libdxfrw: check the typed class table's invariants at compile time

With the registrars driven from one table, the properties they used to
hold simply by being written out one at a time are no longer guaranteed
by construction. Assert them instead: every row needs a non-empty key,
application name, class name and record name, and the key, record name
and class name have to be unique. A copy-paste slip in a new row now
fails the build rather than silently registering the wrong metadata.

Class numbers are deliberately excluded. The check was written to require
them to be unique too, and that immediately failed the build: MLEADERSTYLE
shares 504 with MATERIAL, TABLESTYLE shares 509 with IDBUFFER, and
EVALUATION_GRAPH shares 516 with FIELD. The custom-class range is assigned
per file and registerTypedObjectClass() hands a colliding type the next
free number, so those defaults are a starting point rather than an
identity. That is now recorded at the table.

Author: dxli

libraries/libdxfrw/src/intern/dwgwriter.h

@@ -135,6 +135,60 @@ inline constexpr DwgTypedClassRow kDwgTypedClassRows[] = {
     {"Field", DRW_Field::kDwgClassNum, 0x401, "ACAD", "AcDbField", "FIELD", 0x1F3},
 };
 
+// The table above is now the single source of truth for those registrars, so
+// the properties they used to hold simply by being written out one at a time
+// are checked here at compile time instead: every row needs a non-empty key,
+// application name, class name and record name, and the key, record name and
+// class name have to be unique. A copy-paste slip in a new row then fails the
+// build instead of silently registering the wrong metadata.
+//
+// Class numbers are deliberately NOT required to be unique. The custom-class
+// range (>= 500) is assigned per file, and several types share a default here
+// - MLEADERSTYLE with MATERIAL at 504, TABLESTYLE with IDBUFFER at 509, and
+// EVALUATION_GRAPH with FIELD at 516. registerTypedObjectClass() resolves a
+// collision by handing the later type the next free number, so the defaults
+// are a starting point rather than an identity.
+constexpr bool dwgClassTextEqual(const char* a, const char* b) {
+    if (a == nullptr || b == nullptr)
+        return a == b;
+    while (*a != '\0' && *a == *b) {
+        ++a;
+        ++b;
+    }
+    return *a == *b;
+}
+
+constexpr bool dwgClassTextNonEmpty(const char* text) {
+    return text != nullptr && *text != '\0';
+}
+
+constexpr bool dwgTypedClassRowsAreWellFormed() {
+    constexpr std::size_t count =
+        sizeof(kDwgTypedClassRows) / sizeof(kDwgTypedClassRows[0]);
+    for (std::size_t i = 0; i < count; ++i) {
+        const DwgTypedClassRow& a = kDwgTypedClassRows[i];
+        if (!dwgClassTextNonEmpty(a.key) || !dwgClassTextNonEmpty(a.appName)
+            || !dwgClassTextNonEmpty(a.className)
+            || !dwgClassTextNonEmpty(a.recordName))
+            return false;
+        for (std::size_t j = i + 1; j < count; ++j) {
+            const DwgTypedClassRow& b = kDwgTypedClassRows[j];
+            if (dwgClassTextEqual(a.key, b.key)
+                || dwgClassTextEqual(a.recordName, b.recordName)
+                || dwgClassTextEqual(a.className, b.className))
+                return false;
+        }
+    }
+    return true;
+}
+
+static_assert(dwgTypedClassRowsAreWellFormed(),
+              "kDwgTypedClassRows: every row needs a non-empty key, app name, "
+              "class name and record name, and the key, record name and class "
+              "name must each be unique across the table (class numbers may "
+              "collide and are remapped at registration)");
+
+
 class dwgWriter {
 public:
     /// Construct around an output stream and a populated header.