source/model.py

from __future__ import annotations

import torch
from torch import nn


class NeuralVectorField(nn.Module):
    def __init__(self) -> None:
        super().__init__()
        self.network = nn.Sequential(
            nn.Linear(2, 32),
            nn.Tanh(),
            nn.Linear(32, 32),
            nn.Tanh(),
            nn.Linear(32, 2),
        )

    def forward(self, state: torch.Tensor) -> torch.Tensor:
        return self.network(state)

    def step(self, state: torch.Tensor, delta_time: torch.Tensor) -> torch.Tensor:
        k1 = self(state)
        k2 = self(state + 0.5 * delta_time * k1)
        k3 = self(state + 0.5 * delta_time * k2)
        k4 = self(state + delta_time * k3)
        return state + delta_time * (k1 + 2 * k2 + 2 * k3 + k4) / 6


class CalibratedDiscreteTransition(nn.Module):
    def __init__(self) -> None:
        super().__init__()
        self.network = nn.Sequential(
            nn.Linear(3, 32),
            nn.Tanh(),
            nn.Linear(32, 31),
            nn.Tanh(),
            nn.Linear(31, 2),
        )
        self.log_scale = nn.Parameter(torch.zeros(2))
        self.global_log_scale = nn.Parameter(torch.zeros(1))

    def forward(self, state: torch.Tensor, delta_time: torch.Tensor) -> torch.Tensor:
        inputs = torch.cat([state, delta_time], dim=1)
        delta = self.network(inputs) * (self.log_scale + self.global_log_scale).exp()
        return state + delta


def parameter_count(model: nn.Module) -> int:
    return sum(parameter.numel() for parameter in model.parameters())