Fenwick Point Add in Java

// Fenwick Point Add in Java
//
// PURPOSE
// A point addition updates only Fenwick partial sums containing that position.
//
// METHOD
// Convert the zero-based position to one-based, add delta, then repeatedly move to the
// parent by adding the lowest set bit.
//
// COMPLEXITY
// O(log n) time and O(1) auxiliary space.
//
// CONTRACT
// tree is an existing one-based Fenwick tree of length n+1. 0<=index<n. Mutates tree and
// returns 0. Sums must fit.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// Java 17+: primitive int arrays, Arrays.binarySearch for LIS, and Math exact arithmetic.
// Invalid input throws IllegalArgumentException; arithmetic overflow throws
// ArithmeticException.
//
// EXAMPLE
// Expected return: 0.
// Expected tree after call: [0, 1, 8, 3, 15].
// Run: javac AlgorithmExample.java && java AlgorithmExample
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

class AlgorithmExample {
    static void alg_require(boolean valid) { if (!valid) throw new IllegalArgumentException("invalid algorithm input"); }
    static int alg_size(int n) { alg_require(n>=0 && n<=4000000); return n; }

    static int fenwick_add(int[] tree, int index, int delta) {
        java.util.Objects.requireNonNull(tree, "tree");
        int n = Math.subtractExact(tree.length, 1);
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require((tree.length == Math.addExact(n, 1)));
        alg_require(((index >= 0) && (index < n)));
        index = Math.addExact(index, 1);
        while ((index <= n)) {
            tree[index] = Math.addExact(tree[index], delta);
            index = Math.addExact(index, (index & Math.negateExact(index)));
        }
        return 0;
    }

    public static void main(String[] args) {
        int[] tree = new int[] {0, 1, 3, 3, 10};
        int index = 1;
        int delta = 5;
        int result = fenwick_add(tree, index, delta);
        System.out.println(result);
        // Expected tree: [0, 1, 8, 3, 15]
        System.out.println(java.util.Arrays.toString(tree));
    }
}