<?php

/**
 * Code snippet with math lession
 * @author Nguyen Nhu Tuan <tuanquynh0508@gmail.com>
 */

// 2^3 = 8
echo getExponential(2, 3);
echo '<br/>';

// 2 + 2^2 + 2^3
// = 2 + 4 + 8
// = 14
echo sumOfExponential(2, 3);
echo '<br/>';

// false
$str1 = 'abcba';
if (isSymmetryString($str1)) {
    echo "'$str1' is symmetry <br/>";
} else {
    echo "'$str1' is not symmetry <br/>";
}

// true
$str2 = 'abccba';
if (isSymmetryString($str2)) {
    echo "'$str2' is symmetry <br/>";
} else {
    echo "'$str2' is not symmetry <br/>";
}

#-------------------------------------------------------------------------------
/**
 * Get Exponential
 * Calculate Exponential expression
 * Example: x^n like pow(x, n) function of php
 * @author Nguyen Nhu Tuan <tuanquynh0508@gmail.com>
 *
 * @param  integer $a
 * @param  integer $n
 * @return integer
 */
function getExponential($a, $n)
{
    if ($n === 1) {
        return $a;
    }

    return $a * getExponential($a, $n-1);
}

/**
 * Sum Of Exponential
 * Calculate Sum Of Exponential expression
 * Example: x + x^2 + x^3 + ... + x^n
 * @author Nguyen Nhu Tuan <tuanquynh0508@gmail.com>
 *
 * @param  integer $a
 * @param  integer $n
 * @return integer
 */
function sumOfExponential($a, $n)
{
    if ($n === 1) {
        return $a;
    }

    return pow($a, $n) + sumOfExponential($a, $n-1);
}

/**
 * Is Symmetry String
 * Check string is symmetry or not
 * Example: abcba is not symmetry, but abccba is symmetry
 * @author Nguyen Nhu Tuan <tuanquynh0508@gmail.com>
 *
 * @param  string  $str
 * @return boolean
 */
function isSymmetryString($str)
{
    if (strlen($str) % 2 != 0) {
        return false;
    }

    for ($i=0; $i < strlen($str)/2; $i++) {
        if (substr($str, $i, 1) != substr($str, -($i+1), 1)) {
            return false;
        }
    }

    return true;
}