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Transformation

Suppose, we want to apply certain transforming functions to the immer containers inside a large document type. The most straightforward way would be to simply create new containers with the new data, running the transforming function over each element. However, this approach has some disadvantages:

  • All new containers will be independent, no structural sharing will be preserved, leading to the same data being stored multiple times.
  • The transformation would be applied more times than necessary when some of the data is shared. Example: one vector is built by appending elements to the other vector. Transforming shared elements multiple times could be unnecessary.

Basic example

Let’s consider a simple case using the document from the first-example. The desired transformation would be to multiply each element of the immer::vector<int> by 10.

First, the document value would be created in the same way:

    const auto v1    = vector_one{1, 2, 3};
    const auto v2    = v1.push_back(4).push_back(5).push_back(6);
    const auto value = document{v1, v2};

The next component we need is the pools of all the containers from the value:

    const auto pools = immer::persist::get_output_pools(value);

The get_output_pools function returns the output pools of all immer containers that would be serialized using pools, as controlled by the policy. Here we use the default policy hana_struct_auto_policy which will use pools for all immer containers inside the document type which must be a hana::Struct.

The other required component is the conversion_map:

        const auto conversion_map = hana::make_map(hana::make_pair(
            hana::type_c<vector_one>, [](int val) { return val * 10; }));

This is a hana::map that describes the desired transformations to be applied. The key of the map is an immer container and the value is the function to be applied to each element of the corresponding container type. In this case, it will apply [](int val) { return val * 10; } to each int of the vector_one type, we have two of those in the document.

Having these two parts, we can create new pools with the transformations:

        auto transformed_pools =
            immer::persist::transform_output_pool(pools, conversion_map);

At this point, we can start converting the immer containers and create the transformed document value with them, new_value:

        const auto new_v1 =
            immer::persist::convert_container(pools, transformed_pools, v1);
        const auto expected_new_v1 = vector_one{10, 20, 30};
        REQUIRE(new_v1 == expected_new_v1);

        const auto new_v2 =
            immer::persist::convert_container(pools, transformed_pools, v2);
        const auto expected_new_v2 = vector_one{10, 20, 30, 40, 50, 60};
        REQUIRE(new_v2 == expected_new_v2);

        const auto new_value = document{new_v1, new_v2};

In order to confirm that the structural sharing has been preserved after applying the transformations, let’s serialize the new_value and inspect the JSON:

        const auto policy =
            immer::persist::hana_struct_auto_member_name_policy(document{});
        const auto str =
            immer::persist::cereal_save_with_pools(new_value, policy);
        const auto expected_json = json_t::parse(R"(
{
  "pools": {
    "ints": {
      "B": 5,
      "BL": 1,
      "inners": [
        [0, {"children": [2], "relaxed": false}],
        [3, {"children": [2, 5], "relaxed": false}]
      ],
      "leaves": [[1, [30]], [2, [10, 20]], [4, [50, 60]], [5, [30, 40]]],
      "vectors": [{"root": 0, "tail": 1}, {"root": 3, "tail": 4}]
    }
  },
  "value0": {"ints": 0, "ints2": 1}
}
        )");
        REQUIRE(json_t::parse(str) == expected_json);

And indeed, we can see in the JSON that the node [2, [10, 20]] is reused in both vectors.

Converting types

The transforming function can even return a different type. In the following example, vector<int> is transformed into vector<std::string>. The first two steps are the same as in the previous example:

    const auto v1    = vector_one{1, 2, 3};
    const auto v2    = v1.push_back(4).push_back(5).push_back(6);
    const auto value = document{v1, v2};

    const auto pools = immer::persist::get_output_pools(value);

Only this time the transforming function will convert an integer into a string:

        const auto conversion_map = hana::make_map(hana::make_pair(
            hana::type_c<vector_one>,
            [](int val) -> std::string { return fmt::format("_{}_", val); }));

Then we convert the two vectors the same way as before:

        auto transformed_pools =
            immer::persist::transform_output_pool(pools, conversion_map);

        const auto new_v1 =
            immer::persist::convert_container(pools, transformed_pools, v1);
        const auto expected_new_v1 = vector_str{"_1_", "_2_", "_3_"};
        REQUIRE(new_v1 == expected_new_v1);

        const auto new_v2 =
            immer::persist::convert_container(pools, transformed_pools, v2);
        const auto expected_new_v2 =
            vector_str{"_1_", "_2_", "_3_", "_4_", "_5_", "_6_"};
        REQUIRE(new_v2 == expected_new_v2);

And in order to confirm that the structural sharing has been preserved, we can introduce a new document type with the two vectors being vector<std::string>.

namespace {
struct document_str
{
    vector_str str;
    vector_str str2;

    auto tie() const { return std::tie(str, str2); }

    friend bool operator==(const document_str& left, const document_str& right)
    {
        return left.tie() == right.tie();
    }

    template <class Archive>
    void serialize(Archive& ar)
    {
        ar(CEREAL_NVP(str), CEREAL_NVP(str2));
    }
};
} // namespace
BOOST_HANA_ADAPT_STRUCT(document_str, str, str2);

And serialize it with pools:

        const auto new_value = document_str{new_v1, new_v2};
        const auto policy =
            immer::persist::hana_struct_auto_member_name_policy(document_str{});
        const auto str =
            immer::persist::cereal_save_with_pools(new_value, policy);
        const auto expected_json = json_t::parse(R"(
{
  "pools": {
    "str": {
      "B": 5,
      "BL": 1,
      "inners": [
        [0, {"children": [2], "relaxed": false}],
        [3, {"children": [2, 5], "relaxed": false}]
      ],
      "leaves": [
        [1, ["_3_"]],
        [2, ["_1_", "_2_"]],
        [4, ["_5_", "_6_"]],
        [5, ["_3_", "_4_"]]
      ],
      "vectors": [{"root": 0, "tail": 1}, {"root": 3, "tail": 4}]
    }
  },
  "value0": {"str": 0, "str2": 1}
}
        )");
        REQUIRE(json_t::parse(str) == expected_json);

In the resulting JSON we can confirm that the node [2, ["_1_", "_2_"]] is reused for both vectors.

Hash-based containers

As it was shown, converting vectors is conceptually simple: the transforming function is applied to each element of each node, producing a new node with the transformed elements. When it comes to the hash-based containers, that is set, map and table, their structure is defined by the used hash function, so defining the transformation may become a bit more verbose.

In the following example, we’ll start with a simple case of transforming a map. For a map, only the hash of the key matters and we will not modify the key yet. We will focus on transformations here and not on the structural sharing within the document, so we will use the immer container itself as the document. Let’s define the following policy to indicate that we want to use pools only for our container:

template <class Container>
struct direct_container_policy : immer::persist::value0_serialize_t
{
    template <class T>
    auto get_pool_types(const T&) const
    {
        return boost::hana::tuple_t<Container>;
    }
};

By default, immer uses std::hash for the hash-based containers. While this hash is sufficient for runtime use, it can’t be used for persistence, as noted in the C++ reference:

Note

Hash functions are only required to produce the same result for the same input within a single execution of a program

We will use xxHash as the hash for this example. Let’s create a small map like this:

    using int_map_t =
        immer::map<std::string, int, immer::persist::xx_hash<std::string>>;

    const auto value = int_map_t{{"one", 1}, {"two", 2}};
    const auto pools = immer::persist::get_output_pools(
        value, direct_container_policy<int_map_t>{});

Our goal is to convert the value from int to std::string. Let’s create the conversion_map like this:

    namespace hana     = boost::hana;
    using string_map_t = immer::
        map<std::string, std::string, immer::persist::xx_hash<std::string>>;

    const auto conversion_map = hana::make_map(hana::make_pair(
        hana::type_c<int_map_t>,
        hana::overload(
            [](const std::pair<std::string, int>& item) {
                return std::make_pair(item.first,
                                      fmt::format("_{}_", item.second));
            },
            [](immer::persist::target_container_type_request) {
                return string_map_t{};
            })));

A few important details to note:

  • For maps, the transforming function accepts a pair of key and value, std::pair<std::string, int>.
  • The transforming function must also be able to handle an argument of type immer::persist::target_container_type_request. This is achieved by using hana::overload to combine 2 lambdas into one callable value. When called with that argument, it should return an empty container of the type we’re transforming to. This explicit approach is necessary because there is no reliable way to automatically determine the hash algorithm for the new container. Even though in this case the type of the key doesn’t change (and so the hash remains the same), in other scenarios it might.

Once the conversion_map is defined, the actual conversion is done as before:

    auto transformed_pools =
        immer::persist::transform_output_pool(pools, conversion_map);
    const auto new_value =
        immer::persist::convert_container(pools, transformed_pools, value);
    const auto expected_new = string_map_t{{"one", "_1_"}, {"two", "_2_"}};
    REQUIRE(new_value == expected_new);

And we can see that the original map’s values have been transformed into strings.

Transforming table’s ID

For this example, we’ll transform the type of the ID of the table element while keeping the hash of it the same. This can occur, for instance, if the member that serves as the ID gets wrapped in a wrapper type.

To begin, let’s define an item type for a table:

struct old_item
{
    std::string id;
    int data;

    template <class Archive>
    void serialize(Archive& ar)
    {
        ar(CEREAL_NVP(id), CEREAL_NVP(data));
    }
};

We can create a table value with some data and get the pools for it like this:

    using table_t    = immer::table<old_item,
                                 immer::table_key_fn,
                                 immer::persist::xx_hash<std::string>>;
    const auto value = table_t{old_item{"one", 1}, old_item{"two", 2}};
    const auto pools = immer::persist::get_output_pools(
        value, direct_container_policy<table_t>{});

In this example, we want to change the type of the old_item's ID, which is std::string, while keeping its hash the same. Let’s define a wrapper for std::string and a new_item type like this:

struct new_id_t
{
    std::string id;

    friend bool operator==(const new_id_t& left, const new_id_t& right)
    {
        return left.id == right.id;
    }

    friend std::size_t xx_hash_value(const new_id_t& value)
    {
        return immer::persist::xx_hash<std::string>{}(value.id);
    }
};

struct new_item
{
    new_id_t id;
    std::string data;

    auto tie() const { return std::tie(id, data); }

    friend bool operator==(const new_item& left, const new_item& right)
    {
        return left.tie() == right.tie();
    }
};

We’re also changing the type for data from int to std::string but this change doesn’t affect the structure of the table. We define the xx_hash_value function for the new_id_t type to make it compatible with the immer::persist::xx_hash<new_id_t> hash. Then, we can define the target new_table_t type and the conversion_map that describes how to convert old_item into a new_item.

    using new_table_t = immer::
        table<new_item, immer::table_key_fn, immer::persist::xx_hash<new_id_t>>;

        const auto conversion_map = hana::make_map(hana::make_pair(
            hana::type_c<table_t>,
            hana::overload(
                [](const old_item& item) {
                    return new_item{
                        .id   = new_id_t{item.id},
                        .data = fmt::format("_{}_", item.data),
                    };
                },
                [](immer::persist::target_container_type_request) {
                    return new_table_t{};
                })));

Finally, to convert the value using the defined conversion_map we prepare the converted pools with transform_output_pool and use convert_container to convert the value table.

        auto transformed_pools =
            immer::persist::transform_output_pool(pools, conversion_map);
        const auto new_value =
            immer::persist::convert_container(pools, transformed_pools, value);
        const auto expected_new =
            new_table_t{new_item{{"one"}, "_1_"}, new_item{{"two"}, "_2_"}};
        REQUIRE(new_value == expected_new);

We can see that the new_value table contains the transformed data from the original value table.

Modifying the hash of the ID

If the key of a map, the ID of a table item or an element of a set changes its hash due to a transformation, the transformed hash-based container can no longer keep its shape and it can’t be efficiently transformed by simply applying transformations to its nodes.

immer::persist validates every container it creates from a pool. If such a hash modification occurs, a runtime exception will be thrown because it is not possible to detect this issue during compile-time. Let’s modify the previous example to also change the data of the ID:

        const auto conversion_map = hana::make_map(hana::make_pair(
            hana::type_c<table_t>,
            hana::overload(
                [](const old_item& item) {
                    return new_item{
                        // the ID's data is changed and its hash won't be the
                        // same
                        .id   = new_id_t{item.id + "_key"},
                        .data = fmt::format("_{}_", item.data),
                    };
                },
                [](immer::persist::target_container_type_request) {
                    return new_table_t{};
                })));

Now, if we attempt to convert the original table, a immer::persist::champ::hash_validation_failed_exception will be thrown:

        auto transformed_pools =
            immer::persist::transform_output_pool(pools, conversion_map);
        REQUIRE_THROWS_AS(
            immer::persist::convert_container(pools, transformed_pools, value),
            immer::persist::champ::hash_validation_failed_exception);

Even though such transformation can’t be performed efficiently, on a node level, we can still request these transformations to be applied. This will run for each value of the original container, creating a new independent container that doesn’t use structural sharing:

        const auto conversion_map = hana::make_map(hana::make_pair(
            hana::type_c<table_t>,
            hana::overload(
                [](const old_item& item) {
                    return new_item{
                        // the ID's data is changed and its hash won't be the
                        // same
                        .id   = new_id_t{item.id + "_key"},
                        .data = fmt::format("_{}_", item.data),
                    };
                },
                [](immer::persist::target_container_type_request) {
                    // We know that the hash is changing and requesting to
                    // transform in a less efficient manner
                    return immer::persist::incompatible_hash_wrapper<
                        new_table_t>{};
                })));

We can request for such container-level (as opposed to per-node level) transformation to be performed by wrapping the desired new container type new_table_t in a immer::persist::incompatible_hash_wrapper as the result of the immer::persist::target_container_type_request call.

        auto transformed_pools =
            immer::persist::transform_output_pool(pools, conversion_map);
        const auto new_value =
            immer::persist::convert_container(pools, transformed_pools, value);
        const auto expected_new = new_table_t{new_item{{"one_key"}, "_1_"},
                                              new_item{{"two_key"}, "_2_"}};
        REQUIRE(new_value == expected_new);

We can see that the transformation has been applied, the keys have the _key suffix.

Note

While different transformed containers will not have structural sharing, transforming the same container multiple times will reuse previously transformed data. In other words, transformation will be cached on the container level but not on the nodes level.

        const auto new_value_2 =
            immer::persist::convert_container(pools, transformed_pools, value);
        REQUIRE(new_value_2.impl().root == new_value.impl().root);

Transforming nested containers

Let’s consider a scenario where a transforming function works on an item within an immer container and also needs to transform another immer container. We define the types as follows:

struct nested_t
{
    vector_one ints;

    friend bool operator==(const nested_t& left, const nested_t& right)
    {
        return left.ints == right.ints;
    }

    template <class Archive>
    void serialize(Archive& ar)
    {
        ar(CEREAL_NVP(ints));
    }
};

struct with_nested_t
{
    immer::vector<nested_t> nested;

    friend bool operator==(const with_nested_t& left,
                           const with_nested_t& right)
    {
        return left.nested == right.nested;
    }

    template <class Archive>
    void serialize(Archive& ar)
    {
        ar(CEREAL_NVP(nested));
    }
};

The important property here is that we have a vector<nested_t> where nested_t contains vector<int>, so we can say a vector is nested inside another vector. We can prepare a value with some structural sharing and then serialize it:

    const auto v1    = vector_one{1, 2, 3};
    const auto v2    = v1.push_back(4).push_back(5).push_back(6);
    const auto value = with_nested_t{
        .nested =
            {
                nested_t{.ints = v1},
                nested_t{.ints = v2},
            },
    };

    const auto policy =
        immer::persist::hana_struct_auto_member_name_policy(with_nested_t{});
    const auto str = immer::persist::cereal_save_with_pools(value, policy);

The resulting JSON looks like:

    const auto expected_json = json_t::parse(R"(
{
  "pools": {
    "ints": {
      "B": 5,
      "BL": 1,
      "inners": [
        [0, {"children": [2], "relaxed": false}],
        [3, {"children": [2, 5], "relaxed": false}]
      ],
      "leaves": [[1, [3]], [2, [1, 2]], [4, [5, 6]], [5, [3, 4]]],
      "vectors": [{"root": 0, "tail": 1}, {"root": 3, "tail": 4}]
    },
    "nested": {
      "B": 5,
      "BL": 3,
      "inners": [[0, {"children": [], "relaxed": false}]],
      "leaves": [[1, [{"ints": 0}, {"ints": 1}]]],
      "vectors": [{"root": 0, "tail": 1}]
    }
  },
  "value0": {"nested": 0}
}
    )");

Looking at the JSON we can confirm that the node [2, [1, 2]] is reused. Let’s define a conversion_map like this:

    const auto conversion_map = hana::make_map(
        hana::make_pair(
            hana::type_c<vector_one>,
            [](int val) -> std::string { return fmt::format("_{}_", val); }),
        hana::make_pair(
            hana::type_c<immer::vector<nested_t>>,
            [](const nested_t& item, const auto& convert_container) {
                return new_nested_t{
                    .str =
                        convert_container(hana::type_c<vector_str>, item.ints),
                };
            }));

The transforming function for vector_one is simple as it transforms an int into a std::string. However, the function for the vector<nested_t> is more involved. When we attempt to transform one item of that vector, nested_t, we realize that inside that function we have a vector<int> to deal with. This brings us back to the problems described in the beginning of the transformations-with-pools section. To solve this issue, immer::persist provides an optional second argument to the transforming function, a function called convert_container. This function can be called with two arguments: the desired container type and the immer container to convert. This allows us to access the conversion_map we’re defining. This transformation will be performed using pools and will preserve structural sharing as expected.

Having defined the conversion_map, we apply it in the usual way and get the new_value:

    const auto pools = immer::persist::get_output_pools(value, policy);
    auto transformed_pools =
        immer::persist::transform_output_pool(pools, conversion_map);

    const auto new_value = with_new_nested_t{
        .nested = immer::persist::convert_container(
            pools, transformed_pools, value.nested),
    };

We can verify that the new_value has the expected content:

    const auto expected_new = with_new_nested_t{
        .nested =
            {
                new_nested_t{.str = {"_1_", "_2_", "_3_"}},
                new_nested_t{.str = {"_1_", "_2_", "_3_", "_4_", "_5_", "_6_"}},
            },
    };
    REQUIRE(new_value == expected_new);

And we can serialize it again to confirm that the structural sharing of the nested vectors has been preserved:

    const auto transformed_str = immer::persist::cereal_save_with_pools(
        new_value,
        immer::persist::hana_struct_auto_member_name_policy(
            with_new_nested_t{}));
    const auto expected_transformed_json = json_t::parse(R"(
{
  "pools": {
    "nested": {
      "B": 5,
      "BL": 3,
      "inners": [[0, {"children": [], "relaxed": false}]],
      "leaves": [[1, [{"str": 0}, {"str": 1}]]],
      "vectors": [{"root": 0, "tail": 1}]
    },
    "str": {
      "B": 5,
      "BL": 1,
      "inners": [
        [0, {"children": [2], "relaxed": false}],
        [3, {"children": [2, 5], "relaxed": false}]
      ],
      "leaves": [
        [1, ["_3_"]],
        [2, ["_1_", "_2_"]],
        [4, ["_5_", "_6_"]],
        [5, ["_3_", "_4_"]]
      ],
      "vectors": [{"root": 0, "tail": 1}, {"root": 3, "tail": 4}]
    }
  },
  "value0": {"nested": 0}
}
    )");

We can see that the [2, ["_1_", "_2_"]] node is still being reused in the two vectors.

Serialization
Reference
© Copyright 1980, 2017 Juan Pedro Bolivar Puente. Created using Sphinx.