graph.go

package main

import (
	"bytes"
	"fmt"
)

type Node string

type Graph struct {
	al map[Node]map[Node]bool // adjacency list
}

func NewGraph() Graph {
	return Graph{al: make(map[Node]map[Node]bool)}
}

func (g *Graph) AddNode(n Node) {
	al := g.al
	_, ok := al[n]
	if !ok {
		al[n] = make(map[Node]bool)
	}
}

func (g *Graph) AddEdge(tail Node, head Node) {
	g.AddNode(tail)
	g.AddNode(head)
	g.al[tail][head] = true
}

func (g *Graph) String() string {
	var buffer bytes.Buffer
	buffer.WriteString("digraph {\n")

	ids := make(map[Node]int)
	id := 0
	for node := range g.al {
		id++
		ids[node] = id
		buffer.WriteString(fmt.Sprintf("  %v [label=%v]\n", id, node))
	}

	for tail, heads := range g.al {
		for head := range heads {
			buffer.WriteString(fmt.Sprintf("  %v -> %v\n", ids[tail], ids[head]))
		}
	}

	buffer.WriteString("}\n")
	return buffer.String()
}

func main() {
	g := NewGraph()
	g.AddEdge("one", "two")
	g.AddEdge("two", "three")
	g.AddEdge("three", "one")
	fmt.Println(g)
}