You may know how to get n-th Fibonacci number, but do you know what the fastest way to calculate it is?

The Fibonacci number is defined as:

$F_n = F_{n-1} + F_{n-2}$

where $F_0 = 0, F_1 = 1$.

It can be directly written into the following most common code when we learned what the recursion is:

///////////////////////////////////////////////////////////////////////////////
// Recursive: O(2^n)
uint64_t fibonacci(unsigned int n)
{
return (n <= 1) ? n : fibonacci(n-1) + fibonacci(n-2);
}


However, if you try calculating $F_{100}$, then you will wait a long long time to get the result since it has so many overlapping processes. For example, if we calculate $F_4$, then there are duplicated calculations(overlapping substructures) for $F_2$:

$\begin{matrix} & & & & & & & 4 & & & & & \\ & & & & & & \diagup & & \diagdown & & & & \\ & & & & & \diagup & & & & \diagdown & & & \\ & & & & 3 & & & & & & 2 & & \\ & & & \diagup & & \diagdown & & & & \diagup & & \diagdown & \\ & & 2 & & & & 1 & & 1 & & & & 0 \\ & \diagup & & \diagdown & & & & & & & & & \\ 1 & & & & 0 & & & & & & & & \end{matrix}$

The larger $n$ is, the more overlapping processes we have. As a result, the time-complexity is $O(2^n)$.

### Memoization

To avoid that, we can use a cache to save all the results and check it first before any calculation, so all the $F_k$ we need, for $k \in [0, n]$, will be computed just once. Therefore, the time complexity can be shorten to $O(n)$.

///////////////////////////////////////////////////////////////////////////////
// Recursive with memoization: O(n)
std::vector<uint64_t> mem = { 0, 1 }; // F(k) = mem[k], F(0) = 0, F(1) = 1.
uint64_t fibonacci(unsigned int n)
{
if (n + 1 > mem.size()) { // if n is not calculated yet
mem.push_back(fibonacci(n-1) + fibonacci(n-2));
}
return mem[n];
}


### Dynamic programming

The above implementation needs extra space to save the results, and pay time for memory allocation. If we iteratively calculate $F_n$ from $F_0, F_1$ to $F_2$, $F_3$, … then we can get $F_n$ without extra memory:

The above implementation needs extra space to save the results, and pay time for memory allocation. If we iteratively calculate $F_n$ from $F_0, F_1$ to $F_2$, $F_3$, …, to $F_{n-1}$, $F_n$ or $F_n$, $F_{n+1}$ then we can use only three or four variables to get $F_n$:

///////////////////////////////////////////////////////////////////////////////
// Dynamic programming: O(n)
uint64_t fibonacci(unsigned int n)
{
uint64_t a = 0, b = 1; // a = F(k), b = F(k+1), k = 0 now.
for (unsigned int k = 1 ; k <= n ; ++k) { // loop k from 1 to n.
std::swap(a, b); // a = F(k+1), b = F(k)
b += a; // b = F(k) + F(k+1) = F(k+2)
}
return a;
}


or

///////////////////////////////////////////////////////////////////////////////
// Dynamic programming: O(n)
uint64_t fibonacci(unsigned int n)
{
uint64_t a = 0, b = 1, sum = 0; // a = F(0), b = F(1)
for (unsigned int i = 1 ; i < n ; ++i) { // run if n >= 2
sum = a + b; // sum = F(i+1)
a = b;       // a = F(i)
b = sum;     // b = F(i+1)
}
// Now, i = n, sum = F(n), a = F(n-1), b = F(n)
return (n < 2) ? n : sum;
}


They also run in $O(n)$ with less memory consumption than memoization approach. Furthermore, they avoid the memory overhead for the activation records on the stack segment/space for the recursions. (The recursion will call itself multiple times, so it will push multiple activation records for the same function itself, with different arguments, into the stack segment/space of the process loading the program.)

### Closed-form

In fact, the Fibonacci number can be calculated by the following formula:

$F_n = \frac{1}{\sqrt{5}} \cdot [ (\frac{1 + \sqrt{5}}{2})^n - (\frac{1 - \sqrt{5}}{2})^n ]$

///////////////////////////////////////////////////////////////////////////////
// closed-form: O(log(n))
//   Theoretically, the power of n could be done in O(log(n)), but it's
//   complicated to calculate the floating numbers.
uint64_t fibonacci(unsigned int n)
{
// double sqrt5 = sqrt((double)5);
double sqrt5 = 2.2360679775;
return (pow((1 + sqrt5) / 2, n) - pow((1 - sqrt5) / 2, n)) / sqrt5;
}


Its time-complexity depends on how the power of $n$ is calculated. It could be done in $O(\log n)$ time(we will explain it below). However, the floating point operations limit the calculable number of $n$, and it might block the performance.

### Matrix Algebra

The Fibonacci numbers can be written into the following matrix:

$\vec{F_n} = \begin{bmatrix} F_n \\ F_{F - 1} \end{bmatrix} = \begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix} \cdot \begin{bmatrix} F_{n - 1} \\ F_{n - 2} \end{bmatrix}$

, so it could be easily expanded by the same rule:

\begin{align} \begin{bmatrix} F_{n+1} & F_n \\ F_n & F_{n - 1} \end{bmatrix} &= \begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix} \cdot \begin{bmatrix} F_n & F_{n - 1} \\ F_{n - 1} & F_{n - 2} \end{bmatrix} \\ &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^2 \cdot \begin{bmatrix} F_{n - 1} & F_{n - 2} \\ F_{n - 2} & F_{n - 3} \end{bmatrix} \\ \vdots \\ &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^{n - 1} \cdot \begin{bmatrix} F_2 & F_1 \\ F_1 & F_0 \end{bmatrix} \\ &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^{n - 1} \cdot \begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix} \\ &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^n \end{align}

That is, the Fibonacci matrix turns into a perfect power. Applying exponentiation by squaring :

$k^n = \begin{cases} (k^2)^\frac{n}{2}, & \text{if n is even} \\ k \cdot (k^2)^\frac{n-1}{2}, & \text{if n is odd} \end{cases}$

, we could implement the above idea to:

///////////////////////////////////////////////////////////////////////////////
// Power by matrix exponentiation: O(log(n))
// Matrix A:
//  <---  cols: n  --->
// +-                 -+
// | A11, A12, ... A1n |   ^
// | A21, A22, ... A2n |   |
// | ...               | rows: m
// | ...               |   |
// | Am1, Am2, ... Amn |   v
// +-                 -+
class Matrix
{
public:
Matrix(unsigned int r, unsigned int c,
std::vector<std::vector<uint64_t>> d)
: rows(r)
, cols(c)
, data(d)
{
}

Matrix(unsigned int r, unsigned int c)
: rows(r)
, cols(c)
{
assert(rows && cols);
data.resize(rows);
for (unsigned int i = 0 ; i < rows ; ++i) {
data[i].resize(cols);
}
}

~Matrix()
{
}

uint64_t Read(unsigned int r, unsigned int c)
{
return data[r][c];
}

// friend std::ostream& operator<<(std::ostream& os, const Matrix& m)
// {
//   for (unsigned int i = 0; i < m.rows; ++i) {
//     for (unsigned int j = 0; j < m.cols; ++j) {
//       os << m.data[i][j] << " ";
//     }
//     os << std::endl;
//   }
//   return os;
// }

Matrix operator*(const Matrix& other)
{
assert(cols == other.rows); // Check if they can be multiplied.

Matrix z(rows, other.cols);
for (unsigned int i = 0 ; i < rows ; ++i) {
for (unsigned int j = 0 ; j < other.cols; ++j) {
for (unsigned int k = 0 ; k < cols; ++k) {
z.data[i][j] += data[i][k] * other.data[k][j];
}
}
}

return z;
}

// Calculate the power by fast doubling:
//   k ^ n = (k^2) ^ (n/2)          , if n is even
//        or k * (k^2) ^ ((k-1)/2)  , if n is odd
Matrix pow(unsigned int n)
{
Matrix k(*this); // Copy constructor = Matrix x(rows, cols, data);
Matrix r = Identity(rows);
while (n) {
if (/*n % 2*/n & 1) {
r = r * k;
}
k = k * k;
/*n /= 2*/n >>= 1;
}
return r;
}

private:
Matrix Identity(unsigned int size)
{
Matrix z(size, size);
for (unsigned int i = 0 ; i < size ; ++i) {
z.data[i][i] = 1;
}
return z;
}

unsigned int rows;
unsigned int cols;
std::vector<std::vector<uint64_t>> data;
};

// The Fibonacci matrix can be written into the following equation:
// +-             -+   +-    -+^n
// | F(n+1)   F(n) |   | 1  1 |
// |               | = |      |
// | F(n)   F(n-1) |   | 1  0 |
// +-             -+   +-    -+
uint64_t fibonacci(unsigned int n)
{
Matrix F { 2, 2, {
{ 1, 1 },
{ 1, 0 }
} };

// Using F.data[0][1] since n might be 0.
// (we need to power by n - 1 if we return F.data[0][0].)
F = F.pow(n);
}


Its time-complexity is $O(\log n)$ by halving and halving. Without the floating point operations, the $n$ could be larger than using the closed-form approach.

To make it faster, you can use native array instead of std::vector, but you need to manage the memory usage by yourself. Please read this post to know how to do it.

### Fast doubling

The following equations:

\begin{align} F_{2n+1} &= {F_{n+1}}^2 + {F_n}^2 \\ F_{2n} &= F_n \cdot (F_{n+1} + F_{n-1}) \\ &= F_n \cdot (F_{n+1} + (F_{n+1} - F_n)) \\ &= F_n \cdot (2 \cdot F_{n+1} - F_n) \end{align}

can be derived by applying $2n$ to the above Fibonacci matrix:

\begin{align} \begin{bmatrix} F_{2n+1} & F_{2n} \\ F_{2n} & F_{2n - 1} \end{bmatrix} &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^{2n} \\ &= {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^n \cdot {\begin{bmatrix} 1 & 1 \\ 1 & 0 \end{bmatrix}}^n \\ &= \begin{bmatrix} F_{n+1} & F_n \\ F_n & F_{n - 1} \end{bmatrix} \cdot \begin{bmatrix} F_{n+1} & F_n \\ F_n & F_{n - 1} \end{bmatrix} \\ &= \begin{bmatrix} {F_{n+1}}^2 + {F_n}^2 & F_n \cdot (F_{n+1} + F_{n-1}) \\ F_n \cdot (F_{n+1} + F_{n-1}) & {F_n}^2 + {F_{n-1}}^2 \end{bmatrix} \end{align}

Hence, we could calculate $F_n$ by:

$F_n = \begin{cases} F_{2n'}, & \text{if n is even} \\ F_{2n'+1}, & \text{if n is odd} \end{cases}$

As a consequence, we could use $F_{n'}, F_{n' + 1}$ to compute $F_n$ by the following program: (Please read this post to know how the code is derived.)

uint64_t fibonacci(unsigned int n)
{
// The position of the highest bit of n.
// So we need to loop h times to get the answer.
// Example: n = (Dec)50 = (Bin)00110010, then h = 6.
//                               ^ 6th bit from right side
unsigned int h = 0;
for (unsigned int i = n ; i ; ++h, i >>= 1);

uint64_t a = 0; // F(0) = 0
uint64_t b = 1; // F(1) = 1
// There is only one 1 in the bits of mask. The 1's position is same as
// the highest bit of n(mask = 2^(h-1) at first), and it will be shifted right
// iteratively to do AND operation with n to check n / 2^j is odd
// or even.
for (unsigned int mask = 1 << (h - 1) ; mask ; mask >>= 1) { // Run h times!
// Let j = h-i (looping from i = 1 to i = h),
// n_j = floor(n / 2^j) = n >> j (n_j = n when j = 0), k = floor(n_j / 2),
// then a = F(k), b = F(k+1) now.
uint64_t c = a * (2 * b - a); // F(2k) = F(k) * [ 2 * F(k+1) – F(k) ]
uint64_t d = a * a + b * b;   // F(2k+1) = F(k)^2 + F(k+1)^2

if (mask & n) { // n_j is odd: k = (n_j-1)/2 => n_j = 2k + 1
a = d;        //   F(n_j) = F(2k + 1)
b = c + d;    //   F(n_j + 1) = F(2k + 2) = F(2k) + F(2k + 1)
} else {        // n_j is even: k = n_j/2 => n_j = 2k
a = c;        //   F(n_j) = F(2k)
b = d;        //   F(n_j + 1) = F(2k + 1)
}
}

return a;
}


Its time-complexity is also $O(\log n)$ by halving and halving. In contrast to matrix algebra approach, there is no need for using matrix that contains the duplicated $F_k$, so it will be faster.

### Performance

Approach $F_{45}$ $F_{13100}$ $F_{13500}$ $F_{29108}$
Recursive 7440.61
Memoization 0.034841 3.03045 3.05931 6.10806
Dynamic programming 0.000508 0.052462 0.05395 0.1069
Closed-form 0.030075
Matrix Algebra 0.02013 0.052985 0.052427 0.050423
Fast doubling 0.000446 0.000737 0.000785 0.000724

The above results are the time in millisecond for calculating $F_n$. It will take too long time to get the results from the recursive approach, so we skip it. The closed-form approach is also ignored since the floating point operations only work when $n \leq 97$ in above implementation.

### Conclusion

Although the performance is platform-dependent, it still indicates that:

• The fast doubling approach is always the fastest way and its performance is far far better than others.
• The dynamic programming approach is faster than matrix algebra one when $n$ is small ($n \leq 13000$ here), but slower when $n$ is large.
• Therefore, if you are pretty sure you have a small $n$, and the bottleneck of your algorithm doesn’t depend on the Fibonacci calculation, then dynamic programming is acceptable and it’s easier to implement.

This post is the end of my journey for the Fibonacci calculation. Hope you enjoyed. All the above code are uploaded to gist here. Please clone them to play with it.

I will start another journey for other interesting topics soon. Stay tuned!