🚧 Implement .size_hint() (untested yet).

src/cartesian_power.rs

@@ -13,7 +13,7 @@ where
     I: Iterator,
     I::Item: Clone,
 {
-    pow: usize,
+    pow: u32,
     iter: Option<I>, // Inner iterator. Forget once consumed after 'base' iterations.
     items: Option<Vec<I::Item>>, // Fill from iter. Final length is 'base'.
     // None means that collection has not started yet.
@@ -30,7 +30,7 @@ where
 }
 
 /// Create a new `CartesianPower` from an iterator of clonables.
-pub fn cartesian_power<I>(iter: I, pow: usize) -> CartesianPower<I>
+pub fn cartesian_power<I>(iter: I, pow: u32) -> CartesianPower<I>
 where
     I: Iterator,
     I::Item: Clone,
@@ -62,14 +62,11 @@ where
             items,
             indices,
         } = self;
-        println!(
-            "^{pow}: {} {indices:?}\t\t{:?}",
-            if iter.is_some() { 'S' } else { 'N' },
-            items.as_ref().map(Vec::len)
-        );
+
+        let pow = *pow as usize;
 
         // (weird 'items @' bindings circumvent NLL limitations, unneeded with polonius)
-        match (*pow, iter, &mut *items) {
+        match (pow, iter, &mut *items) {
             // First iteration with degenerated 0th power.
             (0, Some(_), items @ None) => {
                 self.iter = None; // Forget about underlying iteration immediately.
@@ -197,7 +194,9 @@ where
             indices,
         } = self;
 
-        match (*pow, iter, &mut *items, n) {
+        let pow = *pow as usize;
+
+        match (pow, iter, &mut *items, n) {
             // First iteration with degenerated 0th power.
             (0, Some(_), items @ None, 0) => {
                 // Same as .next().
@@ -213,9 +212,7 @@ where
 
             // Subsequent degenerated 0th power iteration.
             // Same as `.next()`.
-            (0, None, items @ None, 0) => {
-                Some((indices, items.insert(Vec::new())))
-            }
+            (0, None, items @ None, 0) => Some((indices, items.insert(Vec::new()))),
             // Saturate.
             (0, None, items, _) => {
                 *items = None;
@@ -264,9 +261,7 @@ where
             }
 
             // Stable iteration in the degenerated case 'base = 0'.
-            (_, None, None, _) => {
-                None
-            }
+            (_, None, None, _) => None,
 
             // Subsequent iteration in the general case.
             // Again, immediate saturation is an option.
@@ -348,7 +343,77 @@ where
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        todo!()
+        let Self {
+            pow,
+            iter,
+            items,
+            indices,
+        } = self;
+
+        // The following case analysis matches implementation of `.next()`.
+        match (*pow, iter, items) {
+            // First iteration with degenerated 0th power.
+            (0, Some(_), None) => (1, Some(1)),
+
+            // Subsequent degenerated 0th power iteration.
+            // Alternating for cycling behaviour.
+            (0, None, Some(_)) => (0, Some(0)),
+            (0, None, None) => (1, Some(1)),
+
+            // First iteration in the general case.
+            (pow, Some(it), None) => {
+                let (a, b) = it.size_hint();
+                (
+                    a.checked_pow(pow).unwrap_or(usize::MAX),
+                    b.map(|b| b.checked_pow(pow)).flatten(),
+                )
+            }
+
+            // Stable iteration in the degenerated case 'base = 0'.
+            (_, None, None) => (0, Some(0)),
+
+            // Subsequent iteration in the general case.
+            (pow, Some(it), Some(items)) => {
+                let already = items.len();
+                let minus_already = |total| total - already;
+                let (a, b) = it.size_hint();
+                (
+                    (a + already)
+                        .checked_pow(pow)
+                        .map(minus_already)
+                        .unwrap_or(usize::MAX),
+                    b.map(|b| (b + already).checked_pow(pow).map(minus_already))
+                        .flatten(),
+                )
+            }
+
+            // Subsequent iteration in the general case after all items have been collected.
+            (pow, None, Some(items)) => {
+                let base = items.len();
+                if indices[0] == base {
+                    // Fresh re-start.
+                    let r = base.checked_pow(pow);
+                    return (r.unwrap_or(usize::MAX), r);
+                }
+                // Count what's left from current indices.
+                // This is subtracting the current iteration number base^pow,
+                // using the complement method.
+                let calc = || -> Option<usize> {
+                    // (closure-wrap to ease interruption on overflow with ?-operator)
+                    let comp = base - 1;
+                    let mut r = 1usize;
+                    for (&i, rank) in indices.iter().rev().zip(0u32..) {
+                        let increment = (comp - i).checked_pow(rank)?;
+                        r = r.checked_add(increment)?;
+                    }
+                    Some(r)
+                };
+                let Some(r) = calc() else {
+                    return (usize::MAX, None);
+                };
+                (r, Some(r))
+            }
+        }
     }
 
     fn count(self) -> usize {
@@ -378,19 +443,19 @@ where
     }
 }
 
+/// Use chars and string to ease testing of every yielded iterator values.
 #[cfg(test)]
 mod tests {
-    //! Use chars and string to ease testing of every yielded iterator values.
 
-    use super::CartesianPower;
     use crate::Itertools;
-    use core::str::Chars;
 
     #[test]
     fn basic() {
-        fn check(origin: &str, pow: usize, expected: &[&str]) {
+        fn check(origin: &str, pow: u32, expected: &[&str]) {
             println!("================== ({origin:?}^{pow})");
             let mut it_act = origin.chars().cartesian_power(pow);
+            // Check size_hint on the fly.
+            let e_hint = expected.len(); // HERE: do.
             // Check thrice that it's cycling.
             for r in 1..=3 {
                 println!("- - {r} - - - - - -");
@@ -473,7 +538,7 @@ mod tests {
 
     #[test]
     fn nth() {
-        fn check(origin: &str, pow: usize, expected: &[(usize, Option<&str>)]) {
+        fn check(origin: &str, pow: u32, expected: &[(usize, Option<&str>)]) {
             println!("================== ({origin:?}^{pow})");
             let mut it = origin.chars().cartesian_power(pow);
             let mut total_n = Vec::new();

src/lib.rs

@@ -1757,7 +1757,7 @@ pub trait Itertools: Iterator {
     /// TODO: illustrative example.
     /// ```
     #[cfg(feature = "use_alloc")]
-    fn cartesian_power(self, pow: usize) -> CartesianPower<Self>
+    fn cartesian_power(self, pow: u32) -> CartesianPower<Self>
     where
         Self: Sized,
         Self::Item: Clone,