Add cell-centric discrete spaces (experimental) (#1994)

## Summary
This PR introduces an alteranative implementation for discrete spaces. This implementation centers on the explicit inclusion of a Cell class. Agents can occupy cells. Cells have connections, specifying their neighbors. The resulting classes offer a cell centric API where agents interact with a cell, and query the cell for its neighbors. 

To capture a collection of cells, and their content (_i.e._, Agents), this PR adds a new CellCollection class. This is an immutable collection of cells with convenient attribute accessors to the cells, or their agents. 

This PR also includes a CellAgent class which extends the default Agent class by adding a `move_to` method that works in conjunction with the new discrete spaces. 

From a performance point of view, the current code is a bit slower in building the grid and cell data structure, but in most use cases this increase in time for model initialization will be more than offset by the faster retrieval of neighboring cells and the agents that occupy them.

## Motive
The PR emerged out of various experiments aimed at improving the performance of the current discrete space code. Moreover, it turned out that a cell centric API resolved various open issues (_e.g._, #1900, #1903, #1953). 

## Implementation
The key idea is to have Cells with connections, and using this to generate neighborhoods for a given radius. So all discrete space classes are in essence a [linked data structure](https://en.wikipedia.org/wiki/Linked_data_structure).

The cell centric API idea is used to implement 4 key discrete space classes: OrthogonalMooreGrid, OrthogonalVonNeumannGrid (alternative for SingleGrid and MultiGrid, and moore and von Neumann neighborhood) , HexGrid (alternative for SingleHexGrid and MultiHexGrid), and Network (alternative for NetworkGrid). Cells have a capacity, so there is no longer a need for seperating Single and Multi grids. Moore and von Neumann reflect different neighborhood connections and so are now implemented as seperate classes. 

---------

Co-authored-by: Jan Kwakkel <j.h.kwakkel@tudelft.nl>

benchmarks/Schelling/schelling.py

@@ -1,12 +1,14 @@
-import mesa
+from mesa import Model
+from mesa.experimental.cell_space import CellAgent, OrthogonalMooreGrid
+from mesa.time import RandomActivation
 
 
-class SchellingAgent(mesa.Agent):
+class SchellingAgent(CellAgent):
     """
     Schelling segregation agent
     """
 
-    def __init__(self, unique_id, model, agent_type):
+    def __init__(self, unique_id, model, agent_type, radius, homophily):
         """
         Create a new Schelling agent.
         Args:
@@ -16,31 +18,32 @@ def __init__(self, unique_id, model, agent_type):
         """
         super().__init__(unique_id, model)
         self.type = agent_type
+        self.radius = radius
+        self.homophily = homophily
 
     def step(self):
         similar = 0
-        for neighbor in self.model.grid.iter_neighbors(
-            self.pos, moore=True, radius=self.model.radius
-        ):
+        neighborhood = self.cell.neighborhood(radius=self.radius)
+        for neighbor in neighborhood.agents:
             if neighbor.type == self.type:
                 similar += 1
 
         # If unhappy, move:
-        if similar < self.model.homophily:
-            self.model.grid.move_to_empty(self)
+        if similar < self.homophily:
+            self.move_to(self.model.grid.select_random_empty_cell())
         else:
             self.model.happy += 1
 
 
-class Schelling(mesa.Model):
+class Schelling(Model):
     """
     Model class for the Schelling segregation model.
     """
 
     def __init__(
         self,
-        width=40,
         height=40,
+        width=40,
         homophily=3,
         radius=1,
         density=0.8,
@@ -51,35 +54,40 @@ def __init__(
         Create a new Schelling model.
 
         Args:
-            width, height: Size of the space.
+            height, width: Size of the space.
             density: Initial Chance for a cell to populated
             minority_pc: Chances for an agent to be in minority class
             homophily: Minimum number of agents of same class needed to be happy
             radius: Search radius for checking similarity
             seed: Seed for Reproducibility
         """
         super().__init__(seed=seed)
-        self.width = width
         self.height = height
+        self.width = width
         self.density = density
         self.minority_pc = minority_pc
-        self.homophily = homophily
-        self.radius = radius
 
-        self.schedule = mesa.time.RandomActivation(self)
-        self.grid = mesa.space.SingleGrid(width, height, torus=True)
+        self.schedule = RandomActivation(self)
+        self.grid = OrthogonalMooreGrid(
+            [height, width],
+            torus=True,
+            capacity=1,
+            random=self.random,
+        )
 
         self.happy = 0
 
         # Set up agents
         # We use a grid iterator that returns
         # the coordinates of a cell as well as
         # its contents. (coord_iter)
-        for _, pos in self.grid.coord_iter():
+        for cell in self.grid:
             if self.random.random() < self.density:
                 agent_type = 1 if self.random.random() < self.minority_pc else 0
-                agent = SchellingAgent(self.next_id(), self, agent_type)
-                self.grid.place_agent(agent, pos)
+                agent = SchellingAgent(
+                    self.next_id(), self, agent_type, radius, homophily
+                )
+                agent.move_to(cell)
                 self.schedule.add(agent)
 
     def step(self):
@@ -93,13 +101,12 @@ def step(self):
 if __name__ == "__main__":
     import time
 
-    # model = Schelling(seed=15, width=40, height=40, homophily=3, radius=1, density=0.625)
+    # model = Schelling(seed=15, height=40, width=40, homophily=3, radius=1, density=0.625)
     model = Schelling(
-        seed=15, width=100, height=100, homophily=8, radius=2, density=0.8
+        seed=15, height=100, width=100, homophily=8, radius=2, density=0.8
     )
 
     start_time = time.perf_counter()
     for _ in range(100):
         model.step()
-
     print(time.perf_counter() - start_time)

benchmarks/WolfSheep/wolf_sheep.py

@@ -9,41 +9,40 @@
     Northwestern University, Evanston, IL.
 """
 
-import mesa
+import math
 
+from mesa import Model
+from mesa.experimental.cell_space import CellAgent, OrthogonalVonNeumannGrid
+from mesa.time import RandomActivationByType
 
-class Animal(mesa.Agent):
-    def __init__(self, unique_id, model, moore, energy, p_reproduce, energy_from_food):
+
+class Animal(CellAgent):
+    def __init__(self, unique_id, model, energy, p_reproduce, energy_from_food):
         super().__init__(unique_id, model)
         self.energy = energy
         self.p_reproduce = p_reproduce
         self.energy_from_food = energy_from_food
-        self.moore = moore
 
     def random_move(self):
-        next_moves = self.model.grid.get_neighborhood(self.pos, self.moore, True)
-        next_move = self.random.choice(next_moves)
-        # Now move:
-        self.model.grid.move_agent(self, next_move)
+        self.move_to(self.cell.neighborhood().select_random_cell())
 
     def spawn_offspring(self):
         self.energy /= 2
         offspring = self.__class__(
             self.model.next_id(),
             self.model,
-            self.moore,
             self.energy,
             self.p_reproduce,
             self.energy_from_food,
         )
-        self.model.grid.place_agent(offspring, self.pos)
+        offspring.move_to(self.cell)
         self.model.schedule.add(offspring)
 
     def feed(self):
         ...
 
     def die(self):
-        self.model.grid.remove_agent(self)
+        self.cell.remove_agent(self)
         self.remove()
 
     def step(self):
@@ -67,8 +66,9 @@ class Sheep(Animal):
 
     def feed(self):
         # If there is grass available, eat it
-        agents = self.model.grid.get_cell_list_contents(self.pos)
-        grass_patch = next(obj for obj in agents if isinstance(obj, GrassPatch))
+        grass_patch = next(
+            obj for obj in self.cell.agents if isinstance(obj, GrassPatch)
+        )
         if grass_patch.fully_grown:
             self.energy += self.energy_from_food
             grass_patch.fully_grown = False
@@ -80,8 +80,7 @@ class Wolf(Animal):
     """
 
     def feed(self):
-        agents = self.model.grid.get_cell_list_contents(self.pos)
-        sheep = [obj for obj in agents if isinstance(obj, Sheep)]
+        sheep = [obj for obj in self.cell.agents if isinstance(obj, Sheep)]
         if len(sheep) > 0:
             sheep_to_eat = self.random.choice(sheep)
             self.energy += self.energy_from_food
@@ -90,7 +89,7 @@ def feed(self):
             sheep_to_eat.die()
 
 
-class GrassPatch(mesa.Agent):
+class GrassPatch(CellAgent):
     """
     A patch of grass that grows at a fixed rate and it is eaten by sheep
     """
@@ -100,7 +99,7 @@ def __init__(self, unique_id, model, fully_grown, countdown):
         Creates a new patch of grass
 
         Args:
-            grown: (boolean) Whether the patch of grass is fully grown or not
+            fully_grown: (boolean) Whether the patch of grass is fully grown or not
             countdown: Time for the patch of grass to be fully grown again
         """
         super().__init__(unique_id, model)
@@ -117,7 +116,7 @@ def step(self):
                 self.countdown -= 1
 
 
-class WolfSheep(mesa.Model):
+class WolfSheep(Model):
     """
     Wolf-Sheep Predation Model
 
@@ -126,7 +125,6 @@ class WolfSheep(mesa.Model):
 
     def __init__(
         self,
-        seed,
         height,
         width,
         initial_sheep,
@@ -136,7 +134,7 @@ def __init__(
         grass_regrowth_time,
         wolf_gain_from_food=13,
         sheep_gain_from_food=5,
-        moore=False,
+        seed=None,
     ):
         """
         Create a new Wolf-Sheep model with the given parameters.
@@ -152,6 +150,7 @@ def __init__(
                                  once it is eaten
             sheep_gain_from_food: Energy sheep gain from grass, if enabled.
             moore:
+            seed
         """
         super().__init__(seed=seed)
         # Set parameters
@@ -161,24 +160,25 @@ def __init__(
         self.initial_wolves = initial_wolves
         self.grass_regrowth_time = grass_regrowth_time
 
-        self.schedule = mesa.time.RandomActivationByType(self)
-        self.grid = mesa.space.MultiGrid(self.height, self.width, torus=False)
+        self.schedule = RandomActivationByType(self)
+        self.grid = OrthogonalVonNeumannGrid(
+            [self.height, self.width],
+            torus=False,
+            capacity=math.inf,
+            random=self.random,
+        )
 
+        # Create sheep:
         for _ in range(self.initial_sheep):
             pos = (
                 self.random.randrange(self.width),
                 self.random.randrange(self.height),
             )
             energy = self.random.randrange(2 * sheep_gain_from_food)
             sheep = Sheep(
-                self.next_id(),
-                self,
-                moore,
-                energy,
-                sheep_reproduce,
-                sheep_gain_from_food,
+                self.next_id(), self, energy, sheep_reproduce, sheep_gain_from_food
             )
-            self.grid.place_agent(sheep, pos)
+            sheep.move_to(self.grid[pos])
             self.schedule.add(sheep)
 
         # Create wolves
@@ -189,21 +189,21 @@ def __init__(
             )
             energy = self.random.randrange(2 * wolf_gain_from_food)
             wolf = Wolf(
-                self.next_id(), self, moore, energy, wolf_reproduce, wolf_gain_from_food
+                self.next_id(), self, energy, wolf_reproduce, wolf_gain_from_food
             )
-            self.grid.place_agent(wolf, pos)
+            wolf.move_to(self.grid[pos])
             self.schedule.add(wolf)
 
         # Create grass patches
         possibly_fully_grown = [True, False]
-        for _agent, pos in self.grid.coord_iter():
+        for cell in self.grid:
             fully_grown = self.random.choice(possibly_fully_grown)
             if fully_grown:
                 countdown = self.grass_regrowth_time
             else:
                 countdown = self.random.randrange(self.grass_regrowth_time)
             patch = GrassPatch(self.next_id(), self, fully_grown, countdown)
-            self.grid.place_agent(patch, pos)
+            patch.move_to(cell)
             self.schedule.add(patch)
 
     def step(self):
@@ -213,7 +213,7 @@ def step(self):
 if __name__ == "__main__":
     import time
 
-    model = WolfSheep(15, 25, 25, 60, 40, 0.2, 0.1, 20)
+    model = WolfSheep(25, 25, 60, 40, 0.2, 0.1, 20, seed=15)
 
     start_time = time.perf_counter()
     for _ in range(100):

mesa/experimental/__init__.py

@@ -1 +1,5 @@
 from .jupyter_viz import JupyterViz, make_text, Slider  # noqa
+from mesa.experimental import cell_space
+
+
+__all__ = ["JupyterViz", "make_text", "Slider", "cell_space"]

mesa/experimental/cell_space/__init__.py

@@ -0,0 +1,23 @@
+from mesa.experimental.cell_space.cell import Cell
+from mesa.experimental.cell_space.cell_agent import CellAgent
+from mesa.experimental.cell_space.cell_collection import CellCollection
+from mesa.experimental.cell_space.discrete_space import DiscreteSpace
+from mesa.experimental.cell_space.grid import (
+    Grid,
+    HexGrid,
+    OrthogonalMooreGrid,
+    OrthogonalVonNeumannGrid,
+)
+from mesa.experimental.cell_space.network import Network
+
+__all__ = [
+    "CellCollection",
+    "Cell",
+    "CellAgent",
+    "DiscreteSpace",
+    "Grid",
+    "HexGrid",
+    "OrthogonalMooreGrid",
+    "OrthogonalVonNeumannGrid",
+    "Network",
+]