From 6f0a9042023b62787922f46baf222c86cb144ef8 Mon Sep 17 00:00:00 2001
From: Gourav Suri <gourav.suri@grabtaxi.com>
Date: Sat, 23 Oct 2021 02:43:40 +0530
Subject: [PATCH] added graph data structure

---
 Graph/Graph.go | 116 +++++++++++++++++++++++++++++++++++++++++++++++++
 README.md      |   1 +
 2 files changed, 117 insertions(+)
 create mode 100644 Graph/Graph.go

diff --git a/Graph/Graph.go b/Graph/Graph.go
new file mode 100644
index 0000000..b99460b
--- /dev/null
+++ b/Graph/Graph.go
@@ -0,0 +1,116 @@
+package Graph
+
+import (
+"errors"
+"sync"
+)
+
+var (
+	// ErrVertexNotFound is returned when an operation is requested on a
+	// non-existent vertex.
+	ErrVertexNotFound = errors.New("vertex not found")
+
+	// ErrSelfLoop is returned when an operation tries to create a disallowed
+	// self loop.
+	ErrSelfLoop = errors.New("self loops not permitted")
+
+	// ErrParallelEdge is returned when an operation tries to create a
+	// disallowed parallel edge.
+	ErrParallelEdge = errors.New("parallel edges are not permitted")
+)
+
+// SimpleGraph is a mutable, non-persistent undirected graph.
+// Parallel edges and self-loops are not permitted.
+// Additional description: https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)#Simple_graph
+type SimpleGraph struct {
+	mutex         sync.RWMutex
+	adjacencyList map[interface{}]map[interface{}]struct{}
+	v, e          int
+}
+
+// V returns the number of vertices in the SimpleGraph
+func (g *SimpleGraph) V() int {
+	g.mutex.RLock()
+	defer g.mutex.RUnlock()
+
+	return g.v
+}
+
+// E returns the number of edges in the SimpleGraph
+func (g *SimpleGraph) E() int {
+	g.mutex.RLock()
+	defer g.mutex.RUnlock()
+
+	return g.e
+}
+
+// AddEdge will create an edge between vertices v and w
+func (g *SimpleGraph) AddEdge(v, w interface{}) error {
+	g.mutex.Lock()
+	defer g.mutex.Unlock()
+
+	if v == w {
+		return ErrSelfLoop
+	}
+
+	g.addVertex(v)
+	g.addVertex(w)
+
+	if _, ok := g.adjacencyList[v][w]; ok {
+		return ErrParallelEdge
+	}
+
+	g.adjacencyList[v][w] = struct{}{}
+	g.adjacencyList[w][v] = struct{}{}
+	g.e++
+	return nil
+}
+
+// Adj returns the list of all vertices connected to v
+func (g *SimpleGraph) Adj(v interface{}) ([]interface{}, error) {
+	g.mutex.RLock()
+	defer g.mutex.RUnlock()
+
+	deg, err := g.Degree(v)
+	if err != nil {
+		return nil, ErrVertexNotFound
+	}
+
+	adj := make([]interface{}, deg)
+	i := 0
+	for key := range g.adjacencyList[v] {
+		adj[i] = key
+		i++
+	}
+	return adj, nil
+}
+
+// Degree returns the number of vertices connected to v
+func (g *SimpleGraph) Degree(v interface{}) (int, error) {
+	g.mutex.RLock()
+	defer g.mutex.RUnlock()
+
+	val, ok := g.adjacencyList[v]
+	if !ok {
+		return 0, ErrVertexNotFound
+	}
+	return len(val), nil
+}
+
+func (g *SimpleGraph) addVertex(v interface{}) {
+	mm, ok := g.adjacencyList[v]
+	if !ok {
+		mm = make(map[interface{}]struct{})
+		g.adjacencyList[v] = mm
+		g.v++
+	}
+}
+
+// NewSimpleGraph creates and returns a SimpleGraph
+func NewSimpleGraph() *SimpleGraph {
+	return &SimpleGraph{
+		adjacencyList: make(map[interface{}]map[interface{}]struct{}),
+		v:             0,
+		e:             0,
+	}
+}
\ No newline at end of file
diff --git a/README.md b/README.md
index 90c81b4..aeda8eb 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,7 @@ There are several data structures and algorithms implemented in this project. Th
 - Binary Tree
 - Hash Table
 - Trie
+- Graph
 
 ##### Searching algorithms
 - Linear Search