Wtf16LeIterator

pub const Wtf16LeIterator = struct {
    bytes: []const u8,
    i: usize,

    pub fn init(s: []const u16) Wtf16LeIterator {
        return Wtf16LeIterator{
            .bytes = mem.sliceAsBytes(s),
            .i = 0,
        };
    }

    /// If the next codepoint is encoded by a surrogate pair, returns the
    /// codepoint that the surrogate pair represents.
    /// If the next codepoint is an unpaired surrogate, returns the codepoint
    /// of the unpaired surrogate.
    pub fn nextCodepoint(it: *Wtf16LeIterator) ?u21 {
        assert(it.i <= it.bytes.len);
        if (it.i == it.bytes.len) return null;
        var code_units: [2]u16 = undefined;
        code_units[0] = mem.readInt(u16, it.bytes[it.i..][0..2], .little);
        it.i += 2;
        surrogate_pair: {
            if (utf16IsHighSurrogate(code_units[0])) {
                if (it.i >= it.bytes.len) break :surrogate_pair;
                code_units[1] = mem.readInt(u16, it.bytes[it.i..][0..2], .little);
                const codepoint = utf16DecodeSurrogatePair(&code_units) catch break :surrogate_pair;
                it.i += 2;
                return codepoint;
            }
        }
        return code_units[0];
    }
}