const (
	REPLACEMENT_CHAR = '\uFFFD';
	MAX_RUNE         = '\U0010FFFF';

	_surr1           = 0xd800;
	_surr2           = 0xdc00;
	_surr3           = 0xe000;
	_surr_self       = 0x10000;
)

proc is_surrogate(r: rune) -> bool {
	return _surr1 <= r && r < _surr3;
}

proc decode_surrogate_pair(r1, r2: rune) -> rune {
	if _surr1 <= r1 && r1 < _surr2 && _surr2 <= r2 && r2 < _surr3 {
		return (r1-_surr1)<<10 | (r2 - _surr2) + _surr_self;
	}
	return REPLACEMENT_CHAR;
}


proc encode_surrogate_pair(r: rune) -> (r1, r2: rune) {
	if r < _surr_self || r > MAX_RUNE {
		return REPLACEMENT_CHAR, REPLACEMENT_CHAR;
	}
	r -= _surr_self;
	return _surr1 + (r>>10)&0x3ff, _surr2 + r&0x3ff;
}

proc encode(d: []u16, s: []rune) {
	var n = len(s);
	for r in s {
		if r >= _surr_self {
			n++;
		}
	}

	var max_n = min(len(d), n);
	n = 0;

	for r in s {
		match r {
		case 0..<_surr1, _surr3..<_surr_self:
			d[n] = u16(r);
			n++;

		case _surr_self..MAX_RUNE:
			var r1, r2 = encode_surrogate_pair(r);
			d[n]    = u16(r1);
			d[n+1]  = u16(r2);
			n += 2;

		case:
			d[n] = u16(REPLACEMENT_CHAR);
			n++;
		}
	}
}