readSmallMessage

pub fn readSmallMessage(ws: *WebSocket) ReadSmallTextMessageError!SmallMessage {
    const in = ws.input;
    while (true) {
        const header = try in.takeArray(2);
        const h0: Header0 = @bitCast(header[0]);
        const h1: Header1 = @bitCast(header[1]);

        switch (h0.opcode) {
            .text, .binary, .pong, .ping => {},
            .connection_close => return error.ConnectionClose,
            .continuation => return error.UnexpectedOpCode,
            _ => return error.UnexpectedOpCode,
        }

        if (!h0.fin) return error.MessageTooBig;
        if (!h1.mask) return error.MissingMaskBit;

        const len: usize = switch (h1.payload_len) {
            .len16 => try in.takeInt(u16, .big),
            .len64 => std.math.cast(usize, try in.takeInt(u64, .big)) orelse return error.MessageTooBig,
            else => @intFromEnum(h1.payload_len),
        };
        if (len > in.buffer.len) return error.MessageTooBig;
        const mask: u32 = @bitCast((try in.takeArray(4)).*);
        const payload = try in.take(len);

        // Skip pongs.
        if (h0.opcode == .pong) continue;

        // The last item may contain a partial word of unused data.
        const floored_len = (payload.len / 4) * 4;
        const u32_payload: []align(1) u32 = @ptrCast(payload[0..floored_len]);
        for (u32_payload) |*elem| elem.* ^= mask;
        const mask_bytes: []const u8 = @ptrCast(&mask);
        for (payload[floored_len..], mask_bytes[0 .. payload.len - floored_len]) |*leftover, m|
            leftover.* ^= m;

        return .{
            .opcode = h0.opcode,
            .data = payload,
        };
    }
}