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<?php |
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declare(strict_types=1); |
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namespace Nyholm\EffectiveInterest; |
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/** |
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* @author Tobias Nyholm <[email protected]> |
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*/ |
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final class Calculator |
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{ |
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/** |
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* @var NewtonRaphson |
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*/ |
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private $newton; |
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/** |
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* @param NewtonRaphson $newton |
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*/ |
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public function __construct(NewtonRaphson $newton = null) |
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{ |
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$this->newton = $newton ?? new NewtonRaphson(); |
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} |
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/** |
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* Get the interest when you know all the payments and their dates. Use this function when you have |
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* administration fees at the first payment and/or when payments are irregular. |
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* |
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* @param int $principal |
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* @param string $startDate in format 'YYYY-mm-dd' |
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* @param array $payments array with payment dates and values ['YYYY-mm-dd'=>int] |
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* @param float $guess A guess what the interest may be. Between zero and one. Example 0.045 |
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* |
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* @return float |
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*/ |
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public function withSpecifiedPayments(int $principal, string $startDate, array $payments, float $guess): float |
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{ |
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list($values, $days) = $this->preparePayments($principal, $startDate, $payments); |
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$fx = function ($x) use ($days, $values) { |
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$sum = 0; |
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foreach ($days as $idx => $day) { |
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$sum += $values[$idx] * pow(1 + $x, ($days[0] - $day) / 365); |
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} |
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return $sum; |
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}; |
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$fdx = function ($x) use ($days, $values) { |
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$sum = 0; |
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foreach ($days as $idx => $day) { |
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$sum += (1 / 365) * ($days[0] - $day) * $values[$idx] * pow(1 + $x, (($days[0] - $day) / 365) - 1); |
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} |
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return $sum; |
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}; |
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return $this->newton->run($fx, $fdx, $guess); |
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} |
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/** |
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* Get the effective interest when the monthly payments are exactly the same. |
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* |
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* @param int $principal The total loan amount (Principal) |
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* @param int $payment The monthly payment |
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* @param int $numberOfMonths The number of months |
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* @param float $guess A guess of what the interest might be. Interest as a number between zero and one. Example 0.045 |
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* |
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* @return float |
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*/ |
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public function withEqualPayments(int $principal, int $payment, int $numberOfMonths, float $guess): float |
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{ |
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View Code Duplication |
$fx = function ($x) use ($principal, $payment, $numberOfMonths) { |
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return $payment - $payment * pow(1 + $x, -1 * $numberOfMonths) - $x * $principal; |
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}; |
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View Code Duplication |
$fdx = function ($x) use ($principal, $payment, $numberOfMonths) { |
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return $numberOfMonths * $payment * pow(1 + $x, -1 * $numberOfMonths - 1) - $principal; |
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}; |
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return 12 * $this->newton->run($fx, $fdx, $guess); |
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} |
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/** |
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* Prepare payment data by separating dates from values and prefix the array with the principal. |
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* |
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* @param int $principal |
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* @param string $startDate |
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* @param array $payments |
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* |
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* @return array |
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*/ |
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private function preparePayments(int $principal, string $startDate, array $payments): array |
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{ |
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$values = [-1 * $principal]; |
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$days = [1]; |
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$startDate = new \DateTimeImmutable($startDate); |
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foreach ($payments as $date => $payment) { |
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$values[] = $payment; |
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$days[] = 1 + $startDate->diff(new \DateTime($date))->days; |
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} |
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return [$values, $days]; |
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} |
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} |
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Nyholm /
effective-interest-rate
