Parse prior hooks.go, if present (prep for more changes).

gengokit/handlers/hooks.go

@@ -2,6 +2,10 @@ package handlers
 
 import (
 	"bytes"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
 	"io"
 
 	"github.com/Unity-Technologies/truss/gengokit"
@@ -12,25 +16,37 @@ const HookPath = "handlers/hooks.gotemplate"
 
 // NewHook returns a new HookRender
 func NewHook(prev io.Reader) (gengokit.Renderable, error) {
-	return &HookRender{
-		prev: prev,
-	}, nil
+	h := new(HookRender)
+	if prev != nil {
+		h.fset = token.NewFileSet()
+		var err error
+		h.ast, err = parser.ParseFile(h.fset, "", prev, parser.ParseComments)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return h, nil
 }
 
 type HookRender struct {
-	prev io.Reader
+	fset *token.FileSet
+	ast  *ast.File
 }
 
 // Render will return the existing file if it exists, otherwise it will return
 // a brand new copy from the template.
 func (h *HookRender) Render(_ string, _ *gengokit.Data) (io.Reader, error) {
-	if h.prev != nil {
-		return h.prev, nil
-	}
 	code := new(bytes.Buffer)
-	code.WriteString(templates.HookHead)
-	for _, hd := range templates.Hooks {
-		code.WriteString(hd.Code)
+	if h.ast == nil {
+		code.WriteString(templates.HookHead)
+		for _, hd := range templates.Hooks {
+			code.WriteString(hd.Code)
+		}
+		return code, nil
+	}
+
+	if err := printer.Fprint(code, h.fset, h.ast); nil != err {
+		return nil, err
 	}
 
 	return code, nil