qubit-function 0.8.3

/*******************************************************************************
 *
 *    Copyright (c) 2025 - 2026.
 *    Haixing Hu, Qubit Co. Ltd.
 *
 *    All rights reserved.
 *
 ******************************************************************************/

use qubit_function::Transformer;

#[test]
fn test_transformer_default_conversions_allow_relaxed_generic_types() {
    #[derive(Clone, Debug, Eq, PartialEq)]
    struct BorrowedRc<'a> {
        value: &'a str,
    }

    #[derive(Clone, Debug)]
    struct BorrowedRcTransformer;

    impl<'a> Transformer<BorrowedRc<'a>, BorrowedRc<'a>> for BorrowedRcTransformer {
        fn apply(&self, value: BorrowedRc<'a>) -> BorrowedRc<'a> {
            value
        }
    }

    fn assert_left(value: BorrowedRc<'_>) {
        assert_eq!(value.value, "left");
    }

    let text = String::from("left");
    let value = || BorrowedRc {
        value: text.as_str(),
    };
    let transformer = BorrowedRcTransformer;

    assert_left(transformer.clone().into_box().apply(value()));
    assert_left(transformer.clone().into_rc().apply(value()));
    assert_left(transformer.clone().into_arc().apply(value()));
    assert_left(qubit_function::TransformerOnce::apply(
        transformer.clone().into_once(),
        value(),
    ));
    assert_left(transformer.clone().into_fn()(value()));

    assert_left(transformer.to_box().apply(value()));
    assert_left(transformer.to_rc().apply(value()));
    assert_left(transformer.to_arc().apply(value()));
    assert_left(qubit_function::TransformerOnce::apply(
        transformer.to_once(),
        value(),
    ));
    assert_left(transformer.to_fn()(value()));
}

// ============================================================================
// BoxTransformer Tests - Immutable, single ownership
// ============================================================================

#[cfg(test)]
mod box_transformer_tests {
    use qubit_function::{
        BoxTransformer,
        Transformer,
    };

    #[test]
    fn test_new_and_apply() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        assert_eq!(double.apply(21), 42);
    }

    #[test]
    fn test_multiple_calls() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        assert_eq!(double.apply(21), 42);
        assert_eq!(double.apply(42), 84);
        assert_eq!(double.apply(10), 20);
    }

    #[test]
    fn test_identity() {
        let identity = BoxTransformer::<i32, i32>::identity();
        assert_eq!(identity.apply(42), 42);
    }

    #[test]
    fn test_constant() {
        let constant = BoxTransformer::constant("hello");
        assert_eq!(constant.apply(123), "hello");
        assert_eq!(constant.apply(456), "hello");
    }

    #[test]
    fn test_with_string() {
        let len = BoxTransformer::new(|s: String| s.len());
        let text = "hello".to_string();
        assert_eq!(len.apply(text), 5);
        // Note: text is consumed by transform
    }

    #[test]
    fn test_captured_variable() {
        let multiplier = 3;
        let multiply = BoxTransformer::new(move |x: i32| x * multiplier);
        assert_eq!(multiply.apply(7), 21);
    }

    #[test]
    fn test_and_then() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        let to_string = BoxTransformer::new(|x: i32| x.to_string());
        let composed = double.and_then(to_string);
        assert_eq!(composed.apply(21), "42");
    }

    #[test]
    fn test_compose() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        let add_one = BoxTransformer::new(|x: i32| x + 1);
        let composed = add_one.and_then(double);
        assert_eq!(composed.apply(5), 12); // (5 + 1) * 2
    }

    #[test]
    fn test_display_with_name() {
        let transformer = BoxTransformer::new_with_name("double", |x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "BoxTransformer(double)");
    }

    #[test]
    fn test_display_without_name() {
        let transformer = BoxTransformer::new(|x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "BoxTransformer");
    }
}

// ============================================================================
// ArcTransformer Tests - Immutable, thread-safe
// ============================================================================

#[cfg(test)]
mod arc_transformer_tests {
    use qubit_function::{
        ArcTransformer,
        Transformer,
    };
    use std::thread;

    #[test]
    fn test_new_and_apply() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        assert_eq!(double.apply(21), 42);
    }

    #[test]
    fn test_clone() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let cloned = double.clone();

        assert_eq!(double.apply(21), 42);
        assert_eq!(cloned.apply(21), 42);
    }

    #[test]
    fn test_thread_safe() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let cloned = double.clone();

        let handle = thread::spawn(move || cloned.apply(21));

        assert_eq!(handle.join().unwrap(), 42);
        assert_eq!(double.apply(21), 42);
    }

    #[test]
    fn test_identity() {
        let identity = ArcTransformer::<i32, i32>::identity();
        assert_eq!(identity.apply(42), 42);
    }

    #[test]
    fn test_constant() {
        let constant = ArcTransformer::constant("hello");
        assert_eq!(constant.apply(123), "hello");
    }

    #[test]
    fn test_multiple_threads() {
        let square = ArcTransformer::new(|x: i32| x * x);

        let handles: Vec<_> = (0..4)
            .map(|i| {
                let sq = square.clone();
                thread::spawn(move || sq.apply(i))
            })
            .collect();

        let results: Vec<_> = handles.into_iter().map(|h| h.join().unwrap()).collect();

        assert_eq!(results, vec![0, 1, 4, 9]);
    }

    #[test]
    fn test_and_then() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let to_string = ArcTransformer::new(|x: i32| x.to_string());
        let composed = double.and_then(to_string);

        // Original double transformer still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(composed.apply(21), "42");
    }

    #[test]
    fn test_compose() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let add_one = ArcTransformer::new(|x: i32| x + 1);
        let composed = add_one.and_then(double);

        assert_eq!(composed.apply(5), 12); // (5 + 1) * 2
    }
}

// ============================================================================
// RcTransformer Tests - Immutable, single-threaded
// ============================================================================

#[cfg(test)]
mod rc_transformer_tests {
    use qubit_function::{
        RcTransformer,
        Transformer,
    };

    #[test]
    fn test_new_and_apply() {
        let double = RcTransformer::new(|x: i32| x * 2);
        assert_eq!(double.apply(21), 42);
    }

    #[test]
    fn test_clone() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let cloned = double.clone();

        assert_eq!(double.apply(21), 42);
        assert_eq!(cloned.apply(21), 42);
    }

    #[test]
    fn test_identity() {
        let identity = RcTransformer::<i32, i32>::identity();
        assert_eq!(identity.apply(42), 42);
    }

    #[test]
    fn test_constant() {
        let constant = RcTransformer::constant("hello");
        assert_eq!(constant.apply(123), "hello");
    }

    #[test]
    fn test_shared_usage() {
        let to_upper = RcTransformer::new(|s: String| s.to_uppercase());

        let func1 = to_upper.clone();
        let func2 = to_upper.clone();

        assert_eq!(to_upper.apply("hello".to_string()), "HELLO");
        assert_eq!(func1.apply("world".to_string()), "WORLD");
        assert_eq!(func2.apply("rust".to_string()), "RUST");
    }

    #[test]
    fn test_and_then() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let to_string = RcTransformer::new(|x: i32| x.to_string());
        let composed = double.and_then(to_string);

        // Original double transformer still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(composed.apply(21), "42");
    }

    #[test]
    fn test_compose() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let add_one = RcTransformer::new(|x: i32| x + 1);
        let composed = add_one.and_then(double);

        assert_eq!(composed.apply(5), 12); // (5 + 1) * 2
    }

    #[test]
    fn test_display_with_name() {
        let transformer = RcTransformer::new_with_name("double", |x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "RcTransformer(double)");
    }

    #[test]
    fn test_display_without_name() {
        let transformer = RcTransformer::new(|x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "RcTransformer");
    }
}

// ============================================================================
// Conditional Transformer Tests
// ============================================================================

#[cfg(test)]
mod box_conditional_tests {
    use qubit_function::{
        FnTransformerOps,
        Transformer,
    };

    #[test]
    fn test_when_or_else() {
        let double_fn = |x: i32| x * 2;
        let negate_fn = |x: i32| -x;
        let conditional = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(negate_fn);
        let result = conditional.into_box();

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
    }

    #[test]
    fn test_when_or_else_with_closure() {
        let double_fn = |x: i32| x * 2;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(|x: i32| -x);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
        assert_eq!(result.apply(0), 0);
    }
}

#[cfg(test)]
mod arc_conditional_tests {
    use qubit_function::{
        FnTransformerOps,
        Transformer,
    };

    #[test]
    fn test_when_or_else() {
        let double_fn = |x: i32| x * 2;
        let negate_fn = |x: i32| -x;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(negate_fn);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
    }

    #[test]
    fn test_when_or_else_with_closure() {
        let double_fn = |x: i32| x * 2;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(|x: i32| -x);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
        assert_eq!(result.apply(0), 0);
    }

    #[test]
    fn test_conditional_or_else() {
        let double_fn = |x: i32| x * 2;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(|x: i32| -x);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
    }
}

#[cfg(test)]
mod rc_conditional_tests {
    use qubit_function::{
        FnTransformerOps,
        Transformer,
    };

    #[test]
    fn test_when_or_else() {
        let double_fn = |x: i32| x * 2;
        let negate_fn = |x: i32| -x;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(negate_fn);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
    }

    #[test]
    fn test_when_or_else_with_closure() {
        let double_fn = |x: i32| x * 2;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(|x: i32| -x);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
        assert_eq!(result.apply(0), 0);
    }

    #[test]
    fn test_conditional_or_else() {
        let double_fn = |x: i32| x * 2;
        let result = FnTransformerOps::when(double_fn, |x: &i32| *x > 0).or_else(|x: i32| -x);

        assert_eq!(result.apply(5), 10);
        assert_eq!(result.apply(-5), 5);
    }
}

// ============================================================================
// Conversion Tests
// ============================================================================

#[cfg(test)]
mod conversion_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };

    #[test]
    fn test_closure_to_box() {
        let double = |x: i32| x * 2;
        let boxed = double.into_box();
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_closure_to_arc() {
        let double = |x: i32| x * 2;
        let arc = double.into_arc();
        assert_eq!(arc.apply(21), 42);
    }

    #[test]
    fn test_closure_to_rc() {
        let double = |x: i32| x * 2;
        let rc = double.into_rc();
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_box_to_fn() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        let func = double.into_fn();
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_arc_to_fn() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let func = double.into_fn();
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_rc_to_fn() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let func = double.into_fn();
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_box_to_rc() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        let rc = double.into_rc();
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_arc_to_box() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let boxed = double.into_box();
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_arc_to_rc() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let rc = double.into_rc();
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_rc_to_box() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let boxed = double.into_box();
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_closure_into_fn() {
        // Test into_fn in impl<F, T, R> Transformer<T, R> for F
        let double = |x: i32| x * 2;
        let func = double.into_fn();
        assert_eq!(func(21), 42);
    }
}

// ============================================================================
// Non-consuming Conversion Tests (to_xxx methods)
// ============================================================================

#[cfg(test)]
mod to_conversion_tests {
    use qubit_function::{
        ArcTransformer,
        RcTransformer,
        Transformer,
    };
    use std::thread;

    // ArcTransformer to_xxx tests
    #[test]
    fn test_arc_to_box() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let boxed = double.to_box();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_arc_to_rc() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let rc = double.to_rc();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_arc_to_arc() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let arc2 = double.to_arc();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(arc2.apply(21), 42);
    }

    #[test]
    fn test_arc_to_fn() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let func = double.to_fn();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(func(21), 42);
    }

    // RcTransformer to_xxx tests
    #[test]
    fn test_rc_to_box() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let boxed = double.to_box();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_rc_to_rc() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let rc2 = double.to_rc();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(rc2.apply(21), 42);
    }

    #[test]
    fn test_rc_to_fn() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let func = double.to_fn();

        // Original still usable
        assert_eq!(double.apply(21), 42);
        assert_eq!(func(21), 42);
    }

    // Test to_xxx with composition
    #[test]
    fn test_arc_to_box_with_composition() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let to_string = ArcTransformer::new(|x: i32| x.to_string());

        let boxed = double.to_box();
        let composed = boxed.and_then(to_string);

        assert_eq!(composed.apply(21), "42");
    }

    #[test]
    fn test_rc_to_box_with_composition() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let to_string = RcTransformer::new(|x: i32| x.to_string());

        let boxed = double.to_box();
        let composed = boxed.and_then(to_string);

        assert_eq!(composed.apply(21), "42");
    }

    // Test multiple conversions
    #[test]
    fn test_arc_multiple_to_conversions() {
        let double = ArcTransformer::new(|x: i32| x * 2);

        let boxed = double.to_box();
        let rc = double.to_rc();
        let func = double.to_fn();

        // All still work
        assert_eq!(double.apply(21), 42);
        assert_eq!(boxed.apply(21), 42);
        assert_eq!(rc.apply(21), 42);
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_rc_multiple_to_conversions() {
        let double = RcTransformer::new(|x: i32| x * 2);

        let boxed = double.to_box();
        let rc2 = double.to_rc();
        let func = double.to_fn();

        // All still work
        assert_eq!(double.apply(21), 42);
        assert_eq!(boxed.apply(21), 42);
        assert_eq!(rc2.apply(21), 42);
        assert_eq!(func(21), 42);
    }

    // Test with different types
    #[test]
    fn test_arc_to_box_with_string() {
        let len = ArcTransformer::new(|s: String| s.len());
        let boxed = len.to_box();

        assert_eq!(len.apply("hello".to_string()), 5);
        assert_eq!(boxed.apply("world".to_string()), 5);
    }

    #[test]
    fn test_rc_to_fn_with_string() {
        let upper = RcTransformer::new(|s: String| s.to_uppercase());
        let func = upper.to_fn();

        assert_eq!(upper.apply("hello".to_string()), "HELLO");
        assert_eq!(func("world".to_string()), "WORLD");
    }

    // Test thread safety with Arc - clone first to get owned values
    #[test]
    fn test_arc_to_fn_thread_safe() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let double1 = double.clone();
        let double2 = double.clone();

        let handle1 = thread::spawn(move || {
            let func = double1.to_fn();
            func(21)
        });
        let handle2 = thread::spawn(move || {
            let func = double2.to_fn();
            func(10)
        });

        assert_eq!(handle1.join().unwrap(), 42);
        assert_eq!(handle2.join().unwrap(), 20);

        // Original still usable
        assert_eq!(double.apply(5), 10);
    }

    // Test that to_xxx creates independent copies
    #[test]
    fn test_arc_to_conversions_are_independent() {
        let double = ArcTransformer::new(|x: i32| x * 2);

        let boxed1 = double.to_box();
        let boxed2 = double.to_box();

        // Both work independently
        assert_eq!(boxed1.apply(21), 42);
        assert_eq!(boxed2.apply(10), 20);
    }

    // ========================================================================
    // Closure / function-pointer Transformer to_xxx Tests
    // ========================================================================

    #[test]
    fn test_fn_ptr_to_box_and_to_fn() {
        fn double(x: i32) -> i32 {
            x * 2
        }
        let fn_ptr: fn(i32) -> i32 = double;

        // to_box() and to_fn() should work for function pointers
        let boxed = fn_ptr.to_box();
        let func = fn_ptr.to_fn();

        assert_eq!(fn_ptr(21), 42);
        assert_eq!(boxed.apply(21), 42);
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_non_capturing_closure_coerced_to_fn_to_box() {
        // Non-capturing closure can be coerced to a fn pointer which is Clone
        let closure = |x: i32| x * 3;
        let fn_ptr: fn(i32) -> i32 = closure;
        let boxed = fn_ptr.to_box();

        assert_eq!(fn_ptr(7), 21);
        assert_eq!(boxed.apply(7), 21);
    }

    #[test]
    fn test_fn_ptr_to_arc_and_rc() {
        fn add_one(x: i32) -> i32 {
            x + 1
        }
        let fn_ptr: fn(i32) -> i32 = add_one;

        let arc = fn_ptr.to_arc();
        let rc = fn_ptr.to_rc();

        assert_eq!(fn_ptr(41), 42);
        assert_eq!(arc.apply(41), 42);
        assert_eq!(rc.apply(41), 42);
    }

    #[test]
    fn test_display_with_name() {
        let transformer = ArcTransformer::new_with_name("double", |x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "ArcTransformer(double)");
    }

    #[test]
    fn test_display_without_name() {
        let transformer = ArcTransformer::new(|x: i32| x * 2);
        let display_str = format!("{}", transformer);
        assert_eq!(display_str, "ArcTransformer");
    }
}

// ============================================================================
// Trait Usage Tests
// ============================================================================

#[cfg(test)]
mod trait_usage_tests {
    use qubit_function::{
        BoxTransformer,
        Transformer,
    };

    #[test]
    fn test_transformer_trait() {
        fn apply_transformer<F: Transformer<i32, i32>>(f: &F, x: i32) -> i32 {
            f.apply(x)
        }

        let double = BoxTransformer::new(|x: i32| x * 2);
        assert_eq!(apply_transformer(&double, 21), 42);
    }

    #[test]
    fn test_closure_as_transformer() {
        fn apply_transformer<F: Transformer<i32, i32>>(f: &F, x: i32) -> i32 {
            f.apply(x)
        }

        let double = |x: i32| x * 2;
        assert_eq!(apply_transformer(&double, 21), 42);
    }

    #[test]
    fn test_with_different_types() {
        fn apply_transformer<T, R, F: Transformer<T, R>>(f: &F, x: T) -> R {
            f.apply(x)
        }

        let to_string = BoxTransformer::new(|x: i32| x.to_string());
        assert_eq!(apply_transformer(&to_string, 42), "42");
    }
}

// ============================================================================
// Complex Composition Tests
// ============================================================================

#[cfg(test)]
mod complex_composition_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };

    #[test]
    fn test_multiple_and_then() {
        let add_one = BoxTransformer::new(|x: i32| x + 1);
        let double = BoxTransformer::new(|x: i32| x * 2);
        let to_string = BoxTransformer::new(|x: i32| x.to_string());
        let composed = add_one.and_then(double).and_then(to_string);
        assert_eq!(composed.apply(5), "12"); // (5 + 1) * 2 = 12
    }

    #[test]
    fn test_multiple_and_then_with_box() {
        let add_one = BoxTransformer::new(|x: i32| x + 1);
        let double = BoxTransformer::new(|x: i32| x * 2);
        let square = BoxTransformer::new(|x: i32| x * x);
        let composed = add_one.and_then(double).and_then(square);
        assert_eq!(composed.apply(5), 144); // ((5 + 1) * 2)^2 = 144
    }

    #[test]
    fn test_arc_multiple_and_then() {
        let add_one = ArcTransformer::new(|x: i32| x + 1);
        let double = ArcTransformer::new(|x: i32| x * 2);
        let to_string = ArcTransformer::new(|x: i32| x.to_string());
        let composed = add_one.and_then(double.clone()).and_then(to_string.clone());
        assert_eq!(composed.apply(5), "12");
        // Original transformers still usable
        assert_eq!(add_one.apply(5), 6);
        assert_eq!(double.apply(5), 10);
    }

    #[test]
    fn test_rc_multiple_and_then() {
        let add_one = RcTransformer::new(|x: i32| x + 1);
        let double = RcTransformer::new(|x: i32| x * 2);
        let square = RcTransformer::new(|x: i32| x * x);
        let composed = add_one.and_then(double.clone()).and_then(square.clone());
        assert_eq!(composed.apply(5), 144); // (5 + 1) * 2 = 12, then 12 * 12 = 144
                                            // Original transformers still usable
        assert_eq!(add_one.apply(5), 6);
        assert_eq!(double.apply(5), 10);
        assert_eq!(square.apply(5), 25);
    }
}

// ============================================================================
// Edge Cases Tests
// ============================================================================

#[cfg(test)]
mod edge_cases_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        Transformer,
    };

    #[test]
    fn test_identity_composition() {
        let double = BoxTransformer::new(|x: i32| x * 2);
        let identity = BoxTransformer::<i32, i32>::identity();
        let composed = double.and_then(identity);
        assert_eq!(composed.apply(21), 42);
    }

    #[test]
    fn test_constant_with_different_types() {
        let constant = BoxTransformer::constant("hello");
        assert_eq!(constant.apply(123), "hello");
        assert_eq!(constant.apply(456), "hello");
        assert_eq!(constant.apply(789), "hello");
    }

    #[test]
    fn test_with_option() {
        let parse = BoxTransformer::new(|s: String| s.parse::<i32>().ok());
        assert_eq!(parse.apply("42".to_string()), Some(42));
        assert_eq!(parse.apply("abc".to_string()), None);
    }

    #[test]
    fn test_with_result() {
        let parse = BoxTransformer::new(|s: String| s.parse::<i32>());
        assert!(parse.apply("42".to_string()).is_ok());
        assert!(parse.apply("abc".to_string()).is_err());
    }

    #[test]
    fn test_with_vec() {
        let split = BoxTransformer::new(|s: String| {
            s.split(',').map(|s| s.to_string()).collect::<Vec<_>>()
        });
        assert_eq!(
            split.apply("a,b,c".to_string()),
            vec!["a".to_string(), "b".to_string(), "c".to_string()]
        );
    }

    #[test]
    fn test_arc_with_large_data() {
        let process = ArcTransformer::new(|v: Vec<i32>| v.iter().sum::<i32>());
        let data = (1..=100).collect::<Vec<_>>();
        assert_eq!(process.apply(data), 5050);
    }
}

// ============================================================================
// Default Implementation Tests
// ============================================================================

#[cfg(test)]
mod default_implementation_tests {
    use qubit_function::{
        BoxTransformer,
        Transformer,
    };
    use std::thread;

    // A custom transformer that only implements the core `transform`
    // method, relying on default implementations for all `into_xxx`
    // methods.
    struct CustomTransformer {
        multiplier: i32,
    }

    impl Transformer<i32, i32> for CustomTransformer {
        fn apply(&self, input: i32) -> i32 {
            input * self.multiplier
        }
    }

    #[test]
    fn test_custom_into_box_uses_default() {
        let custom = CustomTransformer { multiplier: 3 };
        let boxed = custom.into_box();

        // Test that the BoxTransformer works correctly with the
        // default implementation
        assert_eq!(boxed.apply(7), 21);
        assert_eq!(boxed.apply(10), 30);
    }

    #[test]
    fn test_custom_into_rc_uses_default() {
        let custom = CustomTransformer { multiplier: 5 };
        let rc = custom.into_rc();

        // Test that the RcTransformer works correctly with the
        // default implementation
        assert_eq!(rc.apply(4), 20);

        // Test that cloning works
        let rc_clone = rc.clone();
        assert_eq!(rc_clone.apply(6), 30);
    }

    #[test]
    fn test_custom_into_fn_uses_default() {
        let custom = CustomTransformer { multiplier: 7 };
        let func = custom.into_fn();

        // Test that the closure works correctly with the default
        // implementation
        assert_eq!(func(3), 21);
        assert_eq!(func(5), 35);
    }

    #[test]
    fn test_custom_chained_conversions() {
        let custom = CustomTransformer { multiplier: 2 };

        // Convert to Box, then convert to Rc
        let boxed = custom.into_box();
        assert_eq!(boxed.apply(10), 20);

        // Create another custom transformer for the next test
        let custom2 = CustomTransformer { multiplier: 4 };
        let rc = custom2.into_rc();
        let boxed2 = rc.into_box();
        assert_eq!(boxed2.apply(5), 20);
    }

    #[test]
    fn test_custom_with_different_types() {
        // A custom transformer that converts i32 to String
        struct IntToString;

        impl Transformer<i32, String> for IntToString {
            fn apply(&self, input: i32) -> String {
                format!("Number: {}", input)
            }
        }

        let custom = IntToString;
        let boxed = custom.into_box();

        assert_eq!(boxed.apply(42), "Number: 42");
        assert_eq!(boxed.apply(100), "Number: 100");
    }

    #[test]
    fn test_custom_composition_with_default() {
        let custom = CustomTransformer { multiplier: 3 };
        let boxed = custom.into_box();

        // Compose with another transformer
        let to_string = BoxTransformer::new(|x: i32| x.to_string());
        let composed = boxed.and_then(to_string);

        assert_eq!(composed.apply(7), "21");
    }

    #[test]
    fn test_custom_into_arc_uses_default() {
        // A thread-safe custom transformer that implements Send + Sync
        #[derive(Clone)]
        struct ThreadSafeTransformer {
            multiplier: i32,
        }

        impl Transformer<i32, i32> for ThreadSafeTransformer {
            fn apply(&self, input: i32) -> i32 {
                input * self.multiplier
            }
        }

        let custom = ThreadSafeTransformer { multiplier: 4 };
        let arc = custom.into_arc();

        // Test that the ArcTransformer works correctly with the
        // default implementation
        assert_eq!(arc.apply(5), 20);

        // Test that cloning works
        let arc_clone = arc.clone();
        assert_eq!(arc_clone.apply(10), 40);

        // Test thread safety
        let arc2 = arc.clone();
        let handle = thread::spawn(move || arc2.apply(7));
        assert_eq!(handle.join().unwrap(), 28);
        assert_eq!(arc.apply(3), 12);
    }
}

// ============================================================================
// Specialized into_fn Implementation Tests
// ============================================================================

#[cfg(test)]
mod specialized_into_fn_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };
    use std::thread;

    #[test]
    fn test_box_transformer_into_fn_optimized() {
        // Test that BoxTransformer::into_fn uses the optimized
        // implementation that unwraps the Box directly
        let double = BoxTransformer::new(|x: i32| x * 2);
        let func = double.into_fn();

        assert_eq!(func(21), 42);
        assert_eq!(func(10), 20);
        assert_eq!(func(0), 0);
    }

    #[test]
    fn test_box_transformer_into_fn_with_closure_capture() {
        // Test that the optimized implementation works with closures
        // that capture variables
        let multiplier = 3;
        let multiply = BoxTransformer::new(move |x: i32| x * multiplier);
        let func = multiply.into_fn();

        assert_eq!(func(7), 21);
        assert_eq!(func(10), 30);
    }

    #[test]
    fn test_box_transformer_into_fn_type_conversion() {
        // Test that the optimized implementation works with type
        // conversions
        let to_string = BoxTransformer::new(|x: i32| x.to_string());
        let func = to_string.into_fn();

        assert_eq!(func(42), "42");
        assert_eq!(func(100), "100");
    }

    #[test]
    fn test_arc_transformer_into_fn_optimized() {
        // Test that ArcTransformer::into_fn uses the optimized
        // implementation
        let double = ArcTransformer::new(|x: i32| x * 2);
        let func = double.into_fn();

        assert_eq!(func(21), 42);
        assert_eq!(func(10), 20);
    }

    #[test]
    fn test_arc_transformer_into_fn_clone_and_convert() {
        // Test that we can clone an ArcTransformer and convert the
        // clone to a function
        let double = ArcTransformer::new(|x: i32| x * 2);
        let double_clone = double.clone();

        let func = double_clone.into_fn();
        assert_eq!(func(21), 42);

        // Original still usable
        assert_eq!(double.apply(10), 20);
    }

    #[test]
    fn test_arc_transformer_into_fn_thread_safety() {
        // Test that the converted function maintains thread safety
        let square = ArcTransformer::new(|x: i32| x * x);
        let func = square.into_fn();

        let handle = thread::spawn(move || func(7));
        assert_eq!(handle.join().unwrap(), 49);
    }

    #[test]
    fn test_rc_transformer_into_fn_optimized() {
        // Test that RcTransformer::into_fn uses the optimized
        // implementation
        let triple = RcTransformer::new(|x: i32| x * 3);
        let func = triple.into_fn();

        assert_eq!(func(7), 21);
        assert_eq!(func(10), 30);
    }

    #[test]
    fn test_rc_transformer_into_fn_clone_and_convert() {
        // Test that we can clone an RcTransformer and convert the
        // clone to a function
        let add_one = RcTransformer::new(|x: i32| x + 1);
        let add_one_clone = add_one.clone();

        let func = add_one_clone.into_fn();
        assert_eq!(func(41), 42);

        // Original still usable
        assert_eq!(add_one.apply(99), 100);
    }

    #[test]
    fn test_rc_transformer_into_fn_with_shared_state() {
        // Test that multiple clones can be converted to functions
        let negate = RcTransformer::new(|x: i32| -x);
        let clone1 = negate.clone();
        let clone2 = negate.clone();

        let func1 = clone1.into_fn();
        let func2 = clone2.into_fn();

        assert_eq!(func1(42), -42);
        assert_eq!(func2(100), -100);
        assert_eq!(negate.apply(7), -7);
    }

    #[test]
    fn test_closure_into_fn_zero_cost() {
        // Test that closure::into_fn is a zero-cost abstraction that
        // returns the closure itself
        let double = |x: i32| x * 2;
        let func = double.into_fn();

        assert_eq!(func(21), 42);
        assert_eq!(func(10), 20);
    }

    #[test]
    fn test_closure_into_fn_with_capture() {
        // Test that closure::into_fn works with captured variables
        let base = 10;
        let add_base = move |x: i32| x + base;
        let func = add_base.into_fn();

        assert_eq!(func(5), 15);
        assert_eq!(func(32), 42);
    }

    #[test]
    fn test_closure_into_fn_composition() {
        // Test that closures converted to functions can be composed
        let double = |x: i32| x * 2;
        let add_one = |x: i32| x + 1;

        let func1 = double.into_fn();
        let func2 = add_one.into_fn();

        assert_eq!(func2(func1(5)), 11); // (5 * 2) + 1
    }

    #[test]
    fn test_function_pointer_into_fn() {
        // Test that function pointers work with into_fn
        fn square(x: i32) -> i32 {
            x * x
        }

        let func = square.into_fn();
        assert_eq!(func(5), 25);
        assert_eq!(func(7), 49);
    }

    #[test]
    fn test_into_fn_multiple_calls() {
        // Test that the returned function can be called multiple
        // times
        let factorial = BoxTransformer::new(|n: u32| (1..=n).product::<u32>());
        let func = factorial.into_fn();

        assert_eq!(func(0), 1);
        assert_eq!(func(1), 1);
        assert_eq!(func(5), 120);
        assert_eq!(func(5), 120); // Call again with same value
    }

    #[test]
    fn test_into_fn_with_complex_types() {
        // Test into_fn with more complex input/output types
        let parse = ArcTransformer::new(|s: String| s.parse::<i32>());
        let func = parse.into_fn();

        assert_eq!(func("42".to_string()), Ok(42));
        assert!(func("abc".to_string()).is_err());
    }

    #[test]
    fn test_into_fn_preserves_behavior() {
        // Test that into_fn preserves the exact behavior of the
        // original transformer
        let transformer = RcTransformer::new(|x: i32| if x > 0 { x * 2 } else { x * 3 });

        let original_result1 = transformer.apply(5);
        let original_result2 = transformer.apply(-5);

        let func = transformer.into_fn();

        assert_eq!(func(5), original_result1);
        assert_eq!(func(-5), original_result2);
    }
}

// ============================================================================
// Type Conversion Tests
// ============================================================================

#[cfg(test)]
mod type_conversion_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };

    #[test]
    fn test_box_into_box() {
        let add = BoxTransformer::new(|x: i32| x + 10);
        let boxed = add.into_box();
        assert_eq!(boxed.apply(20), 30);
    }

    #[test]
    fn test_box_into_rc() {
        let add = BoxTransformer::new(|x: i32| x + 10);
        let rc = add.into_rc();
        assert_eq!(rc.apply(20), 30);
    }

    #[test]
    fn test_arc_into_arc() {
        let add = ArcTransformer::new(|x: i32| x + 10);
        let arc = add.into_arc();
        assert_eq!(arc.apply(20), 30);
    }

    #[test]
    fn test_arc_into_fn() {
        let add = ArcTransformer::new(|x: i32| x + 10);
        let func = add.into_fn();
        assert_eq!(func(20), 30);
    }

    #[test]
    fn test_rc_into_rc() {
        let add = RcTransformer::new(|x: i32| x + 10);
        let rc = add.into_rc();
        assert_eq!(rc.apply(20), 30);
    }

    #[test]
    fn test_rc_into_fn() {
        let add = RcTransformer::new(|x: i32| x + 10);
        let func = add.into_fn();
        assert_eq!(func(20), 30);
    }

    #[test]
    fn test_box_into_fn() {
        let add = BoxTransformer::new(|x: i32| x + 10);
        let func = add.into_fn();
        assert_eq!(func(20), 30);
    }

    #[test]
    fn test_arc_into_box() {
        let add = ArcTransformer::new(|x: i32| x + 10);
        let boxed = add.into_box();
        assert_eq!(boxed.apply(20), 30);
    }

    #[test]
    fn test_arc_into_rc() {
        let add = ArcTransformer::new(|x: i32| x + 10);
        let rc = add.into_rc();
        assert_eq!(rc.apply(20), 30);
    }

    #[test]
    fn test_rc_into_box() {
        let add = RcTransformer::new(|x: i32| x + 10);
        let boxed = add.into_box();
        assert_eq!(boxed.apply(20), 30);
    }

    #[test]
    fn test_arc_constant_with_clone() {
        let constant = ArcTransformer::constant(42);
        assert_eq!(constant.apply(1), 42);
        assert_eq!(constant.apply(2), 42);
        assert_eq!(constant.apply(3), 42);
    }

    #[test]
    fn test_rc_constant_with_clone() {
        let constant = RcTransformer::constant("test");
        assert_eq!(constant.apply(1), "test");
        assert_eq!(constant.apply(2), "test");
        assert_eq!(constant.apply(3), "test");
    }
}

// ============================================================================
// Transformer Default Implementation Tests - to_xxx() methods
// ============================================================================

#[cfg(test)]
mod transformer_default_to_methods_tests {
    use qubit_function::{
        ArcTransformer,
        RcTransformer,
        Transformer,
    };
    use std::thread;

    // ========================================================================
    // ArcTransformer::to_box() Tests
    // ========================================================================

    #[test]
    fn test_arc_to_box_basic() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let boxed = double.to_box();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted BoxTransformer also usable
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_arc_to_box_multiple_conversions() {
        let triple = ArcTransformer::new(|x: i32| x * 3);
        let boxed1 = triple.to_box();
        let boxed2 = triple.to_box();

        // Multiple conversions all work
        assert_eq!(boxed1.apply(7), 21);
        assert_eq!(boxed2.apply(7), 21);
        assert_eq!(triple.apply(7), 21);
    }

    #[test]
    fn test_arc_to_box_with_string() {
        let length = ArcTransformer::new(|s: String| s.len());
        let boxed = length.to_box();

        assert_eq!(length.apply("hello".to_string()), 5);
        assert_eq!(boxed.apply("world".to_string()), 5);
    }

    #[test]
    fn test_arc_to_box_with_captured_variable() {
        let multiplier = 5;
        let multiply = ArcTransformer::new(move |x: i32| x * multiplier);
        let boxed = multiply.to_box();

        assert_eq!(multiply.apply(8), 40);
        assert_eq!(boxed.apply(8), 40);
    }

    // ========================================================================
    // ArcTransformer::to_rc() Tests
    // ========================================================================

    #[test]
    fn test_arc_to_rc_basic() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let rc = double.to_rc();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted RcTransformer also usable
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_arc_to_rc_multiple_conversions() {
        let add_ten = ArcTransformer::new(|x: i32| x + 10);
        let rc1 = add_ten.to_rc();
        let rc2 = add_ten.to_rc();

        assert_eq!(rc1.apply(5), 15);
        assert_eq!(rc2.apply(5), 15);
        assert_eq!(add_ten.apply(5), 15);
    }

    #[test]
    fn test_arc_to_rc_clone_conversion() {
        let negate = ArcTransformer::new(|x: i32| -x);
        let rc = negate.to_rc();
        let rc_clone = rc.clone();

        assert_eq!(negate.apply(42), -42);
        assert_eq!(rc.apply(42), -42);
        assert_eq!(rc_clone.apply(42), -42);
    }

    // ========================================================================
    // ArcTransformer::to_arc() Tests
    // ========================================================================

    #[test]
    fn test_arc_to_arc_basic() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let arc = double.to_arc();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted ArcTransformer also usable
        assert_eq!(arc.apply(21), 42);
    }

    #[test]
    fn test_arc_to_arc_is_clone() {
        let add_one = ArcTransformer::new(|x: i32| x + 1);
        let arc = add_one.to_arc();

        // to_arc() should be equivalent to clone()
        assert_eq!(add_one.apply(41), 42);
        assert_eq!(arc.apply(41), 42);
    }

    #[test]
    fn test_arc_to_arc_thread_safe() {
        let increment = ArcTransformer::new(|x: i32| x + 1);
        let arc = increment.to_arc();

        let handle = thread::spawn(move || arc.apply(99));

        assert_eq!(handle.join().unwrap(), 100);
        assert_eq!(increment.apply(41), 42);
    }

    // ========================================================================
    // ArcTransformer::to_fn() Tests
    // ========================================================================

    #[test]
    fn test_arc_to_fn_basic() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let func = double.to_fn();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted function also usable
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_arc_to_fn_multiple_calls() {
        let square = ArcTransformer::new(|x: i32| x * x);
        let func = square.to_fn();

        assert_eq!(func(5), 25);
        assert_eq!(func(7), 49);
        assert_eq!(square.apply(3), 9);
    }

    #[test]
    fn test_arc_to_fn_with_closure_composition() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let func = double.to_fn();

        let add_ten = |x: i32| x + 10;
        let result = add_ten(func(5)); // (5 * 2) + 10
        assert_eq!(result, 20);
    }

    // ========================================================================
    // RcTransformer::to_box() Tests
    // ========================================================================

    #[test]
    fn test_rc_to_box_basic() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let boxed = double.to_box();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted BoxTransformer also usable
        assert_eq!(boxed.apply(21), 42);
    }

    #[test]
    fn test_rc_to_box_multiple_conversions() {
        let subtract = RcTransformer::new(|x: i32| x - 5);
        let boxed1 = subtract.to_box();
        let boxed2 = subtract.to_box();

        assert_eq!(boxed1.apply(15), 10);
        assert_eq!(boxed2.apply(15), 10);
        assert_eq!(subtract.apply(15), 10);
    }

    #[test]
    fn test_rc_to_box_with_clone() {
        let negate = RcTransformer::new(|x: i32| -x);
        let rc_clone = negate.clone();
        let boxed = rc_clone.to_box();

        assert_eq!(negate.apply(42), -42);
        assert_eq!(rc_clone.apply(42), -42);
        assert_eq!(boxed.apply(42), -42);
    }

    // ========================================================================
    // RcTransformer::to_rc() Tests
    // ========================================================================

    #[test]
    fn test_rc_to_rc_basic() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let rc = double.to_rc();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted RcTransformer also usable
        assert_eq!(rc.apply(21), 42);
    }

    #[test]
    fn test_rc_to_rc_is_clone() {
        let add_one = RcTransformer::new(|x: i32| x + 1);
        let rc = add_one.to_rc();

        // to_rc() should be equivalent to clone()
        assert_eq!(add_one.apply(41), 42);
        assert_eq!(rc.apply(41), 42);
    }

    #[test]
    fn test_rc_to_rc_multiple_clones() {
        let triple = RcTransformer::new(|x: i32| x * 3);
        let rc1 = triple.to_rc();
        let rc2 = triple.to_rc();
        let rc1_clone = rc1.clone();

        assert_eq!(triple.apply(7), 21);
        assert_eq!(rc1.apply(7), 21);
        assert_eq!(rc2.apply(7), 21);
        assert_eq!(rc1_clone.apply(7), 21);
    }

    // ========================================================================
    // RcTransformer::to_fn() Tests
    // ========================================================================

    #[test]
    fn test_rc_to_fn_basic() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let func = double.to_fn();

        // Original transformer still usable
        assert_eq!(double.apply(21), 42);
        // Converted function also usable
        assert_eq!(func(21), 42);
    }

    #[test]
    fn test_rc_to_fn_multiple_calls() {
        let abs = RcTransformer::new(|x: i32| x.abs());
        let func = abs.to_fn();

        assert_eq!(func(-5), 5);
        assert_eq!(func(5), 5);
        assert_eq!(abs.apply(-10), 10);
    }

    #[test]
    fn test_rc_to_fn_with_captured_state() {
        let offset = 100;
        let add_offset = RcTransformer::new(move |x: i32| x + offset);
        let func = add_offset.to_fn();

        assert_eq!(func(42), 142);
        assert_eq!(add_offset.apply(42), 142);
    }

    // ========================================================================
    // Cross-type conversion tests
    // ========================================================================

    #[test]
    fn test_arc_to_box_to_fn_chain() {
        let double = ArcTransformer::new(|x: i32| x * 2);
        let boxed = double.to_box();
        let func = boxed.into_fn();

        assert_eq!(func(21), 42);
        // Original ArcTransformer still usable
        assert_eq!(double.apply(21), 42);
    }

    #[test]
    fn test_arc_to_rc_to_box_chain() {
        let triple = ArcTransformer::new(|x: i32| x * 3);
        let rc = triple.to_rc();
        let boxed = rc.to_box();

        assert_eq!(boxed.apply(7), 21);
        assert_eq!(rc.apply(7), 21);
        assert_eq!(triple.apply(7), 21);
    }

    #[test]
    fn test_rc_to_box_composition() {
        let double = RcTransformer::new(|x: i32| x * 2);
        let add_one = |x: i32| x + 1;

        let boxed = double.to_box();
        let composed = boxed.and_then(add_one);

        assert_eq!(composed.apply(5), 11); // (5 * 2) + 1
        assert_eq!(double.apply(5), 10);
    }

    // ========================================================================
    // Complex type tests
    // ========================================================================

    #[test]
    fn test_arc_to_box_with_option() {
        let parse = ArcTransformer::new(|s: String| s.parse::<i32>().ok());
        let boxed = parse.to_box();

        assert_eq!(parse.apply("42".to_string()), Some(42));
        assert_eq!(boxed.apply("42".to_string()), Some(42));
        assert_eq!(boxed.apply("abc".to_string()), None);
    }

    #[test]
    fn test_rc_to_fn_with_result() {
        let divide = RcTransformer::new(|x: i32| {
            if x == 0 {
                Err("Division by zero")
            } else {
                Ok(100 / x)
            }
        });
        let func = divide.to_fn();

        assert_eq!(func(10), Ok(10));
        assert_eq!(func(0), Err("Division by zero"));
        assert_eq!(divide.apply(5), Ok(20));
    }

    #[test]
    fn test_arc_to_rc_with_vec() {
        let sort = ArcTransformer::new(|mut v: Vec<i32>| {
            v.sort();
            v
        });
        let rc = sort.to_rc();

        let input1 = vec![3, 1, 4, 1, 5, 9];
        let input2 = vec![3, 1, 4, 1, 5, 9];

        assert_eq!(sort.apply(input1), vec![1, 1, 3, 4, 5, 9]);
        assert_eq!(rc.apply(input2), vec![1, 1, 3, 4, 5, 9]);
    }

    // ========================================================================
    // Identity and constant tests
    // ========================================================================

    #[test]
    fn test_arc_identity_to_box() {
        let identity = ArcTransformer::<i32, i32>::identity();
        let boxed = identity.to_box();

        assert_eq!(identity.apply(42), 42);
        assert_eq!(boxed.apply(42), 42);
    }

    #[test]
    fn test_rc_constant_to_fn() {
        let constant = RcTransformer::constant("fixed");
        let func = constant.to_fn();

        assert_eq!(func(1), "fixed");
        assert_eq!(func(2), "fixed");
        assert_eq!(constant.apply(3), "fixed");
    }

    #[test]
    fn test_arc_constant_to_rc() {
        let constant = ArcTransformer::constant(123);
        let rc = constant.to_rc();

        assert_eq!(constant.apply(999), 123);
        assert_eq!(rc.apply(999), 123);
    }
}

// ============================================================================
// Custom Type with Default to_xxx Implementation Tests
// ============================================================================

#[cfg(test)]
mod custom_transformer_to_methods_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };
    use std::thread;

    /// Custom Transformer implementation for testing default to_xxx methods
    /// This is a simple multiplication transformer
    #[derive(Clone)]
    struct MultiplyTransformer {
        multiplier: i32,
    }

    impl Transformer<i32, i32> for MultiplyTransformer {
        fn apply(&self, input: i32) -> i32 {
            input * self.multiplier
        }
    }

    /// Thread-safe custom Transformer implementation
    #[derive(Clone)]
    struct ThreadSafeMultiplyTransformer {
        multiplier: i32,
    }

    // Manual implementation of Send + Sync
    unsafe impl Send for ThreadSafeMultiplyTransformer {}
    unsafe impl Sync for ThreadSafeMultiplyTransformer {}

    impl Transformer<i32, i32> for ThreadSafeMultiplyTransformer {
        fn apply(&self, input: i32) -> i32 {
            input * self.multiplier
        }
    }

    // ========================================================================
    // Custom Type to_box() Tests
    // ========================================================================

    #[test]
    fn test_custom_to_box_basic() {
        let multiply = MultiplyTransformer { multiplier: 3 };
        let boxed = multiply.to_box();

        // Original transformer still usable
        assert_eq!(multiply.apply(7), 21);
        // Converted BoxTransformer also usable
        assert_eq!(boxed.apply(7), 21);
    }

    #[test]
    fn test_custom_to_box_multiple_conversions() {
        let multiply = MultiplyTransformer { multiplier: 5 };
        let boxed1 = multiply.to_box();
        let boxed2 = multiply.to_box();

        // Multiple conversions all work
        assert_eq!(boxed1.apply(4), 20);
        assert_eq!(boxed2.apply(4), 20);
        assert_eq!(multiply.apply(4), 20);
    }

    #[test]
    fn test_custom_to_box_with_composition() {
        let multiply = MultiplyTransformer { multiplier: 2 };
        let boxed = multiply.to_box();

        // Compose with other transformer
        let add_ten = BoxTransformer::new(|x: i32| x + 10);
        let composed = boxed.and_then(add_ten);

        assert_eq!(composed.apply(5), 20); // (5 * 2) + 10
    }

    // ========================================================================
    // Custom Type to_rc() Tests
    // ========================================================================

    #[test]
    fn test_custom_to_rc_basic() {
        let multiply = MultiplyTransformer { multiplier: 4 };
        let rc = multiply.to_rc();

        // Original transformer still usable
        assert_eq!(multiply.apply(5), 20);
        // Converted RcTransformer also usable
        assert_eq!(rc.apply(5), 20);
    }

    #[test]
    fn test_custom_to_rc_multiple_conversions() {
        let multiply = MultiplyTransformer { multiplier: 7 };
        let rc1 = multiply.to_rc();
        let rc2 = multiply.to_rc();

        assert_eq!(rc1.apply(3), 21);
        assert_eq!(rc2.apply(3), 21);
        assert_eq!(multiply.apply(3), 21);
    }

    #[test]
    fn test_custom_to_rc_clone_and_use() {
        let multiply = MultiplyTransformer { multiplier: 6 };
        let rc = multiply.to_rc();
        let rc_clone = rc.clone();

        assert_eq!(multiply.apply(4), 24);
        assert_eq!(rc.apply(4), 24);
        assert_eq!(rc_clone.apply(4), 24);
    }

    #[test]
    fn test_custom_to_rc_with_composition() {
        let multiply = MultiplyTransformer { multiplier: 3 };
        let rc = multiply.to_rc();

        // Compose with other transformer
        let square = RcTransformer::new(|x: i32| x * x);
        let composed = rc.and_then(square);

        assert_eq!(composed.apply(5), 225); // (5 * 3)^2 = 225
    }

    // ========================================================================
    // Custom Type to_arc() Tests (Thread-Safe)
    // ========================================================================

    #[test]
    fn test_custom_to_arc_basic() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 5 };
        let arc = multiply.to_arc();

        // Original transformer still usable
        assert_eq!(multiply.apply(8), 40);
        // Converted ArcTransformer also usable
        assert_eq!(arc.apply(8), 40);
    }

    #[test]
    fn test_custom_to_arc_multiple_conversions() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 9 };
        let arc1 = multiply.to_arc();
        let arc2 = multiply.to_arc();

        assert_eq!(arc1.apply(2), 18);
        assert_eq!(arc2.apply(2), 18);
        assert_eq!(multiply.apply(2), 18);
    }

    #[test]
    fn test_custom_to_arc_clone_and_use() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 10 };
        let arc = multiply.to_arc();
        let arc_clone = arc.clone();

        assert_eq!(multiply.apply(3), 30);
        assert_eq!(arc.apply(3), 30);
        assert_eq!(arc_clone.apply(3), 30);
    }

    #[test]
    fn test_custom_to_arc_thread_safe() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 7 };
        let arc = multiply.to_arc();

        let handle = thread::spawn(move || arc.apply(6));

        assert_eq!(handle.join().unwrap(), 42);
        assert_eq!(multiply.apply(6), 42);
    }

    #[test]
    fn test_custom_to_arc_with_composition() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 4 };
        let arc = multiply.to_arc();

        // Compose with other transformer
        let double = ArcTransformer::new(|x: i32| x * 2);
        let composed = arc.and_then(double);

        assert_eq!(composed.apply(5), 40); // (5 * 4) * 2 = 40
    }

    // ========================================================================
    // Custom Type to_fn() Tests
    // ========================================================================

    #[test]
    fn test_custom_to_fn_basic() {
        let multiply = MultiplyTransformer { multiplier: 8 };
        let func = multiply.to_fn();

        // Original transformer still usable
        assert_eq!(multiply.apply(5), 40);
        // Converted function also usable
        assert_eq!(func(5), 40);
    }

    #[test]
    fn test_custom_to_fn_multiple_calls() {
        let multiply = MultiplyTransformer { multiplier: 6 };
        let func = multiply.to_fn();

        assert_eq!(func(3), 18);
        assert_eq!(func(7), 42);
        assert_eq!(multiply.apply(10), 60);
    }

    #[test]
    fn test_custom_to_fn_with_closure_composition() {
        let multiply = MultiplyTransformer { multiplier: 5 };
        let func = multiply.to_fn();

        let add_five = |x: i32| x + 5;
        let result = add_five(func(4)); // (4 * 5) + 5 = 25
        assert_eq!(result, 25);
    }

    #[test]
    fn test_custom_to_fn_multiple_conversions() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 9 };
        let func1 = multiply.to_fn();
        let func2 = multiply.to_fn();

        // Multiple conversions all work
        assert_eq!(func1(5), 45);
        assert_eq!(func2(5), 45);
        assert_eq!(multiply.apply(5), 45);
    }

    // ========================================================================
    // Cross-type conversion tests
    // ========================================================================

    #[test]
    fn test_custom_to_box_to_rc_chain() {
        let multiply = MultiplyTransformer { multiplier: 3 };
        let boxed = multiply.to_box();
        let rc = boxed.into_rc();

        assert_eq!(rc.apply(7), 21);
        // Original custom transformer still usable
        assert_eq!(multiply.apply(7), 21);
    }

    #[test]
    fn test_custom_to_rc_to_box_chain() {
        let multiply = MultiplyTransformer { multiplier: 4 };
        let rc = multiply.to_rc();
        let boxed = rc.to_box();

        assert_eq!(boxed.apply(6), 24);
        assert_eq!(rc.apply(6), 24);
        assert_eq!(multiply.apply(6), 24);
    }

    #[test]
    fn test_custom_to_arc_to_rc_chain() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 5 };
        let arc = multiply.to_arc();
        let rc = arc.to_rc();

        assert_eq!(rc.apply(8), 40);
        assert_eq!(arc.apply(8), 40);
        assert_eq!(multiply.apply(8), 40);
    }

    #[test]
    fn test_custom_multiple_to_conversions() {
        let multiply = ThreadSafeMultiplyTransformer { multiplier: 2 };

        let boxed = multiply.to_box();
        let rc = multiply.to_rc();
        let arc = multiply.to_arc();
        let func = multiply.to_fn();

        // All converted types work normally
        assert_eq!(multiply.apply(10), 20);
        assert_eq!(boxed.apply(10), 20);
        assert_eq!(rc.apply(10), 20);
        assert_eq!(arc.apply(10), 20);
        assert_eq!(func(10), 20);
    }

    // ========================================================================
    // Different type transformation tests
    // ========================================================================

    /// Custom Transformer: i32 -> String
    #[derive(Clone)]
    struct IntToStringTransformer {
        prefix: String,
    }

    impl Transformer<i32, String> for IntToStringTransformer {
        fn apply(&self, input: i32) -> String {
            format!("{}{}", self.prefix, input)
        }
    }

    #[test]
    fn test_custom_different_types_to_box() {
        let transformer = IntToStringTransformer {
            prefix: "Number: ".to_string(),
        };
        let boxed = transformer.to_box();

        assert_eq!(transformer.apply(42), "Number: 42");
        assert_eq!(boxed.apply(42), "Number: 42");
    }

    #[test]
    fn test_custom_different_types_to_rc() {
        let transformer = IntToStringTransformer {
            prefix: "Value: ".to_string(),
        };
        let rc = transformer.to_rc();

        assert_eq!(transformer.apply(100), "Value: 100");
        assert_eq!(rc.apply(100), "Value: 100");
    }

    #[test]
    fn test_custom_different_types_to_fn() {
        let transformer = IntToStringTransformer {
            prefix: "Result: ".to_string(),
        };
        let func = transformer.to_fn();

        assert_eq!(transformer.apply(999), "Result: 999");
        assert_eq!(func(999), "Result: 999");
    }

    // ========================================================================
    // Complex state tests
    // ========================================================================

    /// Custom Transformer with complex state
    #[derive(Clone)]
    struct StatefulTransformer {
        base: i32,
        multiplier: i32,
        offset: i32,
    }

    impl Transformer<i32, i32> for StatefulTransformer {
        fn apply(&self, input: i32) -> i32 {
            (input + self.base) * self.multiplier + self.offset
        }
    }

    #[test]
    fn test_stateful_to_box() {
        let transformer = StatefulTransformer {
            base: 5,
            multiplier: 3,
            offset: 10,
        };
        let boxed = transformer.to_box();

        // (10 + 5) * 3 + 10 = 55
        assert_eq!(transformer.apply(10), 55);
        assert_eq!(boxed.apply(10), 55);
    }

    #[test]
    fn test_stateful_to_rc() {
        let transformer = StatefulTransformer {
            base: 2,
            multiplier: 4,
            offset: 1,
        };
        let rc = transformer.to_rc();

        // (5 + 2) * 4 + 1 = 29
        assert_eq!(transformer.apply(5), 29);
        assert_eq!(rc.apply(5), 29);
    }

    #[test]
    fn test_stateful_to_fn() {
        let transformer = StatefulTransformer {
            base: 1,
            multiplier: 2,
            offset: 3,
        };
        let func = transformer.to_fn();

        // (7 + 1) * 2 + 3 = 19
        assert_eq!(transformer.apply(7), 19);
        assert_eq!(func(7), 19);
    }

    #[test]
    fn test_stateful_all_conversions() {
        let transformer = StatefulTransformer {
            base: 3,
            multiplier: 2,
            offset: 5,
        };

        let boxed = transformer.to_box();
        let rc = transformer.to_rc();
        let func = transformer.to_fn();

        // (6 + 3) * 2 + 5 = 23
        let expected = 23;
        assert_eq!(transformer.apply(6), expected);
        assert_eq!(boxed.apply(6), expected);
        assert_eq!(rc.apply(6), expected);
        assert_eq!(func(6), expected);
    }
}

// ============================================================================
// TransformerOnce Tests for BoxTransformer, RcTransformer, ArcTransformer
// ============================================================================

#[cfg(test)]
mod transformer_once_tests {
    use qubit_function::{
        ArcTransformer,
        BoxTransformer,
        RcTransformer,
        Transformer,
    };
    use std::sync::Arc;
    use std::thread;

    // BoxTransformer TransformerOnce Tests
    #[cfg(test)]
    mod box_transformer_once_tests {
        use super::*;

        #[test]
        fn test_box_transformer_apply() {
            let double = BoxTransformer::new(|x: i32| x * 2);
            let result = double.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_box_transformer_into_box() {
            let double = BoxTransformer::new(|x: i32| x * 2);
            let boxed = double.into_box();
            let result = boxed.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_box_transformer_into_fn() {
            let double = BoxTransformer::new(|x: i32| x * 2);
            let func = double.into_fn();
            let result = func(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_box_transformer_string_transformation() {
            let uppercase = BoxTransformer::new(|s: String| s.to_uppercase());
            let result = uppercase.apply("hello".to_string());
            assert_eq!(result, "HELLO");
        }

        #[test]
        fn test_box_transformer_complex_transformation() {
            let parse_and_double =
                BoxTransformer::new(|s: String| s.parse::<i32>().unwrap_or(0) * 2);
            let result = parse_and_double.apply("21".to_string());
            assert_eq!(result, 42);
        }

        #[test]
        fn test_box_transformer_regular_and_once() {
            let double = BoxTransformer::new(|x: i32| x * 2);

            // Regular apply can be called multiple times
            assert_eq!(double.apply(10), 20);
            assert_eq!(double.apply(15), 30);

            // But apply consumes the transformer
            let double = BoxTransformer::new(|x: i32| x * 2);
            let result = double.apply(21);
            assert_eq!(result, 42);
        }
    }

    // RcTransformer TransformerOnce Tests
    #[cfg(test)]
    mod rc_transformer_once_tests {
        use super::*;

        #[test]
        fn test_rc_transformer_apply() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let result = double.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_rc_transformer_into_box() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let boxed = double.into_box();
            let result = boxed.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_rc_transformer_into_fn() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let func = double.into_fn();
            let result = func(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_rc_transformer_string_transformation() {
            let uppercase = RcTransformer::new(|s: String| s.to_uppercase());
            let result = uppercase.apply("hello".to_string());
            assert_eq!(result, "HELLO");
        }

        #[test]
        fn test_rc_transformer_complex_transformation() {
            let parse_and_double =
                RcTransformer::new(|s: String| s.parse::<i32>().unwrap_or(0) * 2);
            let result = parse_and_double.apply("21".to_string());
            assert_eq!(result, 42);
        }

        #[test]
        fn test_rc_transformer_clone_before_apply() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let double_clone = double.clone();

            // Both should work
            assert_eq!(double.apply(21), 42);
            assert_eq!(double_clone.apply(21), 42);
        }

        #[test]
        fn test_rc_transformer_regular_and_once() {
            let double = RcTransformer::new(|x: i32| x * 2);

            // Regular apply can be called multiple times
            assert_eq!(double.apply(10), 20);
            assert_eq!(double.apply(15), 30);

            // Clone before using apply
            let double_clone = double.clone();
            let result = double_clone.apply(21);
            assert_eq!(result, 42);

            // Original is still usable
            assert_eq!(double.apply(5), 10);
        }

        #[test]
        fn test_rc_transformer_to_box() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let boxed = double.to_box();

            // Original transformer still usable
            assert_eq!(double.apply(21), 42);
            // Converted BoxTransformerOnce also usable
            assert_eq!(boxed.apply(21), 42);
        }

        #[test]
        fn test_rc_transformer_to_fn() {
            let double = RcTransformer::new(|x: i32| x * 2);
            let func = double.to_fn();

            // Original transformer still usable
            assert_eq!(double.apply(21), 42);
            // Converted function also usable
            assert_eq!(func(21), 42);
        }

        #[test]
        fn test_rc_transformer_to_box_multiple_conversions() {
            let triple = RcTransformer::new(|x: i32| x * 3);
            let boxed1 = triple.to_box();
            let boxed2 = triple.to_box();

            // Multiple conversions all work
            assert_eq!(boxed1.apply(7), 21);
            assert_eq!(boxed2.apply(7), 21);
            assert_eq!(triple.apply(7), 21);
        }

        #[test]
        fn test_rc_transformer_to_fn_multiple_conversions() {
            let square = RcTransformer::new(|x: i32| x * x);
            let func1 = square.to_fn();
            let func2 = square.to_fn();

            assert_eq!(func1(5), 25);
            assert_eq!(func2(5), 25);
            assert_eq!(square.apply(5), 25);
        }

        #[test]
        fn test_rc_transformer_to_box_with_string() {
            let length = RcTransformer::new(|s: String| s.len());
            let boxed = length.to_box();

            assert_eq!(length.apply("hello".to_string()), 5);
            assert_eq!(boxed.apply("world".to_string()), 5);
        }

        #[test]
        fn test_rc_transformer_to_fn_with_string() {
            let uppercase = RcTransformer::new(|s: String| s.to_uppercase());
            let func = uppercase.to_fn();

            assert_eq!(uppercase.apply("hello".to_string()), "HELLO");
            assert_eq!(func("world".to_string()), "WORLD");
        }
    }

    // ArcTransformer TransformerOnce Tests
    #[cfg(test)]
    mod arc_transformer_once_tests {
        use super::*;

        #[test]
        fn test_arc_transformer_apply() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let result = double.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_into_box() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let boxed = double.into_box();
            let result = boxed.apply(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_into_fn() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let func = double.into_fn();
            let result = func(21);
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_to_box() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let boxed = double.to_box();

            // Original transformer still usable
            assert_eq!(double.apply(21), 42);
            // Converted BoxTransformerOnce also usable
            assert_eq!(boxed.apply(21), 42);
        }

        #[test]
        fn test_arc_transformer_to_fn() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let func = double.to_fn();

            // Original transformer still usable
            assert_eq!(double.apply(21), 42);
            // Converted function also usable
            assert_eq!(func(21), 42);
        }

        #[test]
        fn test_arc_transformer_to_box_multiple_conversions() {
            let triple = ArcTransformer::new(|x: i32| x * 3);
            let boxed1 = triple.to_box();
            let boxed2 = triple.to_box();

            // Multiple conversions all work
            assert_eq!(boxed1.apply(7), 21);
            assert_eq!(boxed2.apply(7), 21);
            assert_eq!(triple.apply(7), 21);
        }

        #[test]
        fn test_arc_transformer_to_fn_multiple_conversions() {
            let square = ArcTransformer::new(|x: i32| x * x);
            let func1 = square.to_fn();
            let func2 = square.to_fn();

            assert_eq!(func1(5), 25);
            assert_eq!(func2(5), 25);
            assert_eq!(square.apply(5), 25);
        }

        #[test]
        fn test_arc_transformer_to_box_with_string() {
            let length = ArcTransformer::new(|s: String| s.len());
            let boxed = length.to_box();

            assert_eq!(length.apply("hello".to_string()), 5);
            assert_eq!(boxed.apply("world".to_string()), 5);
        }

        #[test]
        fn test_arc_transformer_to_fn_with_string() {
            let uppercase = ArcTransformer::new(|s: String| s.to_uppercase());
            let func = uppercase.to_fn();

            assert_eq!(uppercase.apply("hello".to_string()), "HELLO");
            assert_eq!(func("world".to_string()), "WORLD");
        }

        #[test]
        fn test_arc_transformer_to_box_thread_safety() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let double_arc = Arc::new(double);
            let _double_clone = Arc::clone(&double_arc);

            let handle = thread::spawn(move || {
                // Create a new transformer in the thread
                let new_double = ArcTransformer::new(|x: i32| x * 2);
                let boxed = new_double.to_box();
                boxed.apply(21)
            });

            let result = handle.join().unwrap();
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_to_fn_thread_safety() {
            let square = ArcTransformer::new(|x: i32| x * x);
            let square_arc = Arc::new(square);
            let _square_clone = Arc::clone(&square_arc);

            let handle = thread::spawn(move || {
                // Create a new transformer in the thread
                let new_square = ArcTransformer::new(|x: i32| x * x);
                let func = new_square.to_fn();
                func(7)
            });

            let result = handle.join().unwrap();
            assert_eq!(result, 49);
        }

        #[test]
        fn test_arc_transformer_string_transformation() {
            let uppercase = ArcTransformer::new(|s: String| s.to_uppercase());
            let result = uppercase.apply("hello".to_string());
            assert_eq!(result, "HELLO");
        }

        #[test]
        fn test_arc_transformer_complex_transformation() {
            let parse_and_double =
                ArcTransformer::new(|s: String| s.parse::<i32>().unwrap_or(0) * 2);
            let result = parse_and_double.apply("21".to_string());
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_clone_before_apply() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let double_clone = double.clone();

            // Both should work
            assert_eq!(double.apply(21), 42);
            assert_eq!(double_clone.apply(21), 42);
        }

        #[test]
        fn test_arc_transformer_regular_and_once() {
            let double = ArcTransformer::new(|x: i32| x * 2);

            // Regular apply can be called multiple times
            assert_eq!(double.apply(10), 20);
            assert_eq!(double.apply(15), 30);

            // Clone before using apply
            let double_clone = double.clone();
            let result = double_clone.apply(21);
            assert_eq!(result, 42);

            // Original is still usable
            assert_eq!(double.apply(5), 10);
        }

        #[test]
        fn test_arc_transformer_thread_safety() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let double_arc = Arc::new(double);
            let _double_clone = Arc::clone(&double_arc);

            let handle = thread::spawn(move || {
                // Create a new transformer in the thread to demonstrate thread safety
                let new_double = ArcTransformer::new(|x: i32| x * 2);
                new_double.apply(21)
            });

            let result = handle.join().unwrap();
            assert_eq!(result, 42);
        }

        #[test]
        fn test_arc_transformer_into_box_thread_safety() {
            let double = ArcTransformer::new(|x: i32| x * 2);
            let double_arc = Arc::new(double);
            let _double_clone = Arc::clone(&double_arc);

            let handle = thread::spawn(move || {
                // Create a new transformer in the thread
                let new_double = ArcTransformer::new(|x: i32| x * 2);
                let boxed = new_double.into_box();
                boxed.apply(21)
            });

            let result = handle.join().unwrap();
            assert_eq!(result, 42);
        }
    }

    // Cross-type TransformerOnce Tests
    #[cfg(test)]
    mod cross_type_transformer_once_tests {
        use super::*;

        #[test]
        fn test_all_types_apply() {
            let box_double = BoxTransformer::new(|x: i32| x * 2);
            let rc_double = RcTransformer::new(|x: i32| x * 2);
            let arc_double = ArcTransformer::new(|x: i32| x * 2);

            assert_eq!(box_double.apply(21), 42);
            assert_eq!(rc_double.apply(21), 42);
            assert_eq!(arc_double.apply(21), 42);
        }

        #[test]
        fn test_all_types_into_box() {
            let box_double = BoxTransformer::new(|x: i32| x * 2);
            let rc_double = RcTransformer::new(|x: i32| x * 2);
            let arc_double = ArcTransformer::new(|x: i32| x * 2);

            let box_boxed = box_double.into_box();
            let rc_boxed = rc_double.into_box();
            let arc_boxed = arc_double.into_box();

            assert_eq!(box_boxed.apply(21), 42);
            assert_eq!(rc_boxed.apply(21), 42);
            assert_eq!(arc_boxed.apply(21), 42);
        }

        #[test]
        fn test_all_types_into_fn() {
            let box_double = BoxTransformer::new(|x: i32| x * 2);
            let rc_double = RcTransformer::new(|x: i32| x * 2);
            let arc_double = ArcTransformer::new(|x: i32| x * 2);

            let box_func = box_double.into_fn();
            let rc_func = rc_double.into_fn();
            let arc_func = arc_double.into_fn();

            assert_eq!(box_func(21), 42);
            assert_eq!(rc_func(21), 42);
            assert_eq!(arc_func(21), 42);
        }

        #[test]
        fn test_mixed_regular_and_once_usage() {
            // Test that regular apply and apply work together
            let box_transformer = BoxTransformer::new(|x: i32| x * 2);
            let rc_transformer = RcTransformer::new(|x: i32| x * 2);
            let arc_transformer = ArcTransformer::new(|x: i32| x * 2);

            // Regular apply (multiple calls)
            assert_eq!(box_transformer.apply(10), 20);
            assert_eq!(rc_transformer.apply(10), 20);
            assert_eq!(arc_transformer.apply(10), 20);

            // Clone for apply
            let rc_clone = rc_transformer.clone();
            let arc_clone = arc_transformer.clone();

            // Apply once (consuming)
            assert_eq!(rc_clone.apply(21), 42);
            assert_eq!(arc_clone.apply(21), 42);

            // Original transformers still work
            assert_eq!(rc_transformer.apply(5), 10);
            assert_eq!(arc_transformer.apply(5), 10);
        }
    }
}

// ============================================================================
// Transformer Trait Default Methods Tests - into_once, to_once
// ============================================================================

#[cfg(test)]
mod test_transformer_trait_default_methods {
    use qubit_function::TransformerOnce;
    use std::sync::{
        atomic::{
            AtomicUsize,
            Ordering,
        },
        Arc,
    };

    #[test]
    fn test_custom_transformer_into_once() {
        use qubit_function::Transformer;

        let counter = Arc::new(AtomicUsize::new(0));

        struct MyTransformer {
            counter: Arc<AtomicUsize>,
        }

        impl Transformer<i32, i32> for MyTransformer {
            fn apply(&self, value: i32) -> i32 {
                self.counter.fetch_add(1, Ordering::SeqCst);
                value * 2
            }
        }

        let my_transformer = MyTransformer {
            counter: counter.clone(),
        };

        // Test into_once() - should consume the transformer
        let once_transformer = my_transformer.into_once();
        let result = once_transformer.apply(5);
        assert_eq!(result, 10);
        assert_eq!(counter.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_custom_transformer_to_once() {
        use qubit_function::Transformer;

        let counter = Arc::new(AtomicUsize::new(0));

        #[derive(Clone)]
        struct MyTransformer {
            counter: Arc<AtomicUsize>,
        }

        impl Transformer<i32, i32> for MyTransformer {
            fn apply(&self, value: i32) -> i32 {
                self.counter.fetch_add(1, Ordering::SeqCst);
                value * 2
            }
        }

        let my_transformer = MyTransformer {
            counter: counter.clone(),
        };

        // Test to_once() - should not consume the original
        let once_transformer = my_transformer.to_once();
        let result = once_transformer.apply(5);
        assert_eq!(result, 10);
        assert_eq!(counter.load(Ordering::SeqCst), 1);

        // Original transformer should still be usable
        let result2 = my_transformer.apply(3);
        assert_eq!(result2, 6);
        assert_eq!(counter.load(Ordering::SeqCst), 2);
    }
}