# Python — everything is a reference, gc cleans up
a = [1, 2, 3]
b = a              # b and a point to the SAME list
b.append(4)
print(a)            # [1, 2, 3, 4] — surprise! a changed too

# Who owns the list? Both a and b reference it.
# The garbage collector frees it when no references remain.
# You never think about this.

// Rust — every value has exactly ONE owner
let a = vec![1, 2, 3];
let b = a;           // Ownership MOVES from a to b
// println!("{:?}", a); // ❌ Compile error: value used after move

// a no longer exists. b is the sole owner.
println!("{:?}", b); // ✅ [1, 2, 3]

// When b goes out of scope, the Vec is freed. Deterministic. No GC.

1. Each value has exactly ONE owner variable.
2. When the owner goes out of scope, the value is dropped (freed).
3. Ownership can be transferred (moved) but not duplicated (unless Clone).

# Python — assignment copies the reference, not the data
def process(data):
    data.append(42)
    # Original list is modified!

my_list = [1, 2, 3]
process(my_list)
print(my_list)       # [1, 2, 3, 42] — modified by process!

// Rust — passing to a function MOVES ownership (for non-Copy types)
fn process(mut data: Vec<i32>) -> Vec<i32> {
    data.push(42);
    data  // Must return it to give ownership back!
}

let my_vec = vec![1, 2, 3];
let my_vec = process(my_vec);  // Ownership moves in and back out
println!("{:?}", my_vec);      // [1, 2, 3, 42]

// Or better — borrow instead of moving:
fn process_borrowed(data: &mut Vec<i32>) {
    data.push(42);
}

let mut my_vec = vec![1, 2, 3];
process_borrowed(&mut my_vec);  // Lend it temporarily
println!("{:?}", my_vec);       // [1, 2, 3, 42] — still ours

Python:                              Rust:

  a ──────┐                           a ──→ [1, 2, 3]
           ├──→ [1, 2, 3]
  b ──────┘                           After: let b = a;

  (a and b share one object)          a  (invalid, moved)
  (refcount = 2)                      b ──→ [1, 2, 3]
                                      (only b owns the data)

  del a → refcount = 1                drop(b) → data freed
  del b → refcount = 0 → freed        (deterministic, no GC)

stateDiagram-v2
    state "Python (Reference Counting)" as PY {
        [*] --> a_owns: a = [1,2,3]
        a_owns --> shared: b = a
        shared --> b_only: del a (refcount 2→1)
        b_only --> freed: del b (refcount 1→0)
        note right of shared: Both a and b point\nto the SAME object
    }
    state "Rust (Ownership Move)" as RS {
        [*] --> a_owns2: let a = vec![1,2,3]
        a_owns2 --> b_owns: let b = a (MOVE)
        b_owns --> freed2: b goes out of scope
        note right of b_owns: a is INVALID after move\nCompile error if used
    }

// Simple types (integers, floats, bools, chars) are COPIED, not moved
let x = 42;
let y = x;    // x is COPIED to y (both valid)
println!("{x} {y}");  // ✅ 42 42

// Heap-allocated types (String, Vec, HashMap) are MOVED
let s1 = String::from("hello");
let s2 = s1;  // s1 is MOVED to s2
// println!("{s1}");  // ❌ Error: value used after move

// To explicitly copy heap data, use .clone()
let s1 = String::from("hello");
let s2 = s1.clone();  // Deep copy
println!("{s1} {s2}");  // ✅ hello hello (both valid)

Python:                    Rust:
─────────                  ─────
int, float, bool           Copy types (i32, f64, bool, char)
→ copied on assignment     → copied on assignment (similar behavior)
                           (Note: Python caches small ints; Rust copies are always predictable)

list, dict, str            Move types (Vec, HashMap, String)
→ shared reference         → ownership transfer (different behavior!)
→ gc cleans up             → owner drops data
→ clone with list(x)       → clone with x.clone()
   or copy.deepcopy(x)

# Python — accidental aliasing
def remove_duplicates(items):
    seen = set()
    result = []
    for item in items:
        if item not in seen:
            seen.add(item)
            result.append(item)
    return result

original = [1, 2, 2, 3, 3, 3]
alias = original          # Alias, NOT a copy
unique = remove_duplicates(alias)
# original is still [1, 2, 2, 3, 3, 3] — but only because we didn't mutate
# If remove_duplicates modified the input, original would be affected too

use std::collections::HashSet;

// Rust — ownership prevents accidental aliasing
fn remove_duplicates(items: &[i32]) -> Vec<i32> {
    let mut seen = HashSet::new();
    items.iter()
        .filter(|&&item| seen.insert(item))
        .copied()
        .collect()
}

let original = vec![1, 2, 2, 3, 3, 3];
let unique = remove_duplicates(&original); // Borrows — can't modify
// original is guaranteed unchanged — compiler prevented mutation via &

Think of ownership like a physical book:

Python:  Everyone has a photocopy (shared references + GC)
Rust:    One person owns the book. Others can:
         - &book     = look at it (immutable borrow, many allowed)
         - &mut book = write in it (mutable borrow, exclusive)
         - book      = give it away (move)

flowchart TD
    R["Borrowing Rules"] --> IMM["✅ Many &T\n(shared/immutable)"]
    R --> MUT["✅ One &mut T\n(exclusive/mutable)"]
    R --> CONFLICT["❌ &T + &mut T\n(NEVER at same time)"]
    IMM --> SAFE["Multiple readers, safe"]
    MUT --> SAFE2["Single writer, safe"]
    CONFLICT --> ERR["Compile error!"]
    style IMM fill:#d4edda
    style MUT fill:#d4edda
    style CONFLICT fill:#f8d7da
    style ERR fill:#f8d7da,stroke:#dc3545

// Rule 1: You can have MANY immutable borrows OR ONE mutable borrow (not both)

let mut data = vec![1, 2, 3];

// Multiple immutable borrows — fine
let a = &data;
let b = &data;
println!("{:?} {:?}", a, b);  // ✅

// Mutable borrow — must be exclusive
let c = &mut data;
c.push(4);
// println!("{:?}", a);  // ❌ Error: can't use immutable borrow while mutable exists

// This prevents data races at compile time!
// Python has no equivalent — it's why Python dict modified-during-iteration crashes at runtime.

// Lifetimes answer: "How long does this reference live?"
// Usually the compiler infers them. You rarely write them explicitly.

// Simple case — compiler handles it:
fn first_word(s: &str) -> &str {
    s.split_whitespace().next().unwrap_or("")
}
// The compiler knows: the returned &str lives as long as the input &str

// When you need explicit lifetimes (rare):
fn longest<'a>(a: &'a str, b: &'a str) -> &'a str {
    if a.len() > b.len() { a } else { b }
}
// 'a says: "the return value lives as long as both inputs"

// Box<T> — heap allocation with single owner (like Python's normal allocation)
let boxed = Box::new(42);  // Heap-allocated i32

// Rc<T> — reference counted (like Python's refcount!)
use std::rc::Rc;
let shared = Rc::new(vec![1, 2, 3]);
let clone1 = Rc::clone(&shared);  // Increment refcount
let clone2 = Rc::clone(&shared);  // Increment refcount
// All three point to the same Vec. When all are dropped, Vec is freed.
// Similar to Python's reference counting, but Rc does NOT handle cycles —
// use Weak<T> to break cycles (Python's GC handles cycles automatically)

// Arc<T> — atomic reference counting (Rc for multi-threaded code)
use std::sync::Arc;
let thread_safe = Arc::new(vec![1, 2, 3]);
// Use Arc when sharing across threads (Rc is single-threaded)

// RefCell<T> — runtime borrow checking (like Python's "anything goes" model)
use std::cell::RefCell;
let cell = RefCell::new(42);
*cell.borrow_mut() = 99;  // Mutable borrow at runtime (panics if double-borrowed)

fn main() {
    let mut names = vec!["Alice".to_string(), "Bob".to_string()];
    let first = &names[0];
    names.push("Charlie".to_string());
    println!("First: {first}");

    let greeting = make_greeting(names[0]);
    println!("{greeting}");
}

fn make_greeting(name: String) -> String {
    format!("Hello, {name}!")
}

fn main() {
    let mut names = vec!["Alice".to_string(), "Bob".to_string()];
    let first = &names[0];
    println!("First: {first}"); // Use borrow BEFORE mutating
    names.push("Charlie".to_string()); // Now safe — no live immutable borrow

    let greeting = make_greeting(&names[0]); // Pass reference, not owned
    println!("{greeting}");
}

fn make_greeting(name: &str) -> String { // Accept &str, not String
    format!("Hello, {name}!")
}