Program to decode the encoded string according to the pattern shown (JavaScript)

Question

Encoding Input Data: ABC - Output: ZBYX

The encoding happens such that the odd numbered letters of the English alphabet are replaced by their alphabetic opposite, even numbered letters are replaced by a combination of that same letter and it's alphabetic opposite (ie. as shown above 'B' is even numbered alphabet so it got replaced as 'BY', A is replaced by Z, C is replaced by X)

I need to decode the encoded output data to get the input (the reverse logic). I wrote the below function but it doesn't quite give me the expected output for all test cases. (eg: When the input is ZBYX, output comes up correctly as ABC, but in other cases such as:

JQPLO (input) - output comes as QJKL (supposed to come as JKL)
NMLPK (input) - output comes as MNOP (supposed to come as NOP)

)

How should I refactor the below code so that I get the expected output for all test cases?

let alpha="ABCDEFGHIJKLMNOPQRSTUVWXYZ"


function decode(str){
         let answer=""
   

    for(let i=0;i<str.length;i++){
        if(alpha.indexOf(answer[answer.length-1])%2==0){
             if((alpha.indexOf(str[i])+1)%2==0){
                continue
             }
           answer+=alpha[alpha.length-(alpha.indexOf(str[i])+1)]
        }else{
           answer+=alpha[alpha.length-(alpha.indexOf(str[i])+1)]
        }
    }

    return answer
}

Why are you using answer[answer.length-1] instead of answer[i]? — Bergi
– Bergi, Commented Dec 1, 2022 at 8:52

kimbo · Accepted Answer · 2022-12-01 09:13:39Z

The trick for decoding is, if you get to an even letter l AND the next letter is the opposite of l, then you add l to the decoded output and skip the next letter. Otherwise, you need to just add the opposite of l to the output.

I would recommend trying it yourself before reading the code below, but the below code passed all the test cases you provided.

let alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

function isEven(c) {
    return (alpha.indexOf(c) + 1) % 2 === 0;
}

function opposite(c) {
    return alpha[alpha.length - alpha.indexOf(c) - 1];
}

function encode(str) {
    let answer = "";
    for (let i = 0; i < str.length; i++) {
        if (!isEven(str[i])) {
            answer += opposite(str[i]);
        } else {
            answer += str[i];
            answer += opposite(str[i]);
        }
    }
    return answer;
}

function decode(str) {
    let answer = "";
    for (let i = 0; i < str.length; i++) {
        if (isEven(str[i]) && str[i + 1] === opposite(str[i])) {
            answer += str[i];
            i++;
        } else {
            answer += opposite(str[i]);
        }
    }
    return answer;
}

console.log(encode('ABC'));
console.log(decode('ZBYX'));
console.log(decode('NMLPK'));
console.log(decode('JQPLO'));

Layhout · Accepted Answer · 2022-12-01 09:14:27Z

2

const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

const decode = str => {
    str = str.split("").reduce((p, c, i, arr) => {
        if (alphabet.indexOf(arr[i + 1]) % 2 !== 0)
            p += alphabet[25 - alphabet.indexOf(c)];

        return p
    }, "");

    return str;
}

console.log(decode("ZBYX"));
console.log(decode("JQPLO"));
console.log(decode("NMLPK"));

const encode = str => {
    str = str.split("").reduce((p, c) => {
        if (alphabet.indexOf(c) % 2 !== 0)
            p += c + alphabet[25 - alphabet.indexOf(c)];
        else
            p += alphabet[25 - alphabet.indexOf(c)];
        return p
    }, "");

    return str
}

console.log(encode("ABC"));

edited Dec 1, 2022 at 9:14

answered Dec 1, 2022 at 9:07

Layhout

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Comments

jumosbro · Accepted Answer · 2022-12-01 09:20:04Z

I eventually figured out a solution based off my initial program in the question, I just don't know how it works but it passed all test cases; Thanks for all the other answers here, appreciate the effort guys!

here's my updated code:

function decode(str){
try{
    let answer=""
    if(!(/^[A-Z]+$/).test(str)){
        throw Error("Input must only contain upper case letters")
    }

    for(let i=0;i<str.length;i++){
        if(alpha.indexOf(answer[answer.length-1])%2==0){
             if((alpha.indexOf(str[i])+1)%2==0){
                continue
             }
        }else{
            if(str.length>4)answer=answer.slice(-1)  
        }
        answer+=alpha[alpha.length-(alpha.indexOf(str[i])+1)]

    }

    return answer
} catch (err){
    console.log(err)
}
}

I just added a condition to check for the string length given at the input under the else block, but I guess the other answers here were more logical than mine

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