[Lobster] file_identifier support

lobster/flatbuffers.lobster

@@ -175,25 +175,30 @@ class builder:
         while current_vtable.length <= slotnum: current_vtable.push(0)
         current_vtable[slotnum] = head
 
-    def __Finish(root_table:offset, size_prefix:int):
+    def __Finish(root_table:offset, size_prefix:int, file_identifier:string?):
         // Finish finalizes a buffer, pointing to the given root_table
         assert not finished
         assert not nested
         var prep_size = sz_32
+        if file_identifier:
+            prep_size += sz_32
         if size_prefix:
             prep_size += sz_32
         Prep(minalign, prep_size)
+        if file_identifier:
+            assert file_identifier.length == 4
+            buf, head = buf.write_substring_back(head, file_identifier, false)
         PrependUOffsetTRelative(root_table)
         if size_prefix:
             PrependInt32(head)
         finished = true
         return Start()
 
-    def Finish(root_table:offset):
-        return __Finish(root_table, false)
+    def Finish(root_table:offset, file_identifier:string? = nil):
+        return __Finish(root_table, false, file_identifier)
 
-    def FinishSizePrefixed(root_table:offset):
-        return __Finish(root_table, true)
+    def FinishSizePrefixed(root_table:offset, file_identifier:string? = nil):
+        return __Finish(root_table, true, file_identifier)
 
     def PrependBool(x):
         buf, head = buf.write_int8_le_back(head, x)
@@ -299,3 +304,9 @@ class builder:
             // elsewhere.
             assert x.o == head
             Slot(v)
+
+def has_identifier(buf:string, file_identifier:string):
+    assert file_identifier.length == 4
+    return buf.length >= 8 and buf.substring(4, 4) == file_identifier
+
+

samples/sample_binary.lobster

@@ -54,7 +54,7 @@ let orc = MyGame_Sample_MonsterBuilder { b }
     .end()
 
 // Finish the buffer!
-b.Finish(orc)
+b.Finish(orc, "MONS")
 
 // We now have a FlatBuffer that we could store on disk or send over a network.
 

tests/lobstertest.lobster

@@ -17,7 +17,9 @@ import monster_test_generated
 import optional_scalars_generated
 
 def check_read_buffer(buf):
-    // CheckReadBuffer checks that the given buffer is evaluated correctly as the example Monster.
+    // Check that the given buffer is evaluated correctly as the example Monster.
+    assert flatbuffers_has_identifier(buf, "MONS")
+
     let monster = MyGame_Example_GetRootAsMonster(buf)
 
     assert monster.hp == 80
@@ -105,7 +107,7 @@ def make_monster_from_generated_code():
         .add_vector_of_doubles(vector_of_doubles)
         .end()
 
-    b.Finish(mon)
+    b.Finish(mon, "MONS")
 
     return b.SizedCopy()
 
@@ -126,8 +128,10 @@ def test_optional_scalars():
             ss.add_just_enum(optional_scalars_OptionalByte_Two)
             ss.add_maybe_enum(optional_scalars_OptionalByte_Two)
             ss.add_default_enum(optional_scalars_OptionalByte_Two)
-        b.Finish(ss.end())
-        return optional_scalars_GetRootAsScalarStuff(b.SizedCopy())
+        b.Finish(ss.end(), "NULL")
+        let buf = b.SizedCopy()
+        assert flatbuffers_has_identifier(buf, "NULL")
+        return optional_scalars_GetRootAsScalarStuff(buf)
 
     var root = build(true)