divisors

nekolib-src/math/Cargo.toml

@@ -9,3 +9,4 @@ edition = "2021"
 inner = { path = "../inner" }
 modint = { path = "modint" }
 linear_sieve = { path = "linear_sieve" }
+divisors = { path = "divisors" }

nekolib-src/math/divisors/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "divisors"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

nekolib-src/math/divisors/src/lib.rs

@@ -0,0 +1,44 @@
+trait Divisors {
+    fn divisors(self) -> impl Iterator<Item = Self>;
+}
+
+macro_rules! impl_uint {
+    ( $($ty:ty)* ) => { $(
+        impl Divisors for $ty {
+            fn divisors(self) -> impl Iterator<Item = Self> {
+                let n = self;
+                std::iter::successors(
+                    (n >= 1).then_some((1, true)),
+                    move |&(i, asc)| {
+                        if asc {
+                            if let Some(j) = (i + 1..)
+                                .take_while(|j| j * j <= n)
+                                .find(|j| n % j == 0)
+                            {
+                                return Some((j, true));
+                            } else if n / i != i {
+                                return Some((i, false));
+                            }
+                        }
+                        let j = (1..i).rev().find(|&j| n % j == 0)?;
+                        Some((j, false))
+                    },
+                )
+                .map(move |(i, asc)| if asc { i } else { n / i })
+            }
+        }
+    )* };
+}
+
+impl_uint! { u8 u16 u32 u64 u128 usize }
+
+#[test]
+fn sanity_check() {
+    assert!(0_u32.divisors().eq(None));
+
+    for n in 1_u32..=10000 {
+        let expected = (1..=n).filter(|i| n % i == 0);
+        let actual = n.divisors();
+        assert!(actual.eq(expected));
+    }
+}

nekolib-src/math/src/lib.rs

@@ -1,6 +1,7 @@
 use inner::doc_inline_reexport;
 
 doc_inline_reexport! {
+    divisors,
     linear_sieve,
     modint,
 }