posts: generic-flyweight: fix typo

content/posts/2020-07-16-generic-flyweight-cpp/index.md

@@ -16,7 +16,7 @@ favorite: false
 The flyweight is a well-known
 [GoF](https://en.wikipedia.org/wiki/Design_Patterns) design pattern.
 
-It's intent is to minimize memory usage by reducing the number of instantiations
+Its intent is to minimize memory usage by reducing the number of instantiations
 of a given object.
 
 I will show you how to implement a robust flyweight in C++, as well as a way to

content/posts/2020-07-22-polymorphic-flyweight-cpp/index.md

@@ -68,7 +68,7 @@ public:
         const std::type_index lhs_i(lhs);
         const std::type_index rhs_i(rhs);
         if (lhs_i != rhs_i)
-            returh lhs_i < rhs_i;
+            return lhs_i < rhs_i;
         // We are now assured that both classes have the same type
         return less_than(rhs);
     }

content/posts/2024-06-24-union-find/index.md

@@ -78,3 +78,77 @@ each element to be a root and make it its own parent (`_parent[i] == i` for all
 `i`).
 
 The `_rank` field is an optimization which we will touch on in a later section.
+
+### Find
+
+A naive Implementation of `find(...)` is simple enough to write:
+
+```python
+def find(self, elem: int) -> int:
+    # If `elem` is its own parent, then it is the root of the tree
+    if (parent := self._parent[elem]) == elem:
+        return elem
+    # Otherwise, recurse on the parent
+    return self.find(parent)
+```
+
+However, going back up the chain of parents each time we want to find the root
+node (an `O(n)` operation) would make for disastrous performance. Instead we can
+do a small optimization called _path splitting_.
+
+```python
+def find(self, elem: int) -> int:
+    while (parent := self._parent[elem]) != elem:
+        # Replace each parent link by a link to the grand-parent
+        elem, self._parent[elem] = parent, self._parent[parent]
+    return elem
+```
+
+This flattens the chain so that each node links more directly to the root (the
+length is reduced by half), making each subsequent `find(...)` faster.
+
+Other compression schemes exist, along the spectrum between faster shortening
+the chain faster earlier, or updating `_parent` fewer times per `find(...)`.
+
+### Union
+
+A naive implementation of `union(...)` is simple enough to write:
+
+```python
+def union(self, lhs: int, rhs: int) -> int:
+    # Replace both element by their root parent
+    lhs = self.find(lhs)
+    rhs = self.find(rhs)
+    # arbitrarily merge one into the other
+    self._parent[rhs] = lhs
+    # Return the new root
+    return lhs
+```
+
+Once again, improvements can be made. Depending on the order in which we call
+`union(...)`, we might end up creating a long chain from the leaf of the tree to
+the root node, leading to slower `find(...)` operations. If at all possible, we
+would like to keep the trees as shallow as possible.
+
+To do so, we want to avoid merging taller trees into smaller ones, so as to keep
+them as balanced as possible. Since a higher tree will result in a slower
+`find(...)`, keeping the trees balanced will lead to increased performance.
+
+This is where the `_rank` field we mentioned earlier comes in: the _rank_ of an
+element is an upper bound on its height in the tree. By keeping track of this
+_approximate_ height, we can keep the trees balanced when merging them.
+
+```python
+def union(self, lhs: int, rhs: int) -> int:
+    lhs = self.find(lhs)
+    rhs = self.find(rhs)
+    # Always keep `lhs` as the taller tree
+    if (self._rank[lhs] < self._rank[rhs])
+        lhs, rhs = rhs, lhs
+    # Merge the smaller tree into the taller one
+    self._parent[rhs] = lhs
+    # Update the rank when merging trees of approximately the same size
+    if self._rank[lhs] == self._rank[rhs]:
+        self._rank[lhs] += 1
+    return lhs
+```