Turn symbol_inject into a reusable package

Bug: 31559095
Test: m blueprint_tools
Change-Id: I0f7a9f14111af26d753db547c6de313a7079658a
diff --git a/symbol_inject/symbol_inject_test.go b/symbol_inject/symbol_inject_test.go
new file mode 100644
index 0000000..77ec7d9
--- /dev/null
+++ b/symbol_inject/symbol_inject_test.go
@@ -0,0 +1,67 @@
+// Copyright 2018 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package symbol_inject
+
+import (
+	"bytes"
+	"strconv"
+	"testing"
+)
+
+func TestCopyAndInject(t *testing.T) {
+	s := "abcdefghijklmnopqrstuvwxyz"
+	testCases := []struct {
+		offset, size uint64
+		value        string
+		expected     string
+	}{
+		{
+			offset:   0,
+			size:     1,
+			value:    "A",
+			expected: "Abcdefghijklmnopqrstuvwxyz",
+		},
+		{
+			offset:   1,
+			size:     1,
+			value:    "B",
+			expected: "aBcdefghijklmnopqrstuvwxyz",
+		},
+		{
+			offset:   1,
+			size:     1,
+			value:    "BCD",
+			expected: "aBcdefghijklmnopqrstuvwxyz",
+		},
+		{
+			offset:   25,
+			size:     1,
+			value:    "Z",
+			expected: "abcdefghijklmnopqrstuvwxyZ",
+		},
+	}
+
+	for i, testCase := range testCases {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			in := bytes.NewReader([]byte(s))
+			out := &bytes.Buffer{}
+			copyAndInject(in, out, testCase.offset, testCase.size, testCase.value)
+
+			if out.String() != testCase.expected {
+				t.Errorf("expected %s, got %s", testCase.expected, out.String())
+			}
+		})
+	}
+}