Brian Kernighan Population Count in Java

// Brian Kernighan Population Count in Java
//
// PURPOSE
// The expression n AND (n-1) clears the lowest set bit. Repeating it counts the ones in a
// nonnegative integer without visiting zero bits individually.
//
// METHOD
// Clear one set bit per iteration and increment the counter. The loop stops exactly when all
// original set bits have been removed.
//
// COMPLEXITY
// O(k) iterations for k set bits and O(1) auxiliary space under fixed-width arithmetic.
//
// CONTRACT
// n must be nonnegative. Negative integers have different width/sign behavior between
// languages and are excluded.
// 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.
// 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 population_count(int n) {
        alg_require((n >= 0));
        int count = 0;
        while ((n != 0)) {
            n = (n & Math.subtractExact(n, 1));
            count = Math.addExact(count, 1);
        }
        return count;
    }

    public static void main(String[] args) {
        int n = 0;
        int result = population_count(n);
        System.out.println(result);
    }
}