added arena_sb_append_format and arena_sb_append_vformat

arena.h

@@ -141,6 +141,51 @@ void arena_trim(Arena *a);
 // use it a NULL-terminated C string
 #define arena_sb_append_null(a, sb) arena_da_append(a, sb, 0)
 
+#ifndef ARENA_NOSTDIO
+// Append a formatted string to a string builder using a va_list.
+// The terminating NULL character is NOT included in the builder.
+// So that you can use it repeatedly without having annoying zeros in between.
+#define arena_sb_append_vformat(a, sb, format, va_args)                 \
+    do {                                                                \
+        va_list args_copy;                                              \
+        va_copy(args_copy, va_args);                                    \
+        int n = vsnprintf(NULL, 0, format, args_copy);                  \
+        va_end(args_copy);                                              \
+        if ((sb)->count + n+1 > (sb)->capacity) {                       \
+            size_t new_capacity = (sb)->capacity;                       \
+            if (new_capacity == 0) new_capacity = ARENA_DA_INIT_CAP;    \
+            while ((sb)->count + n+1 > new_capacity) new_capacity *= 2; \
+            (sb)->items = cast_ptr((sb)->items)arena_realloc(           \
+                (a), (sb)->items,                                       \
+                (sb)->capacity*sizeof(*(sb)->items),                    \
+                new_capacity*sizeof(*(sb)->items));                     \
+            (sb)->capacity = new_capacity;                              \
+        }                                                               \
+        vsnprintf((sb)->items + (sb)->count, n+1, format, va_args);     \
+        (sb)->count += n;                                               \
+    } while(0)
+
+// Append a formatted string to a string builder using a variable number of macro arguments.
+// The terminating NULL character is NOT included in the builder.
+// So that you can use it repeatedly without having annoying zeros in between.
+#define arena_sb_append_format(a, sb, format, ...)                      \
+    do {                                                                \
+        int n = snprintf(NULL, 0, format, __VA_ARGS__);                 \
+        if ((sb)->count + n+1 > (sb)->capacity) {                       \
+            size_t new_capacity = (sb)->capacity;                       \
+            if (new_capacity == 0) new_capacity = ARENA_DA_INIT_CAP;    \
+            while ((sb)->count + n+1 > new_capacity) new_capacity *= 2; \
+            (sb)->items = cast_ptr((sb)->items)arena_realloc(           \
+                (a), (sb)->items,                                       \
+                (sb)->capacity*sizeof(*(sb)->items),                    \
+                new_capacity*sizeof(*(sb)->items));                     \
+            (sb)->capacity = new_capacity;                              \
+        }                                                               \
+        snprintf((sb)->items + (sb)->count, n+1, format, __VA_ARGS__);  \
+        (sb)->count += n;                                               \
+    } while(0)
+#endif // ARENA_NOSTDIO
+
 #endif // ARENA_H_
 
 #ifdef ARENA_IMPLEMENTATION

examples/001_basic_usage/main.c

@@ -39,6 +39,10 @@ int main()
         // Append sized buffer
         arena_sb_append_buf(&a, &sb, name, sizeof(name));
 
+        // Append using format strings
+        arena_sb_append_format(&a, &sb, "\n%d", 5);
+        arena_sb_append_format(&a, &sb, " %s", "foo");
+
         // Append '\0' to make the built string NULL-terminated
         arena_sb_append_null(&a, &sb);