/**
 * @flow
 */

/**
 * @param {number} codeUnit   A Unicode code-unit, in range [0, 0x10FFFF]
 * @return {boolean}      Whether code-unit is in a surrogate (hi/low) range
 */
function inSurrogateRange(codeUnit) {
  return 0xD800 <= codeUnit && codeUnit <= 0xDFFF;
}


/**
 * Return the length of the original Unicode character at given position in the
 * String by looking into the UTF-16 code unit; that is equal to 1 for any
 * non-surrogate characters in BMP ([U+0000..U+D7FF] and [U+E000, U+FFFF]); and
 * returns 2 for the hi/low surrogates ([U+D800..U+DFFF]), which are in fact
 * representing non-BMP characters ([U+10000..U+10FFFF]).
 *
 * Examples:
 * - '\u0020' => 1
 * - '\u3020' => 1
 * - '\uD835' => 2
 * - '\uD835\uDDEF' => 2
 * - '\uDDEF' => 2
 *
 * @param {string} str  Non-empty string
 * @param {number} pos  Position in the string to look for one code unit
 * @return {number}      Number 1 or 2
 */
function utf16Length(str, pos) {
  return 1 + inSurrogateRange(str.charCodeAt(pos));
}