commit b7856f8e273bfab36ddd6b26fb7c8b4520e97f40
author Paulo Pinheiro <paulovictor.pinheiro@gmail.com> Fri May 26 20:00:33 2023 +0200
committer GitHub <noreply@github.com> Fri May 26 11:00:33 2023 -0700
tree 51c65e567a2c623083d8da1e86c7564429705ea2
parent 0100f6a5779831fa7a651e4b67ef389a8752bd9b

Add Kotlin multiplatform support (#7969)

* [Kotlin] Introduction to Kotlin Multiplaform

The first implementation of the Kotlin code generation was made years
ago at the time Kotlin Multiplaform was not stable and Kotlin is mostly
used on JVM-based targets. For this reason the generated code uses java
based runtime.

That design decision comes with many drawbacks, leaving the code
generated more java-like and making it impossible to use more advanced
features of the Kotlin language.

In this change we are adding two parts: A pure, multi-plaform, Kotlin
runtime and a new code generator to accompany it.

* [Kotlin] Remove scalar sign cast from code generation

Now that we have a new runtime the accepts unsigned types, we don't
need to code generate casting back and from signed scalars. This
MR removes this from both code generations and adds the necessary
API to the runtime.

* [Kotlin] Use offset on public API to represent buffer position

Currently, kotlin was following Java's approach of representing objects,
vectors, tables as "Int" (the position of it in the buffer). This change
replaces naked Int with Offset<T>, offering a type-safe API. So,
instead of

fun Table.createTable(b: FlatBufferBuilder, subTable: Int)

We will have

fun Table.createTable(b: FlatBufferBuilder, subTable: Offset<SubTable>)

Making impossible to accidentally switch parameters.

The performance should be similar to use Int as we are using value
class for Offset and ArrayOffset, which most of the time translate to
Int in the bytecode.

* [Kotlin] Add builder for tables

Add builder constructor to make create of table more ergonomic.
For example the movie sample for the test set could be written as:

Movie.createMovie(fbb,
    mainCharacterType = Character_.MuLan,
    mainCharacter = att) {
    charactersType = charsType
    this.characters = characters
}

instead of:

Movie.startMovie(fbb)
Movie.addMainCharacterType(fbb, Character_.MuLan)
Movie.addMainCharacter(fbb, att as Offset<Any>)
Movie.addCharactersType(fbb, charsType)
Movie.addCharacters(fbb, charsVec)
Movie.endMovie(fbb)

* [Kotlin] Move enum types to value class

Moving to flatbuffer enums to value class adds type safety for parameters
with minimum to no performance impact.

* [Kotlin] Simplify Union parameters to avoid naked casting

Just a small change on the APIs that receive union as parameters,
creating a typealias UnionOffset to avoid using Offset<Any>. To "convert"
an table offset to an union, one just call Offset.toUnion().

* [Kotlin] Apply clang-format on kotlin code generators

* [Kotlin] Update kotlin generator to follow official naming conventions

Updating directory, package and enum naming to follow Kotlin official
convention.

https://kotlinlang.org/docs/coding-conventions.html#naming-rules

* [Kotlin] Add fixes to improve performance

1 - Add benchmark comparing serialization between Java & Kotlin
2 - ReadWriteBuffer does not auto-grow (thus avoid check size in every op)
3 - Add specialized add functions on FlatBufferBuilder to avoid boxing
offsets.
4 - Remove a few Kotlin syntax sugar that generated performance penalties.

* [Kotlin] Remove builder from Kotlin KMP and add some optimizations
to avoid boxing of Offset classes

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>