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CONVERSION_RULES.md

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Conversion rules

C++

Resolving circular dependencies

During porting code from Rust into C++, was found few circulard dependencies:

  • TrackCounts <--> OriginZeroLine
  • Taffy <--> functions from taffy_tree

To resolve them used pattern, described in the next references:

// A_def.h
#ifndef A_DEF_H
#define A_DEF_H

class B;
class A
{
    int _val;
    B *_b;

public:
    A(int val);
    void SetB(B *b);
    void Print();
};

#endif // A_DEF_H
// B_def.h
#ifndef B_DEF_H
#define B_DEF_H

class A;
class B
{
    double _val;
    A* _a;

public:
    B(double val);
    void SetA(A *a);
    void Print();
};

#endif // B_DEF_H
// A.h
#ifndef A_H
#define A_H

#include "A_def.h"
#include "B_def.h"

inline A::A(int val) : _val(val)
{}

inline void A::SetB(B *b) {
    _b = b;
    _b->Print();
}

inline void A::Print() {
    cout << "Type:A val=" << _val << endl;
}

#endif // A_H
// B.h
#ifndef B_H
#define B_H

#include "A_def.h"
#include "B_def.h"

inline B::B(double val) : _val(val)
{}

inline void B::SetA(A *a) {
    _a = a;
    _a->Print();
}

inline void B::Print() {
    cout << "Type:B val=" << _val << endl;
}

#endif // B_H

Scoped enumerations

All scoped enumerations must explicitly contain smallest underlying type (since default type is int) - to achieve:

  1. Rust behaviour.
  2. Smaller memory usage of huge structures that contain many fields with such enums (like Style).
enum class AlignItems : uint8_t // <-- uint8_t required here
{
    Start = 0,
    End,
    FlexStart,
    FlexEnd,
    Center,
    Baseline,
    Stretch
};

Search all enumerations with missing underlying type:

$ grep -Ern "enum class \S+\s*{"

std::reference_wrapper

All places of creation objects of std::reference_wrapper<T> type must use std::ref() or std::cref() for it - to avoid passing reference to local variable, instead of what we need.

For consistency, them really must be used in all places, even when it not really needed - to not check that places it in case of bugs. That functions produces zero overhead, so their use only increases security.

There were a few bugs during the implementation - some tests fails for strange reason (looking ahead - it was due to the lack of std::ref()). Finding the problems by using printf()/println!() to side-by-side compare internal state and result of algorithms excution in Rust and C++ took around two days :D

Examples:

const Vec<ExpectedChildInfo>& children = ...;

auto mapped_children = new_vec_with_capacity< std::tuple<size_t, NodeId, std::reference_wrapper<Style const>> >(children.size());
for(const auto& item : children) {
    const size_t& index = std::get<0>(item);
    const Style&  style = std::get<1>(item);

    // NOTE: `std::cref()` is important here. Otherwise passed style here
    // will wrong: (same/repeated), since it will be reference to local
    // variable, not reference to `item` style.
    mapped_children.push_back( std::make_tuple(index, NodeId::from(index), std::cref(style)) ); 
}
std::tuple<size_t, NodeId, InBothAbsAxis<Line<OriginZeroGridPlacement>>, std::reference_wrapper<Style const>>
operator () (const std::tuple<size_t, NodeId, std::reference_wrapper<Style const>>& t) const
{
    const auto& index = std::get<0>(t);
    const auto& node  = std::get<1>(t);
    const auto& style = std::get<2>(t).get();

    const auto origin_zero_placement = InBothAbsAxis<Line<OriginZeroGridPlacement>> {
        style.grid_column.map<OriginZeroGridPlacement>([this](const GridPlacement& placement) { return placement.into_origin_zero_placement(explicit_col_count); }),
        style.grid_row.map<OriginZeroGridPlacement>([this](const GridPlacement& placement) { return placement.into_origin_zero_placement(explicit_row_count); })
    };

    // NOTE: `std::cref()` is important here! To pass reference to 't.style', 
    // not reference to temporal 'style'
    return std::make_tuple(index, node, origin_zero_placement, std::cref(style));
}
inline TaffyResult<std::reference_wrapper<Style const>> style(NodeId node) const
{
    // `std::cref()` is not needed here (no local variables created), but we
    // use it for consistency.
    return TaffyResult<std::reference_wrapper<Style const>>::Ok( std::cref(this->nodes[node_id_into_key(node)].style) );
}

Search for all places of usage std::reference_wrapper:

$ grep -rn "std::reference_wrapper"

Rust --> C++

Reserved keywords usage

In Rust sources, in some cases used C++ keywords as names for variables or functions. In such cases, we simply use capitalized names instead. For example:

  • new -> New
  • default -> Default
  • auto -> Auto
  • explicit -> Explicit
  • or -> Or
  • template -> Template

Tests in sources

All tests (marked by #[test]) inside of sources, must be implemented as separated tests in ./tests/ directory.

Search for #[test]:

$ grep -rn "#\[test\]"

Traits :: Debug, Default, Display

TODO

$ grep -Ern "Default[^a-zA-Z]"

Constructor with new

All consistency, all Rust functions fn new(Args...) -> Self must be converted in C++ as static New function AND constructor.

Example:

// Rust
pub(crate) struct NodeData {
    pub(crate) style: Style,
    pub(crate) layout: Layout,
    pub(crate) needs_measure: bool,
    pub(crate) cache: Cache,
}

impl NodeData {
    #[must_use]
    pub const fn new(style: Style) -> Self {
        Self { style, cache: Cache::new(), layout: Layout::new(), needs_measure: false }
    }
}
struct NodeData {
    Style style;
    Layout layout;
    bool needs_measure;
    Cache cache;

    // -------------------------------------------------------------------------

    NodeData(
        const Style& style_,
        const Layout& layout_,
        bool needs_measure_,
        const Cache& cache_
    )
        : style(style_)
        , layout(layout_)
        , needs_measure(needs_measure_)
        , cache(cache_)
    {}

    // -------------------------------------------------------------------------

    // Named constructor New (actually - static function) - to be similar to Rust
    static inline NodeData New(const Style& style) {
        return NodeData{ style, Layout::New(), false, Cache::New() };
    }

    // Common constructor - in C++ style
    NodeData(const Style& style)
        : NodeData{ New(style) }
    {}
};

Search for fn new(:

$ grep -Ern "fn\s+new\s*\("

self. convert into this->

For consistency, all occurancies of self.??? in Rust convert into this->??? in C++.

Search for self.:

$ grep -rn "self\."

Iteration utilites

Simple cases occurancies of .any(...), .all(...), .position(...), ..., for readability must be replaced with one-liners:

  • .any(...) :

    • Search:

      $ grep -Ern "\.\s*any\s*\("

    • Example:

      // Rust
pub fn is_empty(&self) -> bool {
    !self.entries.iter().any(|entry| entry.is_some())
}
      // C++
bool is_empty() const {
    return !any(this->entries, [](const Option<CacheEntry>& entry) { return entry.is_some(); });
}

  • .all(...):

    • Search:

      $ grep -Ern "\.\s*all\s*\("

    • Example:

      // Rust
let tracks_all_fixed = spanned_tracks.iter()
    .all(|track| track.max_track_sizing_function.definite_value(axis_parent_size).is_some());
      // C++
const auto tracks_all_fixed =
    all(spanned_tracks, [&](const GridTrack& track) {
        return track.max_track_sizing_function.definite_value(axis_parent_size).is_some();
    });

  • .position(...):

    • Search:

      $ grep -Ern "\.\s*position\s*\("

    • Example:

      let next_row_first_item =
    remaining_items.iter().position(|item| item.placement(other_axis).start != current_row);
      const auto next_row_first_item =
    position(remaining_items, [&](const GridItem& item) { return item.placement(other_axis).start != current_row; });

  • .count() (typically it's .filter(...).count()):

    • Search:

      $ grep -Ern "\.\s*count\s*\("

    • Example:

      let row_baseline_item_count = row_items.iter().filter(|item| item.align_self == AlignSelf::Baseline).count();
      const auto row_baseline_item_count =
    filter_and_count(row_items, [](const GridItem& item) { return item.align_self == AlignSelf::Baseline; });
      let number_of_growable_tracks = tracks.iter()
    .filter(|track| track_is_affected(track))
    .filter(|track| {
        track.infinitely_growable || track.fit_content_limited_growth_limit(axis_inner_node_size) == f32::INFINITY
    })
      const auto number_of_growable_tracks =
    filter_and_filter_and_count(
        tracks,
        [&](const GridTrack& track) { return track_is_affected(track); },
        [&](const GridTrack& track) { return track.infinitely_growable || track.fit_content_limited_growth_limit(axis_inner_node_size) == std::numeric_limits<float>::infinity(); }
    );

  • .sum()

    • .map(...).sum() and .map(...).sum::<???>():

      • Search:

        $ grep -Ern "\.\s*sum\s*\("
        $ grep -Ern "\.\s*sum\s*::\s*<\s*\S+\s*>\s*\("

      • Example:

        let track_sizes: f32 = tracks.iter().map(|track| track.base_size).sum();
        const float track_sizes = map_and_sum<float>(tracks, [](const GridTrack& track) { return track.base_size; });

panic

Rust panic! must be translated into similar macro (not simple assert()).

  • Search:

    $ grep -rn "panic"

Misc

Iterators

Notice, that Rust iterators not the same as C++ iterators:

  • C++ iterators will be named cursors,
  • Rust iterators will be named streams.

Since this library is written in C++11 (for portability) and perfomance also at the head of the table (any overhead for the sake of readability is not acceptable) - where it's possible, we use alternate simple naive C++ implementation (typically it's for/while loops, even it kinda verbose or boilerplate), without unwanted magic from STL (for example, functions from <algrorithm>), or external libraries.

Alternatives that do not suit us:

Exceptions: