Generational indices in C++ vs Rust

genindex.cpp

/*
Copyright 2021 Jeremy Burns

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

#include <functional>
#include <vector>
#include <cstdint>
#include <optional>
#include <tuple>

using std::vector;
using std::optional;
using std::tuple;
using std::reference_wrapper;

struct GenerationalIndex
{
    uint32_t index = 0;
    uint32_t generation = 0;
};

class GenerationalIndexAllocator
{
    struct AllocatorEntry
    {
        bool is_live = false;
        uint32_t generation = 0;
    };

    vector<AllocatorEntry> m_entries;
    vector<uint32_t> m_free_indices;

public:

    GenerationalIndex allocate();
    void deallocate(GenerationalIndex index);
    bool is_live(GenerationalIndex index) const;
};

GenerationalIndex GenerationalIndexAllocator::allocate()
{
    if (m_free_indices.size() > 0)
    {
        uint32_t index = m_free_indices.back();
        m_free_indices.pop_back();

        m_entries[index].generation += 1;
        m_entries[index].is_live = true;

        return { index, m_entries[index].generation };
    }
    else
    {
        m_entries.push_back({ true, 0 });
        return { static_cast<uint32_t>(m_entries.size()) - 1, 0 };
    }
}

void GenerationalIndexAllocator::deallocate(GenerationalIndex index)
{
    if (is_live(index))
    {
        m_entries[index.index].is_live = false;
        m_free_indices.push_back(index.index);
    }
}

bool GenerationalIndexAllocator::is_live(GenerationalIndex index) const
{
    return index.index < m_entries.size() &&
        m_entries[index.index].generation == index.generation &&
        m_entries[index.index].is_live;
}

template<typename T>
class GenerationalIndexArray
{
    struct Entry
    {
        uint32_t generation;
        T value;
    };

    vector<optional<Entry>> m_entries;

public:

    void set(GenerationalIndex index, T value)
    {
        while (m_entries.size() <= index.index)
            m_entries.push_back(std::nullopt);

        uint32_t prev_gen = 0;

        if (auto prev_entry = m_entries[index.index]) 
            prev_gen = prev_entry->generation;

        if (prev_gen > index.generation)
            exit(1);

        m_entries[index.index] = optional<Entry>{{ index.generation, value }};
    }

    void remove(GenerationalIndex index) 
    {
        if (index.index < m_entries.size())
            m_entries[index.index] = std::nullopt;
    }

public:

    T* get(GenerationalIndex index)
    {
        if (index.index >= m_entries.size()) return nullptr;

        if (auto& entry = m_entries[index.index]) 
        {
            if (entry->generation == index.generation)
                return &entry->value;
        }

        return nullptr;
    }

    const T* get(GenerationalIndex index) const
    {
        return const_cast<const T*>(const_cast<GenerationalIndexArray*>(this)->get(index));
    }

    vector<GenerationalIndex> get_all_valid_indices(const GenerationalIndexAllocator& allocator) const
    {
        vector<GenerationalIndex> result;
        
        for (auto i = 0; i < m_entries.size(); ++i)
        {
            const auto& entry = m_entries[i];
            if (!entry) continue;

            GenerationalIndex index = { i, entry->generation };
            
            if (allocator.is_live(index))
                result.push_back(index);
        }

        return result;
    }

    optional<tuple<GenerationalIndex, reference_wrapper<const T>>> get_first_valid_entry(const GenerationalIndexAllocator& allocator) const
    {
        for (auto i = 0; i < m_entries.size(); ++i)
        {
            const auto& entry = m_entries[i];
            if (!entry) continue;

            GenerationalIndex index = { i, entry->generation };
            
            if (allocator.is_live(index))
                return std::make_tuple(index, std::ref(entry->value));
        }

        return std::nullopt;
    }
};

genindex.rs

/*
Copyright 2021 Jeremy Burns

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

#[derive(Copy, Clone, Eq, PartialEq, Hash)]
struct GenerationalIndex {
    index: usize,
    generation: usize,
}

struct AllocatorEntry {
    is_live: bool,
    generation: usize,
}

struct GenerationalIndexAllocator {
    entries: Vec<AllocatorEntry>,
    free: Vec<usize>,
}

impl GenerationalIndexAllocator {
    fn new() -> GenerationalIndexAllocator {
        GenerationalIndexAllocator {
            entries: Vec::new(),
            free: Vec::new(),
        }
    }

    fn allocate(&mut self) -> GenerationalIndex {
        match self.free.pop() {
            Some(index) => {
                self.entries[index].generation += 1;
                self.entries[index].is_live = true;

                GenerationalIndex {
                    index,
                    generation: self.entries[index].generation,
                }
            }
            None => {
                self.entries.push(AllocatorEntry {
                    is_live: true,
                    generation: 0,
                });

                GenerationalIndex {
                    index: self.entries.len() - 1,
                    generation: 0,
                }
            }
        }
    }

    fn deallocate(&mut self, index: GenerationalIndex) -> bool {
        if self.is_live(index) {
            self.entries[index.index].is_live = false;
            self.free.push(index.index);
            true
        } else {
            false
        }
    }

    fn is_live(&self, index: GenerationalIndex) -> bool {
        index.index < self.entries.len()
            && self.entries[index.index].generation == index.generation
            && self.entries[index.index].is_live
    }
}

struct ArrayEntry<T> {
    value: T,
    generation: usize,
}

struct GenerationalIndexArray<T>(Vec<Option<ArrayEntry<T>>>);

impl<T> GenerationalIndexArray<T> {
    fn new() -> GenerationalIndexArray<T> {
        GenerationalIndexArray(Vec::new())
    }

    fn set(&mut self, index: GenerationalIndex, value: T) {
        while self.0.len() <= index.index {
            self.0.push(None);
        }

        let prev_gen = match &self.0[index.index] {
            Some(entry) => entry.generation,
            None => 0,
        };

        if prev_gen > index.generation {
            panic!("Attempted to write to GenerationalIndexArray with an index from previous generation");
        }

        self.0[index.index] = Some(ArrayEntry {
            value,
            generation: index.generation,
        });
    }

    fn remove(&mut self, index: GenerationalIndex) {
        if index.index < self.0.len() {
            self.0[index.index] = None;
        }
    }

    fn get(&self, index: GenerationalIndex) -> Option<&T> {
        if index.index >= self.0.len() {
            return None;
        }

        match &self.0[index.index] {
            Some(entry) => if entry.generation == index.generation {
                Some(&entry.value)
            } else {
                None
            },
            None => None,
        }
    }

    fn get_mut(&mut self, index: GenerationalIndex) -> Option<&mut T> {
        if index.index >= self.0.len() {
            return None;
        }

        match &mut self.0[index.index] {
            Some(entry) => if entry.generation == index.generation {
                Some(&mut entry.value)
            } else {
                None
            },
            None => None,
        }
    }

    fn get_all_valid_indices(
        &self,
        allocator: &GenerationalIndexAllocator,
    ) -> Vec<GenerationalIndex> {
        let mut result = Vec::new();

        for i in 0..self.0.len() {
            if let Some(entry) = &self.0[i] {
                let index = GenerationalIndex {
                    index: i,
                    generation: entry.generation,
                };
                if allocator.is_live(index) {
                    result.push(index);
                }
            }
        }

        result
    }

    fn get_first_valid_entry(
        &self,
        allocator: &GenerationalIndexAllocator,
    ) -> Option<(GenerationalIndex, &T)> {
        for i in 0..self.0.len() {
            if let Some(entry) = &self.0[i] {
                let index = GenerationalIndex {
                    index: i,
                    generation: entry.generation,
                };
                if allocator.is_live(index) {
                    return Some((index, &entry.value));
                }
            }
        }

        None
    }
}

@jaburns Thank you, that's perfect! It's a really useful piece of code :)