Iterative Fibonacci in Java

// Iterative Fibonacci in Java
//
// PURPOSE
// The Fibonacci recurrence needs only the previous two terms. Keeping those terms avoids
// both exponential recursion and a full table.
//
// METHOD
// Maintain F(i) and F(i+1), then advance the pair. Return F(n) after n advances.
//
// COMPLEXITY
// O(n) time and O(1) auxiliary space, under fixed-width arithmetic.
//
// CONTRACT
// n >= 0, with F(0)=0 and F(1)=1. Use n <= 45 for every intermediate value to fit signed
// 32-bit integers in this version.
// 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 fibonacci(int n) {
        alg_require(((n >= 0) && (n <= 1000000)));
        alg_require(((n >= 0) && (n <= 45)));
        int a = 0;
        int b = 1;
        int next_value = 0;
        int i = 0;
        while ((i < n)) {
            next_value = Math.addExact(a, b);
            a = b;
            b = next_value;
            i = Math.addExact(i, 1);
        }
        return a;
    }

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