Fenwick Tree (#6319)

* Enhance fenwick_tree.py

* Change update to add in fenwick_tree.py

* Some changes

* Fix bug

* Add O(N) initializer to FenwickTree

* Add get method to Fenwick Tree

* Change tree in Fenwick Tree

* Add rank query to FenwickTree

* Add get_array method to FenwickTree

* Add some tests

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update data_structures/binary_tree/fenwick_tree.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* change `List` to `list`

Co-authored-by: Christian Clauss <cclauss@me.com>

data_structures/binary_tree/fenwick_tree.py

@@ -1,28 +1,247 @@
+from copy import deepcopy
+
+
 class FenwickTree:
-    def __init__(self, SIZE):  # create fenwick tree with size SIZE
-        self.Size = SIZE
-        self.ft = [0 for i in range(0, SIZE)]
+    """
+    Fenwick Tree
 
-    def update(self, i, val):  # update data (adding) in index i in O(lg N)
-        while i < self.Size:
-            self.ft[i] += val
-            i += i & (-i)
+    More info: https://en.wikipedia.org/wiki/Fenwick_tree
+    """
 
-    def query(self, i):  # query cumulative data from index 0 to i in O(lg N)
-        ret = 0
-        while i > 0:
-            ret += self.ft[i]
-            i -= i & (-i)
-        return ret
+    def __init__(self, arr: list[int] = None, size: int = None) -> None:
+        """
+        Constructor for the Fenwick tree
+
+        Parameters:
+            arr (list): list of elements to initialize the tree with (optional)
+            size (int): size of the Fenwick tree (if arr is None)
+        """
+
+        if arr is None and size is not None:
+            self.size = size
+            self.tree = [0] * size
+        elif arr is not None:
+            self.init(arr)
+        else:
+            raise ValueError("Either arr or size must be specified")
+
+    def init(self, arr: list[int]) -> None:
+        """
+        Initialize the Fenwick tree with arr in O(N)
+
+        Parameters:
+            arr (list): list of elements to initialize the tree with
+
+        Returns:
+            None
+
+        >>> a = [1, 2, 3, 4, 5]
+        >>> f1 = FenwickTree(a)
+        >>> f2 = FenwickTree(size=len(a))
+        >>> for index, value in enumerate(a):
+        ...     f2.add(index, value)
+        >>> f1.tree == f2.tree
+        True
+        """
+        self.size = len(arr)
+        self.tree = deepcopy(arr)
+        for i in range(1, self.size):
+            j = self.next(i)
+            if j < self.size:
+                self.tree[j] += self.tree[i]
+
+    def get_array(self) -> list[int]:
+        """
+        Get the Normal Array of the Fenwick tree in O(N)
+
+        Returns:
+            list: Normal Array of the Fenwick tree
+
+        >>> a = [i for i in range(128)]
+        >>> f = FenwickTree(a)
+        >>> f.get_array() == a
+        True
+        """
+        arr = self.tree[:]
+        for i in range(self.size - 1, 0, -1):
+            j = self.next(i)
+            if j < self.size:
+                arr[j] -= arr[i]
+        return arr
+
+    @staticmethod
+    def next(index: int) -> int:
+        return index + (index & (-index))
+
+    @staticmethod
+    def prev(index: int) -> int:
+        return index - (index & (-index))
+
+    def add(self, index: int, value: int) -> None:
+        """
+        Add a value to index in O(lg N)
+
+        Parameters:
+            index (int): index to add value to
+            value (int): value to add to index
+
+        Returns:
+            None
+
+        >>> f = FenwickTree([1, 2, 3, 4, 5])
+        >>> f.add(0, 1)
+        >>> f.add(1, 2)
+        >>> f.add(2, 3)
+        >>> f.add(3, 4)
+        >>> f.add(4, 5)
+        >>> f.get_array()
+        [2, 4, 6, 8, 10]
+        """
+        if index == 0:
+            self.tree[0] += value
+            return
+        while index < self.size:
+            self.tree[index] += value
+            index = self.next(index)
+
+    def update(self, index: int, value: int) -> None:
+        """
+        Set the value of index in O(lg N)
+
+        Parameters:
+            index (int): index to set value to
+            value (int): value to set in index
+
+        Returns:
+            None
+
+        >>> f = FenwickTree([5, 4, 3, 2, 1])
+        >>> f.update(0, 1)
+        >>> f.update(1, 2)
+        >>> f.update(2, 3)
+        >>> f.update(3, 4)
+        >>> f.update(4, 5)
+        >>> f.get_array()
+        [1, 2, 3, 4, 5]
+        """
+        self.add(index, value - self.get(index))
+
+    def prefix(self, right: int) -> int:
+        """
+        Prefix sum of all elements in [0, right) in O(lg N)
+
+        Parameters:
+            right (int): right bound of the query (exclusive)
+
+        Returns:
+            int: sum of all elements in [0, right)
+
+        >>> a = [i for i in range(128)]
+        >>> f = FenwickTree(a)
+        >>> res = True
+        >>> for i in range(len(a)):
+        ...     res = res and f.prefix(i) == sum(a[:i])
+        >>> res
+        True
+        """
+        if right == 0:
+            return 0
+        result = self.tree[0]
+        right -= 1  # make right inclusive
+        while right > 0:
+            result += self.tree[right]
+            right = self.prev(right)
+        return result
+
+    def query(self, left: int, right: int) -> int:
+        """
+        Query the sum of all elements in [left, right) in O(lg N)
+
+        Parameters:
+            left (int): left bound of the query (inclusive)
+            right (int): right bound of the query (exclusive)
+
+        Returns:
+            int: sum of all elements in [left, right)
+
+        >>> a = [i for i in range(128)]
+        >>> f = FenwickTree(a)
+        >>> res = True
+        >>> for i in range(len(a)):
+        ...     for j in range(i + 1, len(a)):
+        ...         res = res and f.query(i, j) == sum(a[i:j])
+        >>> res
+        True
+        """
+        return self.prefix(right) - self.prefix(left)
+
+    def get(self, index: int) -> int:
+        """
+        Get value at index in O(lg N)
+
+        Parameters:
+            index (int): index to get the value
+
+        Returns:
+            int: Value of element at index
+
+        >>> a = [i for i in range(128)]
+        >>> f = FenwickTree(a)
+        >>> res = True
+        >>> for i in range(len(a)):
+        ...     res = res and f.get(i) == a[i]
+        >>> res
+        True
+        """
+        return self.query(index, index + 1)
+
+    def rank_query(self, value: int) -> int:
+        """
+        Find the largest index with prefix(i) <= value in O(lg N)
+        NOTE: Requires that all values are non-negative!
+
+        Parameters:
+            value (int): value to find the largest index of
+
+        Returns:
+            -1: if value is smaller than all elements in prefix sum
+            int: largest index with prefix(i) <= value
+
+        >>> f = FenwickTree([1, 2, 0, 3, 0, 5])
+        >>> f.rank_query(0)
+        -1
+        >>> f.rank_query(2)
+        0
+        >>> f.rank_query(1)
+        0
+        >>> f.rank_query(3)
+        2
+        >>> f.rank_query(5)
+        2
+        >>> f.rank_query(6)
+        4
+        >>> f.rank_query(11)
+        5
+        """
+        value -= self.tree[0]
+        if value < 0:
+            return -1
+
+        j = 1  # Largest power of 2 <= size
+        while j * 2 < self.size:
+            j *= 2
+
+        i = 0
+
+        while j > 0:
+            if i + j < self.size and self.tree[i + j] <= value:
+                value -= self.tree[i + j]
+                i += j
+            j //= 2
+        return i
 
 
 if __name__ == "__main__":
-    f = FenwickTree(100)
-    f.update(1, 20)
-    f.update(4, 4)
-    print(f.query(1))
-    print(f.query(3))
-    print(f.query(4))
-    f.update(2, -5)
-    print(f.query(1))
-    print(f.query(3))
+    import doctest
+
+    doctest.testmod()