1 files changed, 277 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/permutations.rs b/third_party/rust/itertools/src/permutations.rs
new file mode 100644
index 0000000000..d03b852626
--- /dev/null
+++ b/third_party/rust/itertools/src/permutations.rs
@@ -0,0 +1,277 @@
+use alloc::vec::Vec;
+use std::fmt;
+use std::iter::once;
+
+use super::lazy_buffer::LazyBuffer;
+
+/// An iterator adaptor that iterates through all the `k`-permutations of the
+/// elements from an iterator.
+///
+/// See [`.permutations()`](crate::Itertools::permutations) for
+/// more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct Permutations<I: Iterator> {
+    vals: LazyBuffer<I>,
+    state: PermutationState,
+}
+
+impl<I> Clone for Permutations<I>
+    where I: Clone + Iterator,
+          I::Item: Clone,
+{
+    clone_fields!(vals, state);
+}
+
+#[derive(Clone, Debug)]
+enum PermutationState {
+    StartUnknownLen {
+        k: usize,
+    },
+    OngoingUnknownLen {
+        k: usize,
+        min_n: usize,
+    },
+    Complete(CompleteState),
+    Empty,
+}
+
+#[derive(Clone, Debug)]
+enum CompleteState {
+    Start {
+        n: usize,
+        k: usize,
+    },
+    Ongoing {
+        indices: Vec<usize>,
+        cycles: Vec<usize>,
+    }
+}
+
+enum CompleteStateRemaining {
+    Known(usize),
+    Overflow,
+}
+
+impl<I> fmt::Debug for Permutations<I>
+    where I: Iterator + fmt::Debug,
+          I::Item: fmt::Debug,
+{
+    debug_fmt_fields!(Permutations, vals, state);
+}
+
+pub fn permutations<I: Iterator>(iter: I, k: usize) -> Permutations<I> {
+    let mut vals = LazyBuffer::new(iter);
+
+    if k == 0 {
+        // Special case, yields single empty vec; `n` is irrelevant
+        let state = PermutationState::Complete(CompleteState::Start { n: 0, k: 0 });
+
+        return Permutations {
+            vals,
+            state
+        };
+    }
+
+    let mut enough_vals = true;
+
+    while vals.len() < k {
+        if !vals.get_next() {
+            enough_vals = false;
+            break;
+        }
+    }
+
+    let state = if enough_vals {
+        PermutationState::StartUnknownLen { k }
+    } else {
+        PermutationState::Empty
+    };
+
+    Permutations {
+        vals,
+        state
+    }
+}
+
+impl<I> Iterator for Permutations<I>
+where
+    I: Iterator,
+    I::Item: Clone
+{
+    type Item = Vec<I::Item>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.advance();
+
+        let &mut Permutations { ref vals, ref state } = self;
+
+        match *state {
+            PermutationState::StartUnknownLen { .. } => panic!("unexpected iterator state"),
+            PermutationState::OngoingUnknownLen { k, min_n } => {
+                let latest_idx = min_n - 1;
+                let indices = (0..(k - 1)).chain(once(latest_idx));
+
+                Some(indices.map(|i| vals[i].clone()).collect())
+            }
+            PermutationState::Complete(CompleteState::Ongoing { ref indices, ref cycles }) => {
+                let k = cycles.len();
+                Some(indices[0..k].iter().map(|&i| vals[i].clone()).collect())
+            },
+            PermutationState::Complete(CompleteState::Start { .. }) | PermutationState::Empty => None
+        }
+    }
+
+    fn count(self) -> usize {
+        fn from_complete(complete_state: CompleteState) -> usize {
+            match complete_state.remaining() {
+                CompleteStateRemaining::Known(count) => count,
+                CompleteStateRemaining::Overflow => {
+                    panic!("Iterator count greater than usize::MAX");
+                }
+            }
+        }
+
+        let Permutations { vals, state } = self;
+        match state {
+            PermutationState::StartUnknownLen { k } => {
+                let n = vals.len() + vals.it.count();
+                let complete_state = CompleteState::Start { n, k };
+
+                from_complete(complete_state)
+            }
+            PermutationState::OngoingUnknownLen { k, min_n } => {
+                let prev_iteration_count = min_n - k + 1;
+                let n = vals.len() + vals.it.count();
+                let complete_state = CompleteState::Start { n, k };
+
+                from_complete(complete_state) - prev_iteration_count
+            },
+            PermutationState::Complete(state) => from_complete(state),
+            PermutationState::Empty => 0
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self.state {
+            PermutationState::StartUnknownLen { .. } |
+            PermutationState::OngoingUnknownLen { .. } => (0, None), // TODO can we improve this lower bound?
+            PermutationState::Complete(ref state) => match state.remaining() {
+                CompleteStateRemaining::Known(count) => (count, Some(count)),
+                CompleteStateRemaining::Overflow => (::std::usize::MAX, None)
+            }
+            PermutationState::Empty => (0, Some(0))
+        }
+    }
+}
+
+impl<I> Permutations<I>
+where
+    I: Iterator,
+    I::Item: Clone
+{
+    fn advance(&mut self) {
+        let &mut Permutations { ref mut vals, ref mut state } = self;
+
+        *state = match *state {
+            PermutationState::StartUnknownLen { k } => {
+                PermutationState::OngoingUnknownLen { k, min_n: k }
+            }
+            PermutationState::OngoingUnknownLen { k, min_n } => {
+                if vals.get_next() {
+                    PermutationState::OngoingUnknownLen { k, min_n: min_n + 1 }
+                } else {
+                    let n = min_n;
+                    let prev_iteration_count = n - k + 1;
+                    let mut complete_state = CompleteState::Start { n, k };
+
+                    // Advance the complete-state iterator to the correct point
+                    for _ in 0..(prev_iteration_count + 1) {
+                        complete_state.advance();
+                    }
+
+                    PermutationState::Complete(complete_state)
+                }
+            }
+            PermutationState::Complete(ref mut state) => {
+                state.advance();
+
+                return;
+            }
+            PermutationState::Empty => { return; }
+        };
+    }
+}
+
+impl CompleteState {
+    fn advance(&mut self) {
+        *self = match *self {
+            CompleteState::Start { n, k } => {
+                let indices = (0..n).collect();
+                let cycles = ((n - k)..n).rev().collect();
+
+                CompleteState::Ongoing {
+                    cycles,
+                    indices
+                }
+            },
+            CompleteState::Ongoing { ref mut indices, ref mut cycles } => {
+                let n = indices.len();
+                let k = cycles.len();
+
+                for i in (0..k).rev() {
+                    if cycles[i] == 0 {
+                        cycles[i] = n - i - 1;
+
+                        let to_push = indices.remove(i);
+                        indices.push(to_push);
+                    } else {
+                        let swap_index = n - cycles[i];
+                        indices.swap(i, swap_index);
+
+                        cycles[i] -= 1;
+                        return;
+                    }
+                }
+
+                CompleteState::Start { n, k }
+            }
+        }
+    }
+
+    fn remaining(&self) -> CompleteStateRemaining {
+        use self::CompleteStateRemaining::{Known, Overflow};
+
+        match *self {
+            CompleteState::Start { n, k } => {
+                if n < k {
+                    return Known(0);
+                }
+
+                let count: Option<usize> = (n - k + 1..n + 1).fold(Some(1), |acc, i| {
+                    acc.and_then(|acc| acc.checked_mul(i))
+                });
+
+                match count {
+                    Some(count) => Known(count),
+                    None => Overflow
+                }
+            }
+            CompleteState::Ongoing { ref indices, ref cycles } => {
+                let mut count: usize = 0;
+
+                for (i, &c) in cycles.iter().enumerate() {
+                    let radix = indices.len() - i;
+                    let next_count = count.checked_mul(radix)
+                        .and_then(|count| count.checked_add(c));
+
+                    count = match next_count {
+                        Some(count) => count,
+                        None => { return Overflow; }
+                    };
+                }
+
+                Known(count)
+            }
+        }
+    }
+}