Merge #335

335: Powerset iterator adaptor r=jswrenn a=willcrozi

Implements a [powerset](https://en.wikipedia.org/wiki/Power_set) iterator adaptor that iterates over all subsets of the input iterator's elements. Returns vectors representing these subsets. Internally uses `Combinations` of increasing length.

I've taken the strategy of using a 'position' field that acts both as a means to to detect the special case of the first element and also allows optimal `size_hint()` implementation.

Additionally there is a commit to improve performance that alters `Combinations` implementation slightly. I've added Combinations benchmark as a stand-alone commit to allow checking for performance regressions. `Powerset` performance after this commit improves some cases (with small sizes of `n`) by 10-30%

This is my first attempt at a Rust contribution, happy to put in whatever discussion/work to get this merged. Cheers!

Co-authored-by: Will Crozier <willcrozi@gmail.com>

Author: bors[bot]

Cargo.toml

@@ -67,3 +67,11 @@ harness = false
 [[bench]]
 name = "bench1"
 harness = false
+
+[[bench]]
+name = "combinations"
+harness = false
+
+[[bench]]
+name = "powerset"
+harness = false

benches/combinations.rs

@@ -0,0 +1,125 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+// approximate 100_000 iterations for each combination
+const N1: usize = 100_000;
+const N2: usize = 448;
+const N3: usize = 86;
+const N4: usize = 41;
+const N14: usize = 21;
+
+fn comb_for1(c: &mut Criterion) {
+    c.bench_function("comb for1", move |b| {
+        b.iter(|| {
+            for i in 0..N1 {
+                black_box(vec![i]);
+            }
+        })
+    });
+}
+
+fn comb_for2(c: &mut Criterion) {
+    c.bench_function("comb for2", move |b| {
+        b.iter(|| {
+            for i in 0..N2 {
+                for j in (i + 1)..N2 {
+                    black_box(vec![i, j]);
+                }
+            }
+        })
+    });
+}
+
+fn comb_for3(c: &mut Criterion) {
+    c.bench_function("comb for3", move |b| {
+        b.iter(|| {
+            for i in 0..N3 {
+                for j in (i + 1)..N3 {
+                    for k in (j + 1)..N3 {
+                        black_box(vec![i, j, k]);
+                    }
+                }
+            }
+        })
+    });
+}
+
+fn comb_for4(c: &mut Criterion) {
+    c.bench_function("comb for4", move |b| {
+        b.iter(|| {
+            for i in 0..N4 {
+                for j in (i + 1)..N4 {
+                    for k in (j + 1)..N4 {
+                        for l in (k + 1)..N4 {
+                            black_box(vec![i, j, k, l]);
+                        }
+                    }
+                }
+            }
+        })
+    });
+}
+
+fn comb_c1(c: &mut Criterion) {
+    c.bench_function("comb c1", move |b| {
+        b.iter(|| {
+            for combo in (0..N1).combinations(1) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c2(c: &mut Criterion) {
+    c.bench_function("comb c2", move |b| {
+        b.iter(|| {
+            for combo in (0..N2).combinations(2) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c3(c: &mut Criterion) {
+    c.bench_function("comb c3", move |b| {
+        b.iter(|| {
+            for combo in (0..N3).combinations(3) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c4(c: &mut Criterion) {
+    c.bench_function("comb c4", move |b| {
+        b.iter(|| {
+            for combo in (0..N4).combinations(4) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+fn comb_c14(c: &mut Criterion) {
+    c.bench_function("comb c14", move |b| {
+        b.iter(|| {
+            for combo in (0..N14).combinations(14) {
+                black_box(combo);
+            }
+        })
+    });
+}
+
+criterion_group!(
+    benches,
+    comb_for1,
+    comb_for2,
+    comb_for3,
+    comb_for4,
+    comb_c1,
+    comb_c2,
+    comb_c3,
+    comb_c4,
+    comb_c14,
+);
+criterion_main!(benches);

benches/powerset.rs

@@ -0,0 +1,44 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+// Keep aggregate generated elements the same, regardless of powerset length.
+const TOTAL_ELEMENTS: usize = 1 << 12;
+const fn calc_iters(n: usize) -> usize {
+    TOTAL_ELEMENTS / (1 << n)
+}
+
+fn powerset_n(c: &mut Criterion, n: usize) {
+    let id = format!("powerset {}", n);
+    c.bench_function(id.as_str(), move |b| {
+        b.iter(|| {
+            for _ in 0..calc_iters(n) {
+                for elt in (0..n).powerset() {
+                    black_box(elt);
+                }
+            }
+        })
+    });
+}
+
+fn powerset_0(c: &mut Criterion) { powerset_n(c, 0); }
+
+fn powerset_1(c: &mut Criterion) { powerset_n(c, 1); }
+
+fn powerset_2(c: &mut Criterion) { powerset_n(c, 2); }
+
+fn powerset_4(c: &mut Criterion) { powerset_n(c, 4); }
+
+fn powerset_8(c: &mut Criterion) { powerset_n(c, 8); }
+
+fn powerset_12(c: &mut Criterion) { powerset_n(c, 12); }
+
+criterion_group!(
+    benches,
+    powerset_0,
+    powerset_1,
+    powerset_2,
+    powerset_4,
+    powerset_8,
+    powerset_12,
+);
+criterion_main!(benches);

benches/tuple_combinations.rs

@@ -7,8 +7,8 @@ const N2: usize = 448;
 const N3: usize = 86;
 const N4: usize = 41;
 
-fn comb_for1(c: &mut Criterion) {
-    c.bench_function("comb for1", move |b| {
+fn tuple_comb_for1(c: &mut Criterion) {
+    c.bench_function("tuple comb for1", move |b| {
         b.iter(|| {
             for i in 0..N1 {
                 black_box(i);
@@ -17,8 +17,8 @@ fn comb_for1(c: &mut Criterion) {
     });
 }
 
-fn comb_for2(c: &mut Criterion) {
-    c.bench_function("comb for2", move |b| {
+fn tuple_comb_for2(c: &mut Criterion) {
+    c.bench_function("tuple comb for2", move |b| {
         b.iter(|| {
             for i in 0..N2 {
                 for j in (i + 1)..N2 {
@@ -29,8 +29,8 @@ fn comb_for2(c: &mut Criterion) {
     });
 }
 
-fn comb_for3(c: &mut Criterion) {
-    c.bench_function("comb for3", move |b| {
+fn tuple_comb_for3(c: &mut Criterion) {
+    c.bench_function("tuple comb for3", move |b| {
         b.iter(|| {
             for i in 0..N3 {
                 for j in (i + 1)..N3 {
@@ -43,8 +43,8 @@ fn comb_for3(c: &mut Criterion) {
     });
 }
 
-fn comb_for4(c: &mut Criterion) {
-    c.bench_function("comb for4", move |b| {
+fn tuple_comb_for4(c: &mut Criterion) {
+    c.bench_function("tuple comb for4", move |b| {
         b.iter(|| {
             for i in 0..N4 {
                 for j in (i + 1)..N4 {
@@ -59,8 +59,8 @@ fn comb_for4(c: &mut Criterion) {
     });
 }
 
-fn comb_c1(c: &mut Criterion) {
-    c.bench_function("comb c1", move |b| {
+fn tuple_comb_c1(c: &mut Criterion) {
+    c.bench_function("tuple comb c1", move |b| {
         b.iter(|| {
             for (i,) in (0..N1).tuple_combinations() {
                 black_box(i);
@@ -69,8 +69,8 @@ fn comb_c1(c: &mut Criterion) {
     });
 }
 
-fn comb_c2(c: &mut Criterion) {
-    c.bench_function("comb c2", move |b| {
+fn tuple_comb_c2(c: &mut Criterion) {
+    c.bench_function("tuple comb c2", move |b| {
         b.iter(|| {
             for (i, j) in (0..N2).tuple_combinations() {
                 black_box(i + j);
@@ -79,8 +79,8 @@ fn comb_c2(c: &mut Criterion) {
     });
 }
 
-fn comb_c3(c: &mut Criterion) {
-    c.bench_function("comb c3", move |b| {
+fn tuple_comb_c3(c: &mut Criterion) {
+    c.bench_function("tuple comb c3", move |b| {
         b.iter(|| {
             for (i, j, k) in (0..N3).tuple_combinations() {
                 black_box(i + j + k);
@@ -89,8 +89,8 @@ fn comb_c3(c: &mut Criterion) {
     });
 }
 
-fn comb_c4(c: &mut Criterion) {
-    c.bench_function("comb c4", move |b| {
+fn tuple_comb_c4(c: &mut Criterion) {
+    c.bench_function("tuple comb c4", move |b| {
         b.iter(|| {
             for (i, j, k, l) in (0..N4).tuple_combinations() {
                 black_box(i + j + k + l);
@@ -101,13 +101,13 @@ fn comb_c4(c: &mut Criterion) {
 
 criterion_group!(
     benches,
-    comb_for1,
-    comb_for2,
-    comb_for3,
-    comb_for4,
-    comb_c1,
-    comb_c2,
-    comb_c3,
-    comb_c4,
+    tuple_comb_for1,
+    tuple_comb_for2,
+    tuple_comb_for3,
+    tuple_comb_for4,
+    tuple_comb_c1,
+    tuple_comb_c2,
+    tuple_comb_c3,
+    tuple_comb_c4,
 );
 criterion_main!(benches);

src/combinations.rs

@@ -31,13 +31,8 @@ impl<I> fmt::Debug for Combinations<I>
 pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
     where I: Iterator
 {
-    let mut pool: LazyBuffer<I> = LazyBuffer::new(iter);
-
-    for _ in 0..k {
-        if !pool.get_next() {
-            break;
-        }
-    }
+    let mut pool = LazyBuffer::new(iter);
+    pool.prefill(k);
 
     Combinations {
         indices: (0..k).collect(),
@@ -46,14 +41,53 @@ pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
     }
 }
 
+impl<I: Iterator> Combinations<I> {
+    /// Returns the length of a combination produced by this iterator.
+    #[inline]
+    pub fn k(&self) -> usize { self.indices.len() }
+
+    /// Returns the (current) length of the pool from which combination elements are
+    /// selected. This value can change between invocations of [`next`].
+    ///
+    /// [`next`]: #method.next
+    #[inline]
+    pub fn n(&self) -> usize { self.pool.len() }
+
+    /// Returns a reference to the source iterator.
+    #[inline]
+    pub(crate) fn src(&self) -> &I { &self.pool.it }
+
+    /// Resets this `Combinations` back to an initial state for combinations of length
+    /// `k` over the same pool data source. If `k` is larger than the current length
+    /// of the data pool an attempt is made to prefill the pool so that it holds `k`
+    /// elements.
+    pub(crate) fn reset(&mut self, k: usize) {
+        self.first = true;
+
+        if k < self.indices.len() {
+            self.indices.truncate(k);
+            for i in 0..k {
+                self.indices[i] = i;
+            }
+
+        } else {
+            for i in 0..self.indices.len() {
+                self.indices[i] = i;
+            }
+            self.indices.extend(self.indices.len()..k);
+            self.pool.prefill(k);
+        }
+    }
+}
+
 impl<I> Iterator for Combinations<I>
     where I: Iterator,
           I::Item: Clone
 {
     type Item = Vec<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
         if self.first {
-            if self.pool.is_done() {
+            if self.k() > self.n() {
                 return None;
             }
             self.first = false;

src/lazy_buffer.rs

@@ -24,10 +24,6 @@ where
         self.buffer.len()
     }
 
-    pub fn is_done(&self) -> bool {
-        self.done
-    }
-
     pub fn get_next(&mut self) -> bool {
         if self.done {
             return false;
@@ -44,6 +40,17 @@ where
             }
         }
     }
+
+    pub fn prefill(&mut self, len: usize) {
+        let buffer_len = self.buffer.len();
+
+        if !self.done && len > buffer_len {
+            let delta = len - buffer_len;
+
+            self.buffer.extend(self.it.by_ref().take(delta));
+            self.done = self.buffer.len() < len;
+        }
+    }
 }
 
 impl<I, J> Index<J> for LazyBuffer<I>