Allow overriden types (#116)

This change will look at the destination package that codegen is being built into, and will skip generation of types that are already declared by files not prefixed with `ipldsch_`.

This isn't the cleanest escape-hatch, but it's a start.

schema/gen/go/externUtil.go

@@ -0,0 +1,40 @@
+package gengo
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"path"
+	"strings"
+)
+
+// getExternTypes provides a mapping of all types defined in the destination package.
+// It is used by generate to not duplicate defined types to allow overriding of types.
+func getExternTypes(pth string) (map[string]struct{}, error) {
+	set := token.NewFileSet()
+	packs, err := parser.ParseDir(set, pth, nil, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	types := make(map[string]struct{})
+	for _, pack := range packs {
+		for fname, f := range pack.Files {
+			if strings.HasPrefix(path.Base(fname), "ipldsch_") {
+				continue
+			}
+			for _, d := range f.Decls {
+				if t, isType := d.(*ast.GenDecl); isType {
+					if t.Tok == token.TYPE {
+						for _, s := range t.Specs {
+							ts := s.(*ast.TypeSpec)
+							types[ts.Name.Name] = struct{}{}
+						}
+					}
+				}
+			}
+		}
+	}
+
+	return types, nil
+}

schema/gen/go/generate.go

@@ -20,6 +20,12 @@ func Generate(pth string, pkgName string, ts schema.TypeSystem, adjCfg *AdjunctC
 		EmitInternalEnums(pkgName, f)
 	})
 
+	externs, err := getExternTypes(pth)
+	if err != nil {
+		// Consider warning that duplication may be present due to inability to parse destination.
+		externs = make(map[string]struct{})
+	}
+
 	// Local helper function for applying generation logic to each type.
 	//  We will end up doing this more than once because in this layout, more than one file contains part of the story for each type.
 	applyToEachType := func(fn func(tg TypeGenerator, w io.Writer), f io.Writer) {
@@ -28,7 +34,9 @@ func Generate(pth string, pkgName string, ts schema.TypeSystem, adjCfg *AdjunctC
 		types := ts.GetTypes()
 		keys := make(sortableTypeNames, 0, len(types))
 		for tn := range types {
-			keys = append(keys, tn)
+			if _, exists := externs[tn.String()]; !exists {
+				keys = append(keys, tn)
+			}
 		}
 		sort.Sort(keys)
 		for _, tn := range keys {