[skip ci] add bitset reserve

Author: trcrsired

include/fast_io_dsal/impl/bitvec.h

@@ -85,7 +85,7 @@ class bitvec
 		{
 			current_capacity /= underlying_digits;
 		}
-		constexpr ::std::size_t mxbyteshalf{max_size_bytes() >> 1};
+		constexpr ::std::size_t mxbyteshalf{max_size() >> 1};
 		if (mxbyteshalf < current_capacity)
 		{
 			::fast_io::fast_terminate();
@@ -700,6 +700,28 @@ class bitvec
 		this->destroy_bitvec();
 		this->imp = {};
 	}
+
+private:
+	inline static constexpr size_type bits_to_blocks(size_type bits) noexcept
+	{
+		if constexpr (underlying_digits == 8)
+		{
+			return (bits + 7) >> 3; // ceil(bits / 8)
+		}
+		else
+		{
+			return (bits + (underlying_digits - 1)) / underlying_digits;
+		}
+	}
+
+public:
+	constexpr void reserve(size_type n) noexcept
+	{
+		if (this->imp.end_pos < n)
+		{
+			this->grow_to_new_capacity(bits_to_blocks(n));
+		}
+	}
 };
 
 } // namespace containers

tests/0026.container/0014.bitvec/bitvec.cc

@@ -414,6 +414,45 @@ inline void test_bitvec_flip()
 	::fast_io::io::print("bitvec flip test finished\n");
 }
 
+inline void test_bitvec_reserve_and_push_back_unchecked()
+{
+	::fast_io::io::perr("=== bitvec reserve + push_back_unchecked test ===\n");
+
+	::fast_io::bitvec bv;
+
+	// Reserve space for 5000 bits
+	bv.reserve(5000);
+
+	if (bv.capacity() < 5000)
+	{
+		::fast_io::io::panic("ERROR: reserve did not increase capacity correctly\n");
+	}
+
+	// Now push bits WITHOUT triggering reallocation
+	// Use push_back_unchecked since we know capacity is sufficient
+	for (::std::size_t i{}; i != 5000u; ++i)
+	{
+		bv.push_back_unchecked((i & 1u) != 0u);
+	}
+
+	if (bv.size() != 5000u)
+	{
+		::fast_io::io::panic("ERROR: size mismatch after push_back_unchecked\n");
+	}
+
+	// Verify pattern
+	for (::std::size_t i{}; i != 5000u; ++i)
+	{
+		bool expected = (i & 1u) != 0u;
+		if (bv.test(i) != expected)
+		{
+			::fast_io::io::panicln("ERROR: push_back_unchecked mismatch at index ", i);
+		}
+	}
+
+	::fast_io::io::print("bitvec reserve + push_back_unchecked test finished\n");
+}
+
 int main()
 {
 	test_bitvec_basic();
@@ -424,6 +463,7 @@ int main()
 	test_bitvec_copy_assignment();
 	test_bitvec_move();
 	test_bitvec_flip();
+	test_bitvec_reserve_and_push_back_unchecked();
 
 	::fast_io::io::print("All bitvec tests finished\n");
 }