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//! This crate provides a macro to generate aux macros for on-demand usage. //! //! # Examples //! //! ```rust //! use on_demand::generate_on_demand_macro; //! //! fn foo() { //! generate_on_demand_macro!(a: usize = None, { //! println!("a"); //! 1 //! }); //! generate_on_demand_macro!(b: usize = None, { //! println!("b"); //! let a_data = on_demand_get_a!(); //! 2 + *a_data //! }); //! generate_on_demand_macro!(c: usize = None, { //! println!("c"); //! let a_data = on_demand_get_a!(); //! let b_data = on_demand_get_b!(); //! 3 + *a_data + *b_data //! }); //! //! let c_data = on_demand_get_c!(); //! assert_eq!(*c_data, 6); //! } //! ``` //! //! After calling `generate_on_demand_macro` to the variable (for example, `a`), //! three new macros `on_demand_get_a`, `on_demand_get_a_mut` and `on_demand_into_a` are generated. //! When `on_demand_get_a` is called, it determines whether `a` has been calculated. If it is, //! then returns the reference to its data, otherwise calls the expression given as the second //! parameter of `generate_on_demand_macro`, then assigns the returned value to `a`. The other two //! generated macros do the similar job, but returns the mutable reference to, or takes ownership //! of `a`. //! //! In all, this means calculating `a` lazily using the expression. //! //! # Notes //! //! The returned value of `on_demand_get_a` is essentially a [`Ref`][std::cell::Ref], and //! `on_demand_get_a_mut` is [`RefMut`][std::cell::RefMut]. So remember to dereference it //! when necessary. //! //! When calling this macro, we assign a default value by `generate_on_demand_macro!(a: MyType //! = default_value_of_optional_my_type), { /* block */ });`. The `default_value_of_optional_my_type` should //! be of type `Option<MyType>`. If it is not `None`, then `a` is considered calculated already, and the expression //! given in block will never be called. //! //! There are some crates providing lazy feature, such as [spin](https://crates.io/crates/spin). //! However, those methods are based on closure, which could not handle some situations such as below //! (error handlings are omitted for simplicity): //! //! ```rust //! # use on_demand::generate_on_demand_macro; //! # use std::io::{Seek, Read, SeekFrom}; //! fn foo<T: Seek + Read>(binary: &mut T) { //! generate_on_demand_macro!(a: u32 = None, { //! let mut buf = [0; 4]; //! binary.seek(SeekFrom::Start(0)).unwrap(); //! binary.read(&mut buf).unwrap(); //! u32::from_be_bytes(buf) //! }); //! generate_on_demand_macro!(b: u32 = None, { //! let a_data = on_demand_get_a!(); //! let mut buf = [0; 4]; //! binary.seek(SeekFrom::Start(0)).unwrap(); //! binary.read(&mut buf).unwrap(); //! u32::from_be_bytes(buf) + *a_data //! }); //! generate_on_demand_macro!(c: u32 = None, { //! let a_data = on_demand_get_a!(); //! let b_data = on_demand_get_b!(); //! let mut buf = [0; 4]; //! binary.seek(SeekFrom::Start(0)).unwrap(); //! binary.read(&mut buf).unwrap(); //! u32::from_be_bytes(buf) + *a_data + *b_data //! }); //! let a_data = on_demand_get_a!(); //! let b_data = on_demand_get_b_mut!(); //! drop(b_data); // drop here since c will take ownership //! let c_data = on_demand_into_c!(); //! } //! ``` //! //! `binary` is considered as uniquely borrowed if closure is used, then the borrow checker //! won't allow us do above things. However, macros can do such things. #[doc(hidden)] pub use paste; /// Macro to generate on-demand macro #[macro_export] macro_rules! generate_on_demand_macro { ($var: ident: $Inner: ty = $default_value: expr, $getter: expr) => { let $var: ::std::cell::RefCell<::std::option::Option<$Inner>> = ::std::cell::RefCell::new($default_value); $crate::paste::paste! { #[allow(unused_macros)] macro_rules! [<on_demand_get_ $var>] { () => {{ let is_some = $var.borrow().is_some(); if !is_some { *($var.borrow_mut()) = Some({$getter}); } ::std::cell::Ref::map($var.borrow(), |var| { if let Some(data) = var.as_ref() { data } else { unreachable!() } }) }}; } #[allow(unused_macros)] macro_rules! [<on_demand_get_ $var _mut>] { () => {{ let is_some = $var.borrow().is_some(); if !is_some { *($var.borrow_mut()) = Some({$getter}); } ::std::cell::RefMut::map($var.borrow_mut(), |var| { if let Some(data) = var.as_mut() { data } else { unreachable!() } }) }}; } #[allow(unused_macros)] macro_rules! [<on_demand_into_ $var>] { () => {{ if let Some(data) = $var.into_inner() { data } else { {$getter} } }}; } } }; } #[cfg(test)] mod tests { use super::generate_on_demand_macro; use std::io::{Cursor, Read, Seek, SeekFrom}; #[test] fn test_reader() { let mut binary = Cursor::new([0x0u8, 0x1u8, 0x2u8, 0x3u8]); generate_on_demand_macro!(a: u32 = None, { let mut buf = [0; 4]; binary.seek(SeekFrom::Start(0)).unwrap(); binary.read(&mut buf).unwrap(); u32::from_le_bytes(buf) }); generate_on_demand_macro!(b: u32 = None, { let a_data = on_demand_get_a!(); let mut buf = [0; 4]; binary.seek(SeekFrom::Start(0)).unwrap(); binary.read(&mut buf).unwrap(); u32::from_le_bytes(buf) + *a_data }); generate_on_demand_macro!(c: u32 = None, { let a_data = on_demand_get_a!(); let b_data = on_demand_get_b!(); let mut buf = [0; 4]; binary.seek(SeekFrom::Start(0)).unwrap(); binary.read(&mut buf).unwrap(); u32::from_le_bytes(buf) + *a_data + *b_data }); let a_data = on_demand_get_a!(); assert_eq!(*a_data, 0x3020100); let b_data = on_demand_get_b_mut!(); assert_eq!(*b_data, 0x6040200); drop(b_data); // drop here since c will take ownership let c_data = on_demand_into_c!(); assert_eq!(c_data, 0xc080400); } }