validate

pub fn validate(bytes: []const u8) ValidateError!void {
    if (bytes.len == 0) return error.InvalidHostName;

    // Ignore trailing dot (FQDN).
    const end = if (bytes[bytes.len - 1] == '.') bytes.len - 1 else bytes.len;

    // The accepted maximum length of a hostname, including labels and dots.
    if (end > max_len_without_root) return error.NameTooLong;

    // Hostnames are divided into dot-separated "labels", which:
    //
    // - Start with a letter or digit
    // - Can contain letters, digits, or hyphens
    // - Must end with a letter or digit
    // - Have a minimum of 1 character and a maximum of 63
    var label_len: usize = 0;
    for (bytes[0..end], 0..) |c, i| {
        switch (c) {
            '.' => {
                if (label_len == 0 or label_len > 63) return error.InvalidHostName;
                if (!std.ascii.isAlphanumeric(bytes[i - 1])) return error.InvalidHostName;
                label_len = 0;
            },
            '-' => {
                if (label_len == 0) return error.InvalidHostName;
                label_len += 1;
            },
            else => {
                if (!std.ascii.isAlphanumeric(c)) return error.InvalidHostName;
                label_len += 1;
            },
        }
    }

    // Validate the final label
    if (label_len == 0 or label_len > 63) return error.InvalidHostName;
    if (!std.ascii.isAlphanumeric(bytes[end - 1])) return error.InvalidHostName;
}