structs.py

# Copyright 2021 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Helpers for constructing data classes that are JAX and tree-math enabled."""

import dataclasses
import jax
from tree_math._src.vector import VectorMixin


def struct(cls):
    """Class decorator that enables JAX function transforms as well as tree math.

    Decorating a class with `@struct` makes it a dataclass that is compatible
    with arithmetic infix operators like `+`, `-`, `*` and `/`. The decorated
    class is also a valid pytree, making it compatible with JAX function
    transformations such as `jit` and `grad`.

    Example usage:
    ```
    import jax
    import tree_math

    @tree_math.struct
    class Point:
        x: float
        y: float
        static_field: int = 0  # base case 

    a = Point(0.0, 1.0)
    b = Point(2.0, 3.0)

    a + 3 * b  # Point(6.0, 10.0)
    jax.grad(lambda x, y: x @ y)(a, b)  # Point(2.0, 3.0)
    ```

    Args:
        cls: a class, written with the same syntax as a `dataclass`.

    Returns:
        A wrapped version of `cls` that implements dataclass, pytree and tree_math
        functionality.
    """
    
    # Get static fields from the class if defined
    static_fields = getattr(cls, 'static_fields', [])

    @property
    def fields(self):
        return dataclasses.fields(self)

    def asdict(self):
        return {field.name: getattr(self, field.name) for field in self.fields}

    def astuple(self):
        return tuple(getattr(self, field.name) for field in self.fields if field.name not in static_fields)

    def tree_flatten(self):
        # Flatten only the non-static fields
        children = [getattr(self, field.name) for field in self.fields if field.name not in static_fields]
        return children, None

    @classmethod
    def tree_unflatten(cls, _, children):
        # Create an instance with the provided children and static fields
        instance = cls(*children)
        for field in cls.static_fields:
            setattr(instance, field, getattr(cls, field))  # Set static fields
        return instance

    cls_as_struct = type(cls.__name__,
                         (VectorMixin, dataclasses.dataclass(cls)),
                         {'fields': fields,
                          'asdict': asdict,
                          'astuple': astuple,
                          'replace': dataclasses.replace,
                          'tree_flatten': tree_flatten,
                          'tree_unflatten': tree_unflatten,
                          '__module__': cls.__module__})
    
    return jax.tree_util.register_pytree_node_class(cls_as_struct)