Add ONCE_EACH macro per-value one-time execution

source/lib/code_generation.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2013 Wildfire Games.
+/* Copyright (C) 2026 Wildfire Games.
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -23,6 +23,9 @@
 #ifndef INCLUDED_CODE_GENERATION
 #define INCLUDED_CODE_GENERATION
 
+#include <unordered_set>
+#include <type_traits>
+
 /**
  * package code into a single statement.
  *
@@ -56,6 +59,30 @@ STMT(\
 	}\
 )
 
+/**
+ * execute the code passed as a parameter only the first time each distinct
+ * value of the key expression is seen.
+ *
+ * @param key_expr expression that produces the key to track (e.g., a filename).
+ * @param code_block code to execute the first time each key value is seen.
+ *                   the key expression result is available as 'key' inside the block.
+ *
+ * example:
+ *   ONCE_EACH(filename, LOGWARNING("Bad file %s", key))
+ *
+ * may be called at any time (in particular before main), but is not
+ * thread-safe.
+ **/
+#define ONCE_EACH(key_expr, code_block) \
+STMT( \
+    using KeyType__ = std::decay_t<decltype(key_expr)>; \
+    static std::unordered_set<KeyType__> seen__; \
+    auto key = (key_expr); \
+    if (seen__.insert(key).second) { \
+        code_block; \
+    } \
+)
+
 /**
  * execute the code passed as a parameter except the first time this is
  * reached.