Implement a "Auto-Stack" that overflows a Sequence into a Stack

[lukaslueg]

Most syntax diagrams run long horizontally but not vertically. It's also true - and related - that most syntax diagrams are made up from long Sequences but few Choices (which in turn have long Sequences). An example are the bitflags()- and named-attrs-macros.

It would be very useful to have a AutoStack which acts like a Sequence, yet has a desired aspect ratio (think 16:9). If the resulting geometry runs too long horizontally, it overflows elements into a Stack until the desired aspect ratio is reached or no actions are possible. An example of a Stack is "06_create-virtual-table-stmt_diagram.txt", where a long-running Sequence (manually) overflows into a Stack of three lanes.

To avoid overflowing just one element into the most bottom lane (where the bottom lane is mostly just one element and a long connector), the algorithm probably looks something like:

• Our input is the same as Sequence
• Init Y to 1
• As long as our Stack is wider than the threshold
  • Increment Y
  • Divide the input into Y parts of equal width and those as lanes in our Stack

It's Ok if the resulting AutoStack-struct is immutable, that is, we don't allow modifcation where we have to re-compute everything.

We'd then simply switch Sequence to AutoStack in macro_railroad's diagram generator.