Iterative Segment Tree Range Sum in PHP

<?php
declare(strict_types=1);
// Iterative Segment Tree Range Sum in PHP
//
// PURPOSE
// A segment tree range query combines the small number of disjoint nodes covering a half-
// open interval.
//
// METHOD
// Move both endpoints to leaves. Include a left endpoint when it is a right child and a
// right endpoint when it is about to cross a left child, then move both up.
//
// COMPLEXITY
// O(log n) time, O(1) auxiliary space.
//
// CONTRACT
// tree has 2*n entries; n>=1, 0<=left<=right<=n. Returns sum in [left,right). Empty range
// returns zero.
// 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
// PHP 8.1+ on a 64-bit runtime: packed arrays, strict scalar types, read-only inputs by
// value using copy-on-write, references only for outputs, and checked 32-bit arithmetic.
// Invalid input and overflow throw exceptions. PHP arrays have higher overhead than native
// packed buffers in C/Java.
//
// EXAMPLE
// Expected return: 5.
// Run: php example.php
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

function alg_require(bool $valid): void { if (!$valid) throw new InvalidArgumentException("invalid algorithm input"); }
function alg_checked(int|float $v): int { if ($v < -2147483648 || $v > 2147483647 || floor($v) != $v) throw new OverflowException("32-bit result overflow"); return (int)$v; }
function alg_add(int $a,int $b): int { return alg_checked($a+$b); }
function alg_sub(int $a,int $b): int { return alg_checked($a-$b); }
function alg_mul(int $a,int $b): int { return alg_checked($a*$b); }
function alg_size(int $n): int { alg_require($n>=0 && $n<=4000000); return $n; }

function segment_sum(array $tree, int $left, int $right): int {
    alg_require(array_is_list($tree));
    $n = intdiv(count($tree), 2);
    alg_require((($n >= 0) && ($n <= 1000000)));
    alg_require((count($tree) == alg_mul(2, $n)));
    alg_require((($n >= 1) && ($left >= 0) && ($left <= $right) && ($right <= $n)));
    $left = alg_add($left, $n);
    $right = alg_add($right, $n);
    $total = 0;
    while (($left < $right)) {
        if ((($left % 2) == 1)) {
            $total = alg_add($total, $tree[$left]);
            $left = alg_add($left, 1);
        }
        if ((($right % 2) == 1)) {
            $right = alg_sub($right, 1);
            $total = alg_add($total, $tree[$right]);
        }
        $left = intdiv($left, 2);
        $right = intdiv($right, 2);
    }
    return $total;
}

$tree = [0, 10, 3, 7, 1, 2, 3, 4];
$left = 1;
$right = 3;
$result = segment_sum($tree, $left, $right);
echo $result, PHP_EOL;