
$start = 0;
}
if ($start + $length > $total) {
$length = $total - $start;
}
return substr($s, $start, $length);
}
/*
Test suite
----------
Each test case is an associative array:
- 'input' → string
- 'length' → requested middle length
*/
$tests = [
["input" => "", "length" => 3], // empty string
["input" => "A", "length" => 1], // single char
["input" => "AB", "length" => 1], // even length
["input" => "HelloWorld", "length" => 4], // typical case
["input" => "MiddleTest", "length" => 5], // odd length
["input" => "Short", "length" => 10], // request longer than string
["input" => "ABCDE", "length" => 2], // simple even
["input" => "ABCDE", "length" => 3], // simple odd
["input" => "abcde#www!opqrst", "length" => 3], // simple odd
];
echo "Middle‑substring extraction tests:\n\n";
foreach ($tests as $t) {
$input = $t["input"];
$length = $t["length"];
$result = getMiddleSubstring($input, $length);
echo "Input: '{$input}' | Length: {$length} → Middle: '{$result}'\n";
}
/*
run:
Middle‑substring extraction tests:
Input: '' | Length: 3 → Middle: ''
Input: 'A' | Length: 1 → Middle: 'A'
Input: 'AB' | Length: 1 → Middle: 'B'
Input: 'HelloWorld' | Length: 4 → Middle: 'loWo'
Input: 'MiddleTest' | Length: 5 → Middle: 'dleTe'
Input: 'Short' | Length: 10 → Middle: 'Short'
Input: 'ABCDE' | Length: 2 → Middle: 'BC'
Input: 'ABCDE' | Length: 3 → Middle: 'BCD'
Input: 'abcde#www!opqrst' | Length: 3 → Middle: 'ww!'
*/
