4 ms·

Even the USPS reuses tracking numbers. I had a package show as "delivered" before it even shipped once, because it received a recycled tracking number.

• hgomersall 4y ago
  I don't even reuse references inside a single running instance of software. Using u64 you can generate billions a second and still not run out for hundreds of years.
  • rahimnathwani 4y ago
    Right, but u64 is kind of long for something you might want to read over the phone, or enter using DTMF tones.
    • JonathonW 4y ago
      USPS tracking numbers are at least 22 digits long (depending on shipping method; some can be 27); I don’t think ease of data entry was a priority for them. 22 digits and they somehow still manage to reuse tracking numbers. 10^22 (or 10^20, if we assume some encoding overhead) valid numbers should be able to uniquely identify a lot of mail.
      • HPsquared 4y ago
        They probably encode information in the code itself then, perhaps source/destination or similar. Perhaps to allow "offline" operations?
      • thejosh 4y ago
        A lot of the first digits are actually used for parsing. It's not a fully unique number. USPS labels (and other labels from other couriers) are an incredibly fascinating subject you would never care about until you work in the industry.
      • jsjohnst 4y ago
        For the ones you mention that have 22 digits, the first 8 digits (if memory serves) identify the class of mail service and other metadata. So that leaves you with 10^14 at most, assuming there’s no encoding overhead / check digits. > USPS tracking numbers are at least 22 digits long Actually, that’s wrong. There are shorter ones. Examples: 82 000 000 00 - Global Express Guaranteed EC 000 000 000 US - Priority Mail Express International CP 000 000 000 US - Priority Mail International There’s also foreign shipped packages that will follow various standards that aren’t 22 digits, yet are still technically a valid USPS tracking number.