String Transformation and Frequency Analysis with Alice

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's 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 a product of the ASCII value of the character and its frequency. Our task is to construct a slice 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".

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

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

Our first step involves mapping each character of the input string to the next alphabetical character. For this, we define the nextString as an empty slice of runes, storing 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.

Was that clear? Look at the following function that implements this exact behaviour:

func characterFrequencyEncoding(word string) string {
    nextString := []rune{}
    for _, letter := range word {
        if letter == 'z' {
            nextString = append(nextString, 'a')
        } else {
            nextString = append(nextString, letter+1)
        }
    }
    return string(nextString)
}