import { Assert } from '#self';
import { isLeadingSurrogate, isTrailingSurrogate, type CodePoint } from '#self';

/** https://tc39.es/ecma262/#sec-utf16decodesurrogatepair */
export function UTF16SurrogatePairToCodePoint(lead: number, trail: number): CodePoint {
  // 1. Assert: lead is a leading surrogate and trail is a trailing surrogate.
  Assert(isLeadingSurrogate(lead) && isTrailingSurrogate(trail));
  // 2. Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
  const cp = (lead - 0xD800) * 0x400 + (trail - 0xDC00) + 0x10000;
  // 3. Return the code point cp.
  return cp as CodePoint;
}