removes offset > 0 branches from WeightedShuffle implementation

Changing offset definition to:

    let offset = (index - 1) & BIT_MASK

will avoid all the

    if offset > 0 { /* ... */ }

branches and massively simplifies the implementation.
It also improves performance in the benchmarks:

On master:

    test bench_weighted_shuffle_new     ... bench:      34,313.20 ns/iter (+/- 382.82)
    test bench_weighted_shuffle_shuffle ... bench:     139,443.76 ns/iter (+/- 3,076.36)

On the branch:

    test bench_weighted_shuffle_new     ... bench:      30,306.27 ns/iter (+/- 475.48)
    test bench_weighted_shuffle_shuffle ... bench:     134,762.74 ns/iter (+/- 1,003.62)

gossip/src/weighted_shuffle.rs

@@ -12,7 +12,10 @@ use {
 // Each internal tree node has FANOUT many child nodes with indices:
 //     (index << BIT_SHIFT) + 1 ..= (index << BIT_SHIFT) + FANOUT
 // Conversely, for each node, the parent node is obtained by:
-//     (index - 1) >> BIT_SHIFT
+//     parent: (index - 1) >> BIT_SHIFT
+// and the subtree weight is stored at
+//     offset: (index - 1) & BIT_MASK
+// of its parent node.
 const BIT_SHIFT: usize = 4;
 const FANOUT: usize = 1 << BIT_SHIFT;
 const BIT_MASK: usize = FANOUT - 1;
@@ -32,7 +35,7 @@ pub struct WeightedShuffle<T> {
     // Nodes without children are never accessed and don't need to be
     // allocated, so tree.len() < num_nodes.
     // tree[i][j] is the sum of all weights in the j'th sub-tree of node i.
-    tree: Vec<[T; FANOUT - 1]>,
+    tree: Vec<[T; FANOUT]>,
     // Current sum of all weights, excluding already sampled ones.
     weight: T,
     // Indices of zero weighted entries.
@@ -49,7 +52,7 @@ where
         let zero = <T as Default>::default();
         let (num_nodes, size) = get_num_nodes_and_tree_size(weights.len());
         debug_assert!(size <= num_nodes);
-        let mut tree = vec![[zero; FANOUT - 1]; size];
+        let mut tree = vec![[zero; FANOUT]; size];
         let mut sum = zero;
         let mut zeros = Vec::default();
         let mut num_negative: usize = 0;
@@ -78,11 +81,9 @@ where
             // updating the sub-tree sums along the way.
             let mut index = num_nodes + k; // leaf node
             while index != 0 {
-                let offset = index & BIT_MASK;
+                let offset = (index - 1) & BIT_MASK;
                 index = (index - 1) >> BIT_SHIFT; // parent node
-                if offset > 0 {
-                    tree[index][offset - 1] += weight;
-                }
+                tree[index][offset] += weight;
             }
         }
         if num_negative > 0 {
@@ -112,12 +113,10 @@ where
         // updating the sub-tree sums along the way.
         let mut index = self.num_nodes + k; // leaf node
         while index != 0 {
-            let offset = index & BIT_MASK;
+            let offset = (index - 1) & BIT_MASK;
             index = (index - 1) >> BIT_SHIFT; // parent node
-            if offset > 0 {
-                debug_assert!(self.tree[index][offset - 1] >= weight);
-                self.tree[index][offset - 1] -= weight;
-            }
+            debug_assert!(self.tree[index][offset] >= weight);
+            self.tree[index][offset] -= weight;
         }
     }
 
@@ -131,52 +130,32 @@ where
         // weight of the subtree which contains the target leaf node.
         let mut index = 0; // root
         let mut weight = self.weight;
-        'outer: while index < self.tree.len() {
-            for (j, &node) in self.tree[index].iter().enumerate() {
+        while let Some(tree) = self.tree.get(index) {
+            for (j, &node) in tree.iter().enumerate() {
                 if val < node {
                     // Traverse to the j'th subtree of self.tree[index].
                     weight = node;
                     index = (index << BIT_SHIFT) + j + 1;
-                    continue 'outer;
+                    break;
                 } else {
                     debug_assert!(weight >= node);
                     weight -= node;
                     val -= node;
                 }
             }
-            // Traverse to the right-most subtree of self.tree[index].
-            index = (index << BIT_SHIFT) + FANOUT;
         }
         (index - self.num_nodes, weight)
     }
 
     pub fn remove_index(&mut self, k: usize) {
-        // Traverse the tree from the leaf node upwards to the root, while
-        // maintaining the sum of weights of subtrees *not* containing the leaf
-        // node.
-        let mut index = self.num_nodes + k; // leaf node
-        let mut weight = <T as Default>::default(); // zero
-        while index != 0 {
-            let offset = index & BIT_MASK;
-            index = (index - 1) >> BIT_SHIFT; // parent node
-            if offset > 0 {
-                if self.tree[index][offset - 1] != weight {
-                    self.remove(k, self.tree[index][offset - 1] - weight);
-                } else {
-                    self.remove_zero(k);
-                }
-                return;
-            }
-            // The leaf node is in the right-most subtree of self.tree[index].
-            for &node in &self.tree[index] {
-                weight += node;
-            }
-        }
-        // The leaf node is the right-most node of the whole tree.
-        if self.weight != weight {
-            self.remove(k, self.weight - weight);
-        } else {
+        let zero = <T as Default>::default();
+        let index = self.num_nodes + k; // leaf node
+        let offset = (index - 1) & BIT_MASK;
+        let index = (index - 1) >> BIT_SHIFT; // parent node
+        if self.tree[index][offset] == zero {
             self.remove_zero(k);
+        } else {
+            self.remove(k, self.tree[index][offset]);
         }
     }