Refactor algorithms structure.

src/algorithms/math/fibonacci/README.md

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+# Fibonacci Number
+
+In mathematics, the Fibonacci numbers are the numbers in the following 
+integer sequence, called the Fibonacci sequence, and characterized by 
+the fact that every number after the first two is the sum of the two 
+preceding ones:
+
+`0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...`

src/algorithms/math/fibonacci/__test__/fibonacci.test.js

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+import fibonacci from '../fibonacci';
+
+describe('fibonacci', () => {
+  it('should calculate fibonacci correctly', () => {
+    expect(fibonacci(1)).toBe(1);
+    expect(fibonacci(2)).toBe(1);
+    expect(fibonacci(3)).toBe(2);
+    expect(fibonacci(4)).toBe(3);
+    expect(fibonacci(5)).toBe(5);
+    expect(fibonacci(6)).toBe(8);
+    expect(fibonacci(7)).toBe(13);
+    expect(fibonacci(8)).toBe(21);
+    expect(fibonacci(20)).toBe(6765);
+  });
+});

src/algorithms/math/fibonacci/fibonacci.js

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+// Calculate fibonacci number at specific position using Dynamic Programming approach.
+export default function fibonacci(numberPosition) {
+  if (numberPosition === 1) {
+    return 1;
+  }
+
+  let iterationsCounter = numberPosition - 1;
+
+  // Calculated fibonacci number.
+  let fib = null;
+  // Previous fibonacci number.
+  let fibPrev = 1;
+  // Before previous fibonacci number.
+  let fibPrevPrev = 0;
+
+  while (iterationsCounter) {
+    // Calculate current value using two previous ones.
+    fib = fibPrev + fibPrevPrev;
+    // Shift previous values.
+    fibPrevPrev = fibPrev;
+    fibPrev = fib;
+    iterationsCounter -= 1;
+  }
+
+  return fib;
+}