php Substitution Encoding Algorithm using cesar cipher - php

Question

hello i am stack with this exercise as part of a test, how can i solve it or hint to solve it
/**
* Class SubstitutionEncodingAlgorithm
*/
class SubstitutionEncodingAlgorithm implements EncodingAlgorithm {
/**
* #var array
*/
private $substitutions;
/**
* SubstitutionEncodingAlgorithm constructor.
* #param $substitutions
*/
public function __construct(array $substitutions) {
$this->substitutions = array();
}
/**
* Encodes text by substituting character with another one provided in the pair.
* For example pair "ab" defines all "a" chars will be replaced with "b" and all "b" chars will be replaced with "a"
* Examples:
* substitutions = ["ab"], input = "aabbcc", output = "bbaacc"
* substitutions = ["ab", "cd"], input = "adam", output = "bcbm"
*
* #param string $text
* #return string
*/
public function encode($text) {
/**
* #todo: Implement it
*/
}
}
that what i ve tried so far in the encode () function but its not working, what i am doing wrong ?
public function encode($text) {
$length = strlen($text);
$newstr = '';
for ($i = 0; $i < $length; $i++) {
if (is_array($this->substitutions) && in_array(strtoupper($text[$i]), array_flip($this->substitutions)))
$newstr .= $this->substitutions[strtoupper($text[$i])];
}
return $newstr;
}
i understand that it is the cesar algorithm to be implemented so far, any help would be appreciated on how to do it

Answer 1

You could take the substitutions array and split it into two arrays, e.g.
$swapA = array();
$swapB = array();
//for each item in the substitutions array take the first char
// and place in swapA and the second/last char and place in swapB
foreach($substitutions as $sub)
{
$swapA = substr($sub,0,1);
$swapB = substr($sub,1,1);
}
// the str_replace will replace the all characters in $text chars
// from position x in swapA with chars in the same position in swapB
$output = str_replace($swapA, $swapB, $text);

Answer 2

$swapA = array();
$swapB = array();
$output = '';
$aText = str_split($text);
foreach($this->substitutions as $sub)
{
$swapA[] = substr($sub,0,1);
$swapB[] = substr($sub,1,1);
}
foreach ($aText as $letter) {
if (in_array(strtolower($letter, $swapA)) {
$positionOccurence = array_search ($letter, $swapA);
$replaced = $swapB[$positionOccurence];
$output .= str_replace($letter, $replaced, $letter);
} elseif (in_array(strtolower($letter), $swapB)) {
$positionOccurence = array_search ($letter, $swapB);
$replaced = $swapA[$positionOccurence];
$output .= str_replace($letter, $replaced, $letter);
} else {
$output .= $letter;
}
}
return $output;

Answer 3

My try - it is just for one byte per char text:
private function encodeText (string $text) : string {
$result = '';
for ($i = 0, $e = strlen ($text); $i < $e; $i ++) {
foreach ($this->substitutions as $substitution) {
$strpos = strpos ($substitution, $text {$i});
if ($strpos !== false) {
$result .= $strpos == 0 ? $substitution {1} : $substitution {0};
continue 2;
}
}
$result .= $text {$i};
}
return $result;
}
Other solution is much more faster and simpler:
first create in constructor array like:
foreach ($substitutions as $substitution) {
$this->substitutions ['from'] .= $substitution {0} . $substitution {1} . strtoupper($substitution {0} . $substitution {1});
$this->substitutions ['to'] .= $substitution {1} . $substitution {0} . strtoupper($substitution {1} . $substitution {0});
}
and then simply make translation:
public function encode($text)
{
return strtr ($text, $this->substitutions ['from'], $this->substitutions ['to']);
}

Answer 4

The problem is this:
public function __construct(array $substitutions) {
$this->substitutions = array();
}
It's an empty array.
Change to:
public function __construct(array $substitutions) {
$this->substitutions = $substitutions;
}
then test your logic.

Answer 5

Here is a more precise answer with edge cases in mind.
public function encode(string $text): string
{
$swapA = array();
$swapB = array();
$output = '';
$aText = str_split($text);
foreach ($this->substitutions as $sub) {
if (strlen($sub) == 2 && ($sub[0] != $sub[1])) {
$swapA[] = $sub[0];
$swapB[] = $sub[1];
} else {
throw new InvalidArgumentException ("Must have 2 different characters");
}
}
foreach ($aText as $letter) {
if (in_array(strtolower($letter), $swapA)) {
$positionOccurence = array_search($letter, $swapA);
if (strtolower($letter) != $letter) {
$replaced = strtoupper($swapB[$positionOccurence]);
} else {
$replaced = $swapB[$positionOccurence];
}
$output .= str_replace($letter, $replaced, $letter);
} elseif (in_array(strtolower($letter), $swapB)) {
$positionOccurence = array_search($letter, $swapB);
if (strtolower($letter) != $letter) {
$replaced = strtoupper($swapA[$positionOccurence]);
} else {
$replaced = $swapA[$positionOccurence];
}
$output .= str_replace($letter, $replaced, $letter);
} else {
$output .= $letter;
}
}
return $output;
}