Get next alphabet string (exemples: 'A' -> 'B', 'Z' -> 'AA', 'AZ' -> 'BA') with a variable increment [closed]

Question

I'm trying to make a function that returns the next string. So if I input 'A' it returns 'B' etc. And i would like to pass as a parameter also an increment so 'AE' with an increment of 30 would be 'BI' and 'CM'(edit: it was 'CM' not 'ZM') would be 'AAQ'.

edit:

(input -> output)

Increment of 1

"A" -> "B"

"B" -> "C"

...

"Z" -> "AA"

...

"AZ" -> "BZ"

Increment of 30(the 30 is an example it can be anything)

"A" -> "AE"

"AE" -> "BI"

"BI" -> "CM"

"CM" -> "DQ"

Essentially, I want the next excel column. But i want to be able to increment. So with an increment of 30 i don't want the next one I want the 30 after if that makes sense.

Answer 1

You're looking for an implementation of the bijective base 26 system, also known as "bijective hexavigesimal". There are many ways to do this; here's one possibility, involving converting such values to numbers before performing calculations on them, and then converting back from numbers afterward:

const baseCode = "A".charCodeAt(0) - 1;

function toBb26(n: number): string {
  let chars: string[] = [];
  while (n > 0) {
    const d = ((n - 1) % 26) + 1;
    chars.unshift(String.fromCharCode(baseCode + d));
    n = (n - d) / 26;
  }
  return chars.join("");
}

function fromBb26(x: string): number {
  let ret = 0;
  for (const c of Array.from(x)) {
    ret *= 26;
    ret += (c.charCodeAt(0) - baseCode);
  }
  return ret;
}

function increment(x: string, n: number=1) {
  return toBb26(fromBb26(x) + n);
}

console.log(increment("A")) // B
console.log(increment("AE", 30)) // BI
console.log(increment("ZM", 30)) // AAQ

I haven't rigorously tested this, and it might not be performance-optimized for your use case, and there are probably existing JS libraries that do this, etc. etc.

Playground link to code