Version 2 of Pack

[mattcox]

• If you're writing data whose layout the reader already knows (three doubles, for example) nothing extra is stored. But when a section is variable in size, you can now write it before knowing how big it is, and Pack fills in the size once it's done. In Pack 1 the only way was to build that section separately in memory and copy it in, which meant holding the largest one twice on every save.
• A reader can now tell where it is in the data. That makes it possible to skip a section you don't recognise, so a file written by a newer version still opens in an older one.
• If a recorded size doesn't match what's actually there, you get an error at that point. In Pack 1 it would read on into the next section and produce nonsense.
• Writing to a file and writing to memory produce exactly the same bytes, with no difference in how you call it.
• Byte order and text encoding are stated when you create the packer, and can't be changed elsewhere in the program. In Pack 1 both could vary by machine, or by any other code that happened to change a shared setting and silently change the file.
• Using Int no longer compiles. It's a different size on different devices, so a file written with it only reads back on a matching machine. Pack 1 only warned in documentation, and the problem never showed up on the machine you tested on.
• Large amounts of data are much faster. Arrays of numbers copy in one go rather than a byte at a time.