Use standard golang.org/x/tools/go/callgraph package (#24)

* Delete `callgraph` package in favor of standard one
* Add `callgraphutil` package
* Add comments to test other algorithms

callgraphutil/aliases.go

@@ -0,0 +1,9 @@
+package callgraphutil
+
+import "golang.org/x/tools/go/callgraph"
+
+// Nodes is a handy alias for a slice of callgraph.Nodes.
+type Nodes = []*callgraph.Node
+
+// Edges is a handy alias for a slice of callgraph.Edges.
+type Edges = []*callgraph.Edge

callgraphutil/calls.go

@@ -0,0 +1,35 @@
+package callgraphutil
+
+import "golang.org/x/tools/go/callgraph"
+
+// CalleesOf returns nodes that are called by the caller node.
+func CalleesOf(caller *callgraph.Node) Nodes {
+	calleesMap := make(map[*callgraph.Node]bool)
+	for _, e := range caller.Out {
+		calleesMap[e.Callee] = true
+	}
+
+	// Convert map to slice.
+	calleesSlice := make([]*callgraph.Node, 0, len(calleesMap))
+	for callee := range calleesMap {
+		calleesSlice = append(calleesSlice, callee)
+	}
+
+	return calleesSlice
+}
+
+// CallersOf returns nodes that call the callee node.
+func CallersOf(callee *callgraph.Node) Nodes {
+	uniqCallers := make(map[*callgraph.Node]bool)
+	for _, e := range callee.In {
+		uniqCallers[e.Caller] = true
+	}
+
+	// Convert map to slice.
+	callersSlice := make(Nodes, 0, len(uniqCallers))
+	for caller := range uniqCallers {
+		callersSlice = append(callersSlice, caller)
+	}
+
+	return callersSlice
+}

callgraphutil/doc.go

@@ -0,0 +1,3 @@
+// Package callgraphutil implements utilities for golang.org/x/tools/go/callgraph
+// including path searching, graph construction, printing, and more.
+package callgraphutil