Support complex entity

[termoshtt]

Consider following schema:

ENTITY a;
  x: REAL;
END_ENTITY;

ENTITY by SUBTYPE_OF a;
  y: REAL;
END_ENTITY;

ENTITY bz SUBTYPE_OF a;
  z: REAL;
END_ENTITY;

STEP exchange format allows complex entity like

#1 = (A(1.0), BY(2.0), BZ(3.0));

This #1 should be regarded as { a: { x: 1.0, y: 2.0, z: 3.0} } (in JSON format).

We may have two way to implement this:

Define a "composed" type instead of "any" type

struct ByExtension {
  y: f64,
}

struct BzExtension {
  z: f64,
}

struct AComposed {
  x: f64,
  by_extension: Option<Box<ByExtension>>,
  bz_extension: Option<Box<BzExtension>>,
}

Option<Box<Extension>> is used because it increases the size_of::<AComposed>(), although this hurts memory locality. In this case x and y will be placed in different memory pages, and it may cause performance issue.

Generate concrete types for every combination and add them to "any" type

To keep every components continuous in memory, we can define every possible struct:

struct ByBz {
  x: f64,
  y: f64,
  z: f64,
}

enum AAny {
  A(Box<A>),
  By(Box<By>),
  Bz(Box<Bz>),
  ByBz(Box<ByBz>),
}

This is better in terms of memory, but the compile of generated Rust code becomes slower and slower.

[termoshtt]

This is better in terms of memory, but the compile of generated Rust code becomes slower and slower.

In AP201, there exists a case where the number of subtypes of a supertype becomes 15, thus we have to generate 2^15 = 32768 structs as Rust code. It is unacceptable.

[termoshtt]

This issue becomes out-dated since I learn a lot from ISO document after I created this issue.

[alteous]

I have a draft implementation of complex entity instantiation on a branch of my fork alteous@c76e159. I avoided the combinatorial problem by simply having multiple entity types sharing the same ID. Would you consider this acceptable solution?

I have several other fixes on the fixes branch and data section export on the export branch. With these changes it's possible to read and write a complete AP203 document. Would you like a pull request for any of them?