Add format utility

tests/test_arena_allocator.cpp

@@ -1,7 +1,9 @@
 #define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
 #include "arena_allocator.hpp"
+#include "format.hpp"
 #include <cstring>
 #include <doctest/doctest.h>
+#include <string>
 #include <vector>
 
 TEST_CASE("ArenaAllocator basic construction") {
@@ -532,3 +534,67 @@ TEST_CASE("ArenaAllocator realloc functionality") {
     }
   }
 }
+
+TEST_CASE("format function fallback codepath") {
+  SUBCASE("single-pass optimization success") {
+    ArenaAllocator arena(128);
+    auto result = format(arena, "Hello %s! Number: %d", "World", 42);
+    CHECK(result == "Hello World! Number: 42");
+    CHECK(result.length() == 23);
+  }
+
+  SUBCASE("fallback when speculative formatting fails") {
+    // Create arena with limited space to force fallback
+    ArenaAllocator arena(16);
+
+    // Consume most space to leave insufficient room for speculative formatting
+    arena.allocate<char>(10);
+    CHECK(arena.available_in_current_block() == 6);
+
+    // Format string larger than available space - should trigger fallback
+    std::string long_string = "This is a very long string that won't fit";
+    auto result = format(arena, "Prefix: %s with %d", long_string.c_str(), 123);
+
+    std::string expected =
+        "Prefix: This is a very long string that won't fit with 123";
+    CHECK(result == expected);
+    CHECK(result.length() == expected.length());
+  }
+
+  SUBCASE("edge case - exactly available space") {
+    ArenaAllocator arena(32);
+    arena.allocate<char>(20); // Leave 12 bytes
+    CHECK(arena.available_in_current_block() == 12);
+
+    // Format that needs exactly available space (should still use fallback due
+    // to null terminator)
+    auto result = format(arena, "Test%d", 123); // "Test123" = 7 chars
+    CHECK(result == "Test123");
+    CHECK(result.length() == 7);
+  }
+
+  SUBCASE("allocate_remaining_space postcondition") {
+    // Test empty arena
+    ArenaAllocator empty_arena(64);
+    auto space1 = empty_arena.allocate_remaining_space();
+    CHECK(space1.allocated_bytes >= 1);
+    CHECK(space1.allocated_bytes == 64);
+
+    // Test full arena (should create new block)
+    ArenaAllocator full_arena(32);
+    full_arena.allocate<char>(32); // Fill completely
+    auto space2 = full_arena.allocate_remaining_space();
+    CHECK(space2.allocated_bytes >= 1);
+    CHECK(space2.allocated_bytes == 32); // New block created
+  }
+
+  SUBCASE("format error handling") {
+    ArenaAllocator arena(64);
+
+    // Test with invalid format (should return empty string_view)
+    // Note: This is hard to trigger reliably across platforms,
+    // so we focus on successful cases in the other subcases
+    auto result = format(arena, "Valid format: %d", 42);
+    CHECK(result == "Valid format: 42");
+  }
+}