function NativeCalculator::doSub

Performs the subtraction of two non-signed large integers.

1 call to NativeCalculator::doSub()

NativeCalculator::add in vendor/brick/math/src/Internal/Calculator/NativeCalculator.php

Adds two numbers.

File

vendor/brick/math/src/Internal/Calculator/NativeCalculator.php, line 324

Class

NativeCalculator

Calculator implementation using only native PHP code.

Namespace

Brick\Math\Internal\Calculator

Code

private function doSub(string $a, string $b) : string {
    if ($a === $b) {
        return '0';
    }
    // Ensure that we always subtract to a positive result: biggest minus smallest.
    $cmp = $this->doCmp($a, $b);
    $invert = $cmp === -1;
    if ($invert) {
        $c = $a;
        $a = $b;
        $b = $c;
    }
    [
        $a,
        $b,
        $length,
    ] = $this->pad($a, $b);
    $carry = 0;
    $result = '';
    $complement = 10 ** $this->maxDigits;
    for ($i = $length - $this->maxDigits;; $i -= $this->maxDigits) {
        $blockLength = $this->maxDigits;
        if ($i < 0) {
            $blockLength += $i;
            
            /** @psalm-suppress LoopInvalidation */
            $i = 0;
        }
        
        /** @psalm-var numeric-string $blockA */
        $blockA = \substr($a, $i, $blockLength);
        
        /** @psalm-var numeric-string $blockB */
        $blockB = \substr($b, $i, $blockLength);
        $sum = $blockA - $blockB - $carry;
        if ($sum < 0) {
            $sum += $complement;
            $carry = 1;
        }
        else {
            $carry = 0;
        }
        $sum = (string) $sum;
        $sumLength = \strlen($sum);
        if ($sumLength < $blockLength) {
            $sum = \str_repeat('0', $blockLength - $sumLength) . $sum;
        }
        $result = $sum . $result;
        if ($i === 0) {
            break;
        }
    }
    // Carry cannot be 1 when the loop ends, as a > b
    assert($carry === 0);
    $result = \ltrim($result, '0');
    if ($invert) {
        $result = $this->neg($result);
    }
    return $result;
}