thai sorting -> port form java to typescipt

thai-sort.tsx

// https://thai-notes.com/notes/sortingthai.html => java to typescript
const SARA_E = 0x0e40;
const SARA_AI_MAIMALAI = 0x0e44;
const MAITAIKHU = 0x0e47;
const THANTHAKHAT = 0x0e4c; // a.k.a. "garan"

export function isLeadingVowel(c: string) {
  // Returns true if character is in the range from SARA E to SARA AI MAIMALAI,
  // i.e. if the character is a leading vowel

  return c.charCodeAt(0) >= SARA_E && c.charCodeAt(0) <= SARA_AI_MAIMALAI;
}

export function isToneMark(c: string) {
  // Returns true if character is in the range from MAITHAIKHU to THANTHAKHAT
  // which includes the four tone marks.  I.e. all "above" symbols

  return c.charCodeAt(0) >= MAITAIKHU && c.charCodeAt(0) <= THANTHAKHAT;
}

function setCharAt(str: string, index: number, chr: string) {
  if (index > str.length - 1) return str;
  return str.substring(0, index) + chr + str.substring(index + 1);
}

export function getThaiComparisonString(s: string) {
  // Convert String to a character array
  const chars = s;

  // Swap all leading vowels with next character
  for (let i = 0; i < chars.length; i++) {
    if (isLeadingVowel(chars[i])) {
      const c = chars[i];
      setCharAt(chars, i, chars[i + 1]);
      setCharAt(chars, i + 1, c);
      i++;
    }
  }

  // The String for comparison is built in to parts, here referred to
  // as "head" and "tail".  "tail" always begins with "00".
  let head = "";
  let tail = "00";

  // Add each character to the "head" unless it's a tone mark,
  // MAITHAIKHU, or THANTHAKHAT, in which case add a 2 digit
  // String to "tail" representing its original position from the
  // END of the original String, then append the mark itself to "tail".

  for (let i = 0; i < chars.length; i++) {
    if (isToneMark(chars[i])) {
      const pos = chars.length - i;
      tail += pos >= 10 ? "" + pos : "0" + pos;
      tail += chars[i];
    } else {
      head += chars[i];
    }
  }

  // Return the Comparison string
  return head + tail;
}

export function HowtoUse(a: string, b: string) {

  const csA = getThaiComparisonString(a);
  const csB = getThaiComparisonString(b);

  return csA.localeCompare(csB, "th");
}