mask example

 {
        /** @var array<int, ParagonIE_Sodium_Core32_Int32> $h */
        $h = array();
        for ($i = 0; $i < 10; ++$i) {
            if (!($f[$i] instanceof ParagonIE_Sodium_Core32_Int32)) {
                throw new TypeError('Expected Int32');
            }
            if (!($g[$i] instanceof ParagonIE_Sodium_Core32_Int32)) {
                throw new TypeError('Expected Int32');
            }
            $h[$i] = $f[$i]->xorInt32(
                $f[$i]->xorInt32($g[$i])->mask($b)
            );
        }
        /** @var array<int, ParagonIE_Sodium_Core32_Int32> $h */
        return ParagonIE_Sodium_Core32_Curve25519_Fe::fromArray($h);
    }

    /** * Create a copy of a field element. * * @internal You should not use this directly from another application * * @param ParagonIE_Sodium_Core32_Curve25519_Fe $f * @return ParagonIE_Sodium_Core32_Curve25519_Fe */

        if (self::strlen($key) < 32) {
            throw new InvalidArgumentException(
                'Poly1305 requires a 32-byte key'
            );
        }
        /* r &= 0xffffffc0ffffffc0ffffffc0fffffff */
        $this->r = array(
            // st->r[0] = ...             ParagonIE_Sodium_Core32_Int32::fromReverseString(self::substr($key, 0, 4))
                ->setUnsignedInt(true)
                ->mask(0x3ffffff),
            // st->r[1] = ...             ParagonIE_Sodium_Core32_Int32::fromReverseString(self::substr($key, 3, 4))
                ->setUnsignedInt(true)
                ->shiftRight(2)
                ->mask(0x3ffff03),
            // st->r[2] = ...             ParagonIE_Sodium_Core32_Int32::fromReverseString(self::substr($key, 6, 4))
                ->setUnsignedInt(true)
                ->shiftRight(4)
                ->mask(0x3ffc0ff),
            // st->r[3] = ...