[Go] Implements a SharedStrings function (#5733)

* Adds the sharedstring implementation for go

* Reimplemented testcase according to request

go/builder.go

@@ -17,6 +17,8 @@ type Builder struct {
 	head      UOffsetT
 	nested    bool
 	finished  bool
+
+	sharedStrings map[string]UOffsetT
 }
 
 const fileIdentifierLength = 4
@@ -33,7 +35,6 @@ func NewBuilder(initialSize int) *Builder {
 	b.head = UOffsetT(initialSize)
 	b.minalign = 1
 	b.vtables = make([]UOffsetT, 0, 16) // sensible default capacity
-
 	return b
 }
 
@@ -307,6 +308,20 @@ func (b *Builder) EndVector(vectorNumElems int) UOffsetT {
 	return b.Offset()
 }
 
+// CreateSharedString Checks if the string is already written
+// to the buffer before calling CreateString
+func (b *Builder) CreateSharedString(s string) UOffsetT {
+	if b.sharedStrings == nil {
+		b.sharedStrings = make(map[string]UOffsetT)
+	}
+	if v, ok := b.sharedStrings[s]; ok {
+		return v
+	}
+	off := b.CreateString(s)
+	b.sharedStrings[s] = off
+	return off
+}
+
 // CreateString writes a null-terminated string as a vector.
 func (b *Builder) CreateString(s string) UOffsetT {
 	b.assertNotNested()

tests/go_test.go

@@ -28,6 +28,7 @@ import (
 	"reflect"
 	"sort"
 	"testing"
+	"testing/quick"
 
 	flatbuffers "github.com/google/flatbuffers/go"
 )
@@ -77,6 +78,7 @@ func TestAll(t *testing.T) {
 	CheckByteStringIsNestedError(t.Fatalf)
 	CheckStructIsNotInlineError(t.Fatalf)
 	CheckFinishedBytesError(t.Fatalf)
+	CheckSharedStrings(t.Fatalf)
 	
 	// Verify that GetRootAs works for non-root tables
 	CheckGetRootAsForNonRootTable(t.Fatalf)
@@ -1375,6 +1377,29 @@ func CheckStringIsNestedError(fail func(string, ...interface{})) {
 	b.CreateString("foo")
 }
 
+func CheckSharedStrings(fail func(string, ...interface{})) {
+	f := func(strings []string) bool {
+		b := flatbuffers.NewBuilder(0)
+		for _, s1 := range strings {
+			for _, s2 := range strings {
+				off1 := b.CreateSharedString(s1)
+				off2 := b.CreateSharedString(s2)
+
+				if (s1 == s2) && (off1 != off2) {
+					return false
+				}
+				if (s1 != s2) && (off1 == off2) {
+					return false
+				}
+			}
+		}
+		return true
+	}
+	if err := quick.Check(f, nil); err != nil {
+		fail("expected same offset")
+	}
+}
+
 // CheckByteStringIsNestedError verifies that a bytestring can not be created
 // inside another object.
 func CheckByteStringIsNestedError(fail func(string, ...interface{})) {