Use an explicit header size rather than an attribute offset

The total size of the header fields (minus the attributes) are declared
up front at the start of the header. This will prevent future additions
to the header from having to introduce their own "attribute_offset"
field variations.

This also rearranges the padding so that all header fields are
strictly aligned according to their type, and the header field total
is a multiple of 16.

Fix #49

README-CHANGES.xml

@@ -2,6 +2,18 @@
 <c:changelog xmlns:c="http://schemas.io7m.com/changelog/2.0.0">
   <c:project>com.io7m.smfj</c:project>
 
+  <c:release c:ticket-system="com.github.io7m.smf">
+    <c:date>2017-06-18</c:date>
+    <c:version>0.13.0</c:version>
+
+    <c:item>
+      <c:date>2017-06-18</c:date>
+      <c:type-code-change/>
+      <c:ticket>49</c:ticket>
+      <c:summary>Replace the attribute_offset field.</c:summary>
+    </c:item>
+  </c:release>
+
   <c:release c:ticket-system="com.github.io7m.smf">
     <c:date>2017-06-18</c:date>
     <c:version>0.12.0</c:version>

com.io7m.smfj.format.binary/src/main/java/com/io7m/smfj/format/binary/v1/SMFBv1Headers.java

@@ -214,16 +214,8 @@ private static void skipAheadToAttributes(
     {
       final long position_now =
         reader.position();
-      final long position_attribute_offset_start =
-        Math.addExact(
-          position_start,
-          (long) view.metaAttributeOffsetOffsetFromCursor());
-      final long position_attribute_offset_value =
-        (long) view.getAttributeOffset();
       final long attribute_offset =
-        Math.addExact(
-          position_attribute_offset_start,
-          position_attribute_offset_value);
+        Math.addExact(position_start, (long) view.getFieldsSize());
       final long seek =
         Math.subtractExact(attribute_offset, position_now);
 

com.io7m.smfj.format.binary/src/main/java/com/io7m/smfj/format/binary/v1/SMFBv1Serializer.java

@@ -138,7 +138,7 @@ public void serializeHeader(
 
         final SMFTriangles triangles = in_header.triangles();
         view.setAttributeCount(in_header.attributesInOrder().size());
-        view.setAttributeOffset(8);
+        view.setFieldsSize(buffer.length);
         view.setTriangleCount(triangles.triangleCount());
         view.setTriangleIndexSizeBits(triangles.triangleIndexSizeBits());
         view.setVertexCount(in_header.vertexCount());

com.io7m.smfj.format.binary/src/main/jpra/com/io7m/smfj/format/binary/v1.jpr

@@ -26,13 +26,14 @@
    [field meta_size                 [integer unsigned 32]])]
 
 [record SMFBv1_0Header
-  ([field schema                   SMFBv1SchemaID]
+  ([field fields_size              [integer unsigned 32]]
+   [field schema                   SMFBv1SchemaID]
+   [padding-octets                 4]
    [field vertex_count             [integer unsigned 64]]
    [field triangle_count           [integer unsigned 64]]
    [field triangle_index_size_bits [integer unsigned 32]]
    [field attribute_count          [integer unsigned 32]]
-   [field attribute_offset         [integer unsigned 32]]
    [field coordinate_system        SMFBv1CoordinateSystems]
-   [padding-octets                 2])]
+   [padding-octets                 14])]
 
 [package-end]

com.io7m.smfj.specification/src/main/resources/com/io7m/smfj/specification/binary-smf-header.txt

@@ -1,9 +1,10 @@
 [record SMFBv1_0Header
-  ([field schema                   SMFBv1SchemaID]
+  ([field fields_size              [integer unsigned 32]]
+   [field schema                   SMFBv1SchemaID]
+   [padding-octets                 4]
    [field vertex_count             [integer unsigned 64]]
    [field triangle_count           [integer unsigned 64]]
    [field triangle_index_size_bits [integer unsigned 32]]
    [field attribute_count          [integer unsigned 32]]
-   [field attribute_offset         [integer unsigned 32]]
    [field coordinate_system        SMFBv1CoordinateSystems]
-   [padding-octets                 2])]
+   [padding-octets                 14])]

com.io7m.smfj.specification/src/main/resources/com/io7m/smfj/specification/binary-smf.sdi

@@ -28,6 +28,12 @@ The [term [type type] smf] section begins with the following structure:
 [formal-item [title "SMF Header"] [id smfb.sections.smf.header.def]]
 [verbatim [include "binary-smf-header.txt"]]
 
+The [term [type field] fields_size] field specifies the total size of the
+header fields, after which a list of attributes will be declared. The purpose
+of this field is to allow for forward compatibility: Future updates to this
+specification may add extra fields to the end of the header structure, and the
+value of this field allows older implementations to skip those newer fields.
+
 [paragraph]
 The [term [type field] schema] field specifies the
 [link [target smf_model.schema_id] schema identifier] for the model:
@@ -54,13 +60,7 @@ individual vertex indices within each triangle.
 
 [paragraph]
 The [term [type field] attribute_count] field specifies the number of
-[link [target smf_model.attribute] attributes] that will be declared. The
-[term [type field] attribute_offset] field specifies a relative offset in
-octets, from the start of the [term [type field] attribute_offset] field,
-at which a list of attributes will be declared. The purpose of this field
-is to allow for forward compatibility: Future updates to this specification
-may add extra fields to the end of the header structure, and the value of
-this field allows older implementations to skip those newer fields.
+[link [target smf_model.attribute] attributes] that will be declared.
 
 [paragraph]
 The [term [type field] coordinate_system] field specifies the