jcoglan · August 20, 2020 23:33 · Aug 20, 2020
diff --git a/combinations.rb b/combinations.rb
@@ -0,0 +1,11 @@
+def combos(items, k, s = 0, combo = [])
+  return [combo] if k == 0
+
+  (s .. items.size - k).flat_map do |i|
+    combos(items, k - 1, i + 1, combo + [items[i]])
+  end
+end
+
+combos([:a, :b, :c, :d, :e, :f], 3).each do |combo|
+  p combo
+end
diff --git a/combinations.rs b/combinations.rs
@@ -0,0 +1,29 @@
+use std::iter;
+
+fn combos<T>(items: &[T], k: usize) -> impl Iterator<Item = Vec<&T>> {
+    combo_helper(items, Vec::new(), k)
+}
+
+fn combo_helper<'a, T>(
+    items: &'a [T],
+    combo: Vec<&'a T>,
+    k: usize,
+) -> Box<dyn Iterator<Item = Vec<&'a T>> + 'a> {
+    if k == 0 {
+        return Box::new(iter::once(combo));
+    }
+
+    let iter = (0..=items.len() - k).flat_map(move |i| {
+        let mut cc = combo.clone();
+        cc.push(&items[i]);
+        combo_helper(&items[i + 1..], cc, k - 1)
+    });
+
+    Box::new(iter)
+}
+
+fn main() {
+    for combo in combos(&['a', 'b', 'c', 'd', 'e', 'f'], 3) {
+        println!("{:?}", combo);
+    }
+}
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