Improved Graph Implementations

graphs/graph_list.py

@@ -1,8 +1,9 @@
 #!/usr/bin/env python3
 
-# Author: OMKAR PATHAK, Nwachukwu Chidiebere
+# Original Authors: OMKAR PATHAK, Nwachukwu Chidiebere
+# Redesigned and reimplemented by Vikram Nithyanandam
 
-# Use a Python dictionary to construct the graph.
+# Use an adjacency list via Python dictionary to construct the graph.
 from __future__ import annotations
 
 from pprint import pformat
@@ -14,137 +15,126 @@
 class GraphAdjacencyList(Generic[T]):
     """
     Adjacency List type Graph Data Structure that accounts for directed and undirected
-    Graphs.  Initialize graph object indicating whether it's directed or undirected.
-
-    Directed graph example:
-    >>> d_graph = GraphAdjacencyList()
-    >>> print(d_graph)
-    {}
-    >>> d_graph.add_edge(0, 1)
-    {0: [1], 1: []}
-    >>> d_graph.add_edge(1, 2).add_edge(1, 4).add_edge(1, 5)
-    {0: [1], 1: [2, 4, 5], 2: [], 4: [], 5: []}
-    >>> d_graph.add_edge(2, 0).add_edge(2, 6).add_edge(2, 7)
-    {0: [1], 1: [2, 4, 5], 2: [0, 6, 7], 4: [], 5: [], 6: [], 7: []}
-    >>> d_graph
-    {0: [1], 1: [2, 4, 5], 2: [0, 6, 7], 4: [], 5: [], 6: [], 7: []}
-    >>> print(repr(d_graph))
-    {0: [1], 1: [2, 4, 5], 2: [0, 6, 7], 4: [], 5: [], 6: [], 7: []}
-
-    Undirected graph example:
-    >>> u_graph = GraphAdjacencyList(directed=False)
-    >>> u_graph.add_edge(0, 1)
-    {0: [1], 1: [0]}
-    >>> u_graph.add_edge(1, 2).add_edge(1, 4).add_edge(1, 5)
-    {0: [1], 1: [0, 2, 4, 5], 2: [1], 4: [1], 5: [1]}
-    >>> u_graph.add_edge(2, 0).add_edge(2, 6).add_edge(2, 7)
-    {0: [1, 2], 1: [0, 2, 4, 5], 2: [1, 0, 6, 7], 4: [1], 5: [1], 6: [2], 7: [2]}
-    >>> u_graph.add_edge(4, 5)
-    {0: [1, 2],
-     1: [0, 2, 4, 5],
-     2: [1, 0, 6, 7],
-     4: [1, 5],
-     5: [1, 4],
-     6: [2],
-     7: [2]}
-    >>> print(u_graph)
-    {0: [1, 2],
-     1: [0, 2, 4, 5],
-     2: [1, 0, 6, 7],
-     4: [1, 5],
-     5: [1, 4],
-     6: [2],
-     7: [2]}
-    >>> print(repr(u_graph))
-    {0: [1, 2],
-     1: [0, 2, 4, 5],
-     2: [1, 0, 6, 7],
-     4: [1, 5],
-     5: [1, 4],
-     6: [2],
-     7: [2]}
-     >>> char_graph = GraphAdjacencyList(directed=False)
-     >>> char_graph.add_edge('a', 'b')
-     {'a': ['b'], 'b': ['a']}
-     >>> char_graph.add_edge('b', 'c').add_edge('b', 'e').add_edge('b', 'f')
-     {'a': ['b'], 'b': ['a', 'c', 'e', 'f'], 'c': ['b'], 'e': ['b'], 'f': ['b']}
-     >>> char_graph
-     {'a': ['b'], 'b': ['a', 'c', 'e', 'f'], 'c': ['b'], 'e': ['b'], 'f': ['b']}
+    Graphs. Initialize graph object indicating whether it's directed or undirected.
     """
 
-    def __init__(self, directed: bool = True) -> None:
+    def __init__(
+        self, vertices: list[T] = [], edges: list[list[T]] = [], directed: bool = True
+    ) -> None:
         """
         Parameters:
         directed: (bool) Indicates if graph is directed or undirected. Default is True.
         """
-
-        self.adj_list: dict[T, list[T]] = {}  # dictionary of lists
+        self.adj_list: dict[T, list[T]] = {}  # dictionary of lists of T
         self.directed = directed
 
+        for vertex in vertices:
+            self.add_vertex(vertex)
+
+        for edge in edges:
+            if len(edge) != 2:
+                raise ValueError(f"Invalid input: {edge} is the wrong length.")
+            self.add_edge(edge[0], edge[1])
+
+    def add_vertex(self, vertex: T) -> GraphAdjacencyList[T]:
+        """
+        Adds a vertex to the graph. If the given vertex already exists, a ValueError will
+        be thrown.
+        """
+        if not self.contains_vertex(vertex):
+            self.adj_list[vertex] = []
+        else:
+            raise ValueError(f"Incorrect input: {vertex} is already in the graph.")
+
     def add_edge(
         self, source_vertex: T, destination_vertex: T
     ) -> GraphAdjacencyList[T]:
         """
-        Connects vertices together. Creates and Edge from source vertex to destination
-        vertex.
-        Vertices will be created if not found in graph
+        Creates an edge from source vertex to destination vertex. If any given vertex doesn't exist
+        or the edge already exists, a ValueError will be thrown.
         """
-
-        if not self.directed:  # For undirected graphs
-            # if both source vertex and destination vertex are both present in the
-            # adjacency list, add destination vertex to source vertex list of adjacent
-            # vertices and add source vertex to destination vertex list of adjacent
-            # vertices.
-            if source_vertex in self.adj_list and destination_vertex in self.adj_list:
-                self.adj_list[source_vertex].append(destination_vertex)
+        if (
+            self.contains_vertex(source_vertex)
+            and self.contains_vertex(destination_vertex)
+            and not self.contains_edge(source_vertex, destination_vertex)
+        ):
+            self.adj_list[source_vertex].append(destination_vertex)
+            if not self.directed:
                 self.adj_list[destination_vertex].append(source_vertex)
-            # if only source vertex is present in adjacency list, add destination vertex
-            # to source vertex list of adjacent vertices, then create a new vertex with
-            # destination vertex as key and assign a list containing the source vertex
-            # as it's first adjacent vertex.
-            elif source_vertex in self.adj_list:
-                self.adj_list[source_vertex].append(destination_vertex)
-                self.adj_list[destination_vertex] = [source_vertex]
-            # if only destination vertex is present in adjacency list, add source vertex
-            # to destination vertex list of adjacent vertices, then create a new vertex
-            # with source vertex as key and assign a list containing the source vertex
-            # as it's first adjacent vertex.
-            elif destination_vertex in self.adj_list:
-                self.adj_list[destination_vertex].append(source_vertex)
-                self.adj_list[source_vertex] = [destination_vertex]
-            # if both source vertex and destination vertex are not present in adjacency
-            # list, create a new vertex with source vertex as key and assign a list
-            # containing the destination vertex as it's first adjacent vertex also
-            # create a new vertex with destination vertex as key and assign a list
-            # containing the source vertex as it's first adjacent vertex.
-            else:
-                self.adj_list[source_vertex] = [destination_vertex]
-                self.adj_list[destination_vertex] = [source_vertex]
-        else:  # For directed graphs
-            # if both source vertex and destination vertex are present in adjacency
-            # list, add destination vertex to source vertex list of adjacent vertices.
-            if source_vertex in self.adj_list and destination_vertex in self.adj_list:
-                self.adj_list[source_vertex].append(destination_vertex)
-            # if only source vertex is present in adjacency list, add destination
-            # vertex to source vertex list of adjacent vertices and create a new vertex
-            # with destination vertex as key, which has no adjacent vertex
-            elif source_vertex in self.adj_list:
-                self.adj_list[source_vertex].append(destination_vertex)
-                self.adj_list[destination_vertex] = []
-            # if only destination vertex is present in adjacency list, create a new
-            # vertex with source vertex as key and assign a list containing destination
-            # vertex as first adjacent vertex
-            elif destination_vertex in self.adj_list:
-                self.adj_list[source_vertex] = [destination_vertex]
-            # if both source vertex and destination vertex are not present in adjacency
-            # list, create a new vertex with source vertex as key and a list containing
-            # destination vertex as it's first adjacent vertex. Then create a new vertex
-            # with destination vertex as key, which has no adjacent vertex
+        else:
+            raise ValueError(
+                f"Incorrect input: Either {source_vertex} or {destination_vertex} does not exist \
+                  OR the requested edge already exists between them."
+            )
+        return self
+
+    def remove_vertex(self, vertex: T) -> GraphAdjacencyList[T]:
+        """
+        Removes the given vertex from the graph and deletes all incoming and outgoing edges from
+        the given vertex as well. If the given vertex does not exist, a ValueError will be thrown.
+        """
+        if self.contains_vertex(vertex):
+            if not self.directed:
+                # If not directed, find all neighboring vertices and delete all references of
+                # edges connecting to the given vertex
+                for neighbor in self.adj_list[vertex]:
+                    self.adj_list[neighbor].remove(vertex)
             else:
-                self.adj_list[source_vertex] = [destination_vertex]
-                self.adj_list[destination_vertex] = []
+                # If directed, search all neighbors of all vertices and delete all references of
+                # edges connecting to the given vertex
+                for edge_list in self.adj_list.values():
+                    if vertex in edge_list:
+                        edge_list.remove(vertex)
 
-        return self
+            # Finally, delete the given vertex and all of its outgoing edge references
+            self.adj_list.pop(vertex)
+        else:
+            raise ValueError(f"Incorrect input: {vertex} does not exist in this graph.")
+
+    def remove_edge(
+        self, source_vertex: T, destination_vertex: T
+    ) -> GraphAdjacencyList[T]:
+        """
+        Removes the edge between the two vertices. If any given vertex doesn't exist
+        or the edge does not exist, a ValueError will be thrown.
+        """
+        if (
+            self.contains_vertex(source_vertex)
+            and self.contains_vertex(destination_vertex)
+            and self.contains_edge(source_vertex, destination_vertex)
+        ):
+            self.adj_list[source_vertex].remove(destination_vertex)
+            if not self.directed:
+                self.adj_list[destination_vertex].remove(source_vertex)
+        else:
+            raise ValueError(
+                f"Incorrect input: Either {source_vertex} or {destination_vertex} do not exist \
+                OR the requested edge does not exists between them."
+            )
+
+    def contains_vertex(self, vertex: T) -> bool:
+        """
+        Returns True if the graph contains the vertex, False otherwise.
+        """
+        return vertex in self.adj_list
+
+    def contains_edge(self, source_vertex: T, destination_vertex: T) -> bool:
+        """
+        Returns True if the graph contains the edge from the source_vertex to the
+        destination_vertex, False otherwise. If any given vertex doesn't exist, a
+        ValueError will be thrown.
+        """
+        if self.contains_vertex(source_vertex) and self.contains_vertex(
+            destination_vertex
+        ):
+            return True if destination_vertex in self.adj_list[source_vertex] else False
+        else:
+            raise ValueError(
+                f"Incorrect input: Either {source_vertex} or {destination_vertex} does not exist."
+            )
+
+    def clear_graph(self) -> None:
+        self.adj_list: dict[T, list[T]] = {}
 
     def __repr__(self) -> str:
         return pformat(self.adj_list)