func abc(a int) int {
	if a == 0 {
		return a * 2
	}
	
	a++
	return a
}

package main

import (
	"bytes"
	"fmt"
	"go/ast"
	"go/format"
	"go/parser"
	"go/token"
	"io"
	"os"
	"strings"

	"golang.org/x/tools/go/ast/inspector"
	"golang.org/x/tools/go/cfg"
)

// PrintCFG print control flow graph with graphviz dot format.
func printCFG(w io.Writer, graph *cfg.CFG) {
	if graph == nil {
		return
	}

	fset := token.NewFileSet()

	fmt.Fprintln(w, "digraph structs {")
	fmt.Fprintln(w, "\tnode [shape=Mrecord]")

	// output nodes
	for _, b := range graph.Blocks {
		var labels []string
		labels = append(labels, fmt.Sprintf("<name> %s", b.String()))

		var codes []string
		for _, n := range b.Nodes {
			codes = append(codes, formatNode(fset, n))
		}
		fmt.Fprintf(w, "\t"+`%d [label="%s" tooltip="%s"]`+"\n",
			b.Index,
			strings.Join(labels, "|"),
			strings.Join(codes, "\n"),
		)
	}

        // output eges
	for _, b := range graph.Blocks {
		for _, s := range b.Succs {
			fmt.Fprintf(w, "\t"+`%d -> %d`+"\n", b.Index, s.Index)
		}
	}

	// output edges
	fmt.Fprintln(w, "}")
}

func formatNode(fset *token.FileSet, n ast.Node) string {
	var buf bytes.Buffer
	format.Node(&buf, fset, n)
	return fmt.Sprintf("%d:", n.Pos()) + buf.String()
}

func Debug(source string) {
	fset := token.NewFileSet()
	file, err := parser.ParseFile(fset, "demo.go", source, parser.ParseComments)
	if err != nil {
		panic(err)
	}

	nodeFilter := []ast.Node{(*ast.FuncDecl)(nil)}
	inspector.New([]*ast.File{file}).Preorder(nodeFilter, func(n ast.Node) {
		fn, ok := n.(*ast.FuncDecl)
		if !ok {
			return
		}
		graph := cfg.New(fn.Body, func(_ *ast.CallExpr) bool { return false })
		printCFG(os.Stdout, graph)
	})
}

func main() {
	src := `
	package testdata

	func abc(a int) int {
		if a == 0 {
			return a * 2
		}
	
		a++
		return a
	}
	`
	Debug(src)
}