rust持续学习声明宏

news2024/9/22 23:26:54

学习记录，都是学自圣经，macrobook啥的
https://doc.rust-lang.org/reference/macros-by-example.html

macro_rules! bar {
    (3) => {
        println!("3");
    };
    (4) => {
        println!("4");
    };
}

这个是入门例子，有点像match
调用就是bar!(xxxx)
bar!(3)会输出3这样

macro_rules! foo {
    ($input:tt) => {
        bar!($input);
    };
}
macro_rules! bar {
    (3) => {
        println!("3");
    };
    (4) => {
        println!("4");
    };
}

第二个例子，在第一个宏调用另外的宏，
这有个需要注意的地方，$input只有ident,tt,lifetime可以传递出去

然后还能正则表达式

macro_rules! vec_strs {
    (
        $(
            $element:expr
        )
        ,
        *
    ) => {
        {
            let mut v = Vec::new();

            $(
                v.push(format!("{}", $element));
            )*

            v
        }
    };
}

fn main() {
    let s = vec_strs![1, "a", true, 3.14159f32];
    assert_eq!(s, &["1", "a", "true", "3.14159"]);
}

这个就是匹配括号里的内容多次并替换为后面的括号里的逻辑，然后把东西push进去vec
复杂一点的斐波那契数列，为Recurrence实现一个iterator
返回Recurrence 的那里，mem匹配多次inits得到0,1，
next_val 求值那里ind，seq的类型要ident才能通过

macro_rules! count_exprs {
    () => (0);
    ($head:expr) => (1);
    ($head:expr, $($tail:expr),*) => (1 + count_exprs!($($tail),*));
}

macro_rules! recurrence {
    ( $seq:ident [ $ind:ident ]: $sty:ty = $($inits:expr),+ ; ... ; $recur:expr ) => {
        {
            use std::ops::Index;

            const MEM_SIZE: usize = count_exprs!($($inits),+);

            struct Recurrence {
                mem: [$sty; MEM_SIZE],
                pos: usize,
            }

            struct IndexOffset<'a> {
                slice: &'a [$sty; MEM_SIZE],
                offset: usize,
            }

            impl<'a> Index<usize> for IndexOffset<'a> {
                type Output = $sty;

                #[inline(always)]
                fn index<'b>(&'b self, index: usize) -> &'b $sty {
                    use std::num::Wrapping;

                    let index = Wrapping(index);
                    let offset = Wrapping(self.offset);
                    let window = Wrapping(MEM_SIZE);

                    let real_index = index - offset + window;
                    &self.slice[real_index.0]
                }
            }

            impl Iterator for Recurrence {
                type Item = $sty;

                #[inline]
                fn next(&mut self) -> Option<$sty> {
                    if self.pos < MEM_SIZE {
                        let next_val = self.mem[self.pos];
                        self.pos += 1;
                        Some(next_val)
                    } else {
                        let next_val = {
                            let $ind = self.pos;
                            let $seq = IndexOffset { slice: &self.mem, offset: $ind };
                            $recur
                        };

                        {
                            use std::mem::swap;

                            let mut swap_tmp = next_val;
                            for i in (0..MEM_SIZE).rev() {
                                swap(&mut swap_tmp, &mut self.mem[i]);
                            }
                        }

                        self.pos += 1;
                        Some(next_val)
                    }
                }
            }

            Recurrence { mem: [$($inits),+], pos: 0 }
        }
    };
}

fn main() {
    let fib = recurrence![a[n]: u64 = 0, 1; ...; a[n-2] + a[n-1]];

    for e in fib.take(10) { println!("{}", e) }
}