nix/src/libutil/types.hh

#pragma once

#include "ref.hh"

#include <list>
#include <set>
#include <string>
#include <limits>
#include <map>
#include <variant>
#include <vector>

namespace nix {

typedef std::list<std::string> Strings;
typedef std::set<std::string> StringSet;
typedef std::map<std::string, std::string> StringMap;
typedef std::map<std::string, std::string> StringPairs;

/* Paths are just strings. */
typedef std::string Path;
typedef std::string_view PathView;
typedef std::list<Path> Paths;
typedef std::set<Path> PathSet;

typedef std::vector<std::pair<std::string, std::string>> Headers;

/* Helper class to run code at startup. */
template<typename T>
struct OnStartup
{
    OnStartup(T && t) { t(); }
};

/* Wrap bools to prevent string literals (i.e. 'char *') from being
   cast to a bool in Attr. */
template<typename T>
struct Explicit {
    T t;

    bool operator ==(const Explicit<T> & other) const
    {
        return t == other.t;
    }
};


/* This wants to be a little bit like rust's Cow type.
   Some parts of the evaluator benefit greatly from being able to reuse
   existing allocations for strings, but have to be able to also use
   newly allocated storage for values.

   We do not define implicit conversions, even with ref qualifiers,
   since those can easily become ambiguous to the reader and can degrade
   into copying behaviour we want to avoid. */
class BackedStringView {
private:
    std::variant<std::string, std::string_view> data;

    /* Needed to introduce a temporary since operator-> must return
       a pointer. Without this we'd need to store the view object
       even when we already own a string. */
    class Ptr {
    private:
        std::string_view view;
    public:
        Ptr(std::string_view view): view(view) {}
        const std::string_view * operator->() const { return &view; }
    };

public:
    BackedStringView(std::string && s): data(std::move(s)) {}
    BackedStringView(std::string_view sv): data(sv) {}
    template<size_t N>
    BackedStringView(const char (& lit)[N]): data(std::string_view(lit)) {}

    BackedStringView(const BackedStringView &) = delete;
    BackedStringView & operator=(const BackedStringView &) = delete;

    /* We only want move operations defined since the sole purpose of
       this type is to avoid copies. */
    BackedStringView(BackedStringView && other) = default;
    BackedStringView & operator=(BackedStringView && other) = default;

    bool isOwned() const
    {
        return std::holds_alternative<std::string>(data);
    }

    std::string toOwned() &&
    {
        return isOwned()
            ? std::move(std::get<std::string>(data))
            : std::string(std::get<std::string_view>(data));
    }

    std::string_view operator*() const
    {
        return isOwned()
            ? std::get<std::string>(data)
            : std::get<std::string_view>(data);
    }
    Ptr operator->() const { return Ptr(**this); }
};

/* Provides an indexable container like vector<> with memory overhead
   guarantees like list<> by allocating storage in chunks of ChunkSize
   elements instead of using a contiguous memory allocation like vector<>
   does. Not using a single vector that is resized reduces memory overhead
   on large data sets by on average (growth factor)/2, mostly
   eliminates copies within the vector during resizing, and provides stable
   references to its elements. */
template<typename T, size_t ChunkSize>
class ChunkedVector {
private:
    uint32_t size_ = 0;
    std::vector<std::vector<T>> chunks;

    /* keep this out of the ::add hot path */
    [[gnu::noinline]]
    auto & addChunk()
    {
        if (size_ >= std::numeric_limits<uint32_t>::max() - ChunkSize)
            abort();
        chunks.emplace_back();
        chunks.back().reserve(ChunkSize);
        return chunks.back();
    }

public:
    ChunkedVector(uint32_t reserve)
    {
        chunks.reserve(reserve);
        addChunk();
    }

    uint32_t size() const { return size_; }

    std::pair<T &, uint32_t> add(T value)
    {
        const auto idx = size_++;
        auto & chunk = [&] () -> auto & {
            if (auto & back = chunks.back(); back.size() < ChunkSize)
                return back;
            return addChunk();
        }();
        auto & result = chunk.emplace_back(std::move(value));
        return {result, idx};
    }

    const T & operator[](uint32_t idx) const
    {
        return chunks[idx / ChunkSize][idx % ChunkSize];
    }
};

}