Hello! Are you ready for an exciting voyage into the wonderful realm of strings and data structures? Today, we will assist Alice, an aspiring cryptographer, with an intriguing string manipulation task. She loves playing with strings and has come up with a unique string encoding scheme. I assure you, this will be an enlightening journey that will stretch your programming muscles. Let's get started!

Alice has devised a unique way of encoding words. She takes a word and replaces each character with the next character in the alphabetical order. In other words, given a string word, for each character, if it is not z, she replaces it with the character that comes next alphabetically. For the character z, she replaces it with a.

Another element of Alice's algorithm involves frequency analysis. After shifting the characters, she counts the frequency of each character in the new string. Then, she creates an association of each character with its frequency and ASCII value. Each character maps to a number, which is the product of the ASCII value of the character and its frequency. Our task is to construct a list containing these products, sorted in descending order.

Example

For the input string "banana", the output should be [294, 222, 99].

The string "banana" will be shifted to "cbobob".

Calculating the product of frequency and ASCII value for each character:

• The ASCII value for c is 99; it appears once in the string, so its product is 99 * 1 = 99.
• The ASCII value for b is 98; it appears three times in the string, so its product is 98 * 3 = 294.
• The ASCII value for o is 111; it appears twice in the string, so its product is 111 * 2 = 222.

Collecting these products into a list gives [99, 294, 222]. Sorting this list in descending order results in [294, 222, 99].

Our first step involves mapping each character of the input string to the next alphabetical character. We initialize an empty string $nextString to store the shift operation result. We then iterate over each character of the input string. If a character is not z, we replace it with the next alphabetical character. If it is z, we replace it with a.

Here's the updated function in PHP:

php
Copy to clipboard
1<?php
2
3function characterFrequencyEncoding($word) {
4    $nextString = "";
5    for ($i = 0; $i < strlen($word); $i++) {
6        $letter = $word[$i];
7        $nextString .= ($letter === 'z') ? 'a' : chr(ord($letter) + 1);
8    }
9    return $nextString;
10}

The next step is to track the frequency of each character in $nextString. We start by initializing an empty associative array $frequencyMap. Then, we iterate over $nextString. If the current character exists in $frequencyMap, we increment its frequency by 1. If it doesn't exist, we add it to $frequencyMap with a frequency of 1.

Incorporating this step into the function, our code now looks like this:

php
Copy to clipboard
1<?php
2
3function countFrequency($nextString) {
4    $frequencyMap = array();
5    for ($i = 0; $i < strlen($nextString); $i++) {
6        $letter = $nextString[$i];
7        if (array_key_exists($letter, $frequencyMap)) {
8            $frequencyMap[$letter]++;
9        } else {
10            $frequencyMap[$letter] = 1;
11        }
12    }
13    return $frequencyMap;
14}

Next, we calculate the numerical representation for each unique character. We initialize an empty array $combinedValues to store these numbers. For each character in $frequencyMap, we calculate the product of its ASCII representation and its frequency in $nextString and append this to $combinedValues.

Here's the updated function:

php
Copy to clipboard
1<?php
2
3function buildProductList($frequencyMap) {
4    $combinedValues = array();
5    foreach ($frequencyMap as $character => $frequency) {
6        $combinedValues[] = ord($character) * $frequency;
7    }
8    return $combinedValues;
9}

The final step is to sort the array $combinedValues in descending order using rsort.

Here's our complete function:

php
Copy to clipboard
1<?php
2
3function characterFrequencyEncoding($word) {
4    // Step 1: Mapping each character to the next alphabetical character
5    $nextString = "";
6    for ($i = 0; $i < strlen($word); $i++) {
7        $letter = $word[$i];
8        $nextString .= ($letter === 'z') ? 'a' : chr(ord($letter) + 1);
9    }
10
11    // Step 2: Counting the frequency of characters in nextString
12    $frequencyMap = array();
13    for ($i = 0; $i < strlen($nextString); $i++) {
14        $letter = $nextString[$i];
15        if (array_key_exists($letter, $frequencyMap)) {
16            $frequencyMap[$letter]++;
17        } else {
18            $frequencyMap[$letter] = 1;
19        }
20    }
21
22    // Step 3: Building the product list
23    $combinedValues = array();
24    foreach ($frequencyMap as $character => $frequency) {
25        $combinedValues[] = ord($character) * $frequency;
26    }
27
28    // Step 4: Sorting the final values in descending order
29    rsort($combinedValues);
30
31    // Return the sorted list
32    return $combinedValues;
33}
34
35// Example usage
36$word = "banana";
37$result = characterFrequencyEncoding($word);
38foreach ($result as $value) {
39    echo $value . " ";
40}
41// Prints:
42// 294 222 99

Well done! You've successfully tackled an intricate problem that required you to exercise multiple topics, such as string manipulation, associative array processing, and array sorting. This task underscored the importance of reusing already calculated values. I encourage you to apply what you've learned today to other tasks. Many more exciting challenges are waiting for you in the upcoming practice sessions. Happy coding!