feat: kata/the-millionth-fibonacci-kata

kata/3-kyu/index.md

@@ -1,3 +1,4 @@
 - [Battleship field validator](battleship-field-validator "52bb6539a4cf1b12d90005b7")
 - [Make a spiral](make-a-spiral "534e01fbbb17187c7e0000c6")
 - [Path Finder #3: the Alpinist](path-finder-number-3-the-alpinist "576986639772456f6f00030c")
+- [The Millionth Fibonacci Kata](the-millionth-fibonacci-kata "53d40c1e2f13e331fc000c26")

kata/3-kyu/the-millionth-fibonacci-kata/README.md

@@ -0,0 +1,37 @@
+# [The Millionth Fibonacci Kata](https://www.codewars.com/kata/the-millionth-fibonacci-kata "https://www.codewars.com/kata/53d40c1e2f13e331fc000c26")
+
+The year is 1214. One night, Pope Innocent III awakens to find the archangel Gabriel floating before him. Gabriel thunders to the pope:
+
+> Gather all the learned men in Pisa, especially Leonardo Fibonacci. In order for the crusades in the holy lands to be successful, these
+> men must calculate the *millionth* number in Fibonacci's recurrence. Fail to do this, and your armies will never reclaim the holy land. It
+> is His will.
+
+The angel then vanishes in an explosion of white light.
+
+Pope Innocent III sits in his bed in awe. *How much is a million?* he thinks to himself. He was never very good at math.
+
+He tries writing the number down, but because everyone in Europe is still using Roman numerals at this moment in history, he cannot
+represent this number. If he only knew about the invention of zero, it might make this sort of thing easier.
+
+He decides to go back to bed. He consoles himself, *The Lord would never challenge me thus; this must have been some deceit by the devil. A
+pretty horrendous nightmare, to be sure.*
+
+Pope Innocent III's armies would go on to conquer Constantinople (now Istanbul), but they would never reclaim the holy land as he desired.
+
+---------------------------
+
+In this kata you will have to calculate `fib(n)` where:
+
+	fib(0) := 0
+	fib(1) := 1
+	fib(n + 2) := fib(n + 1) + fib(n)
+
+Write an algorithm that can handle `n` up to `2000000`.
+
+Your algorithm must output the exact integer answer, to full precision. Also, it must correctly handle negative numbers as input.
+
+**HINT I**: Can you rearrange the equation `fib(n + 2) = fib(n + 1) + fib(n)` to find `fib(n)` if you already know `fib(n + 1)` and
+`fib(n + 2)`? Use this to reason what value `fib` has to have for negative values.
+
+**HINT II**:
+See https://web.archive.org/web/20220614001843/https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book-Z-H-11.html#%_sec_1.2.4

kata/3-kyu/the-millionth-fibonacci-kata/main/Fibonacci.java

@@ -0,0 +1,16 @@
+import static java.math.BigInteger.*;
+
+import java.math.BigInteger;
+
+interface Fibonacci {
+  static BigInteger fib(BigInteger n) {
+    BigInteger result = fib(ONE, ZERO, ZERO, ONE, n.abs());
+    return n.signum() < 0 && !n.testBit(0) ? result.negate() : result;
+  }
+
+  private static BigInteger fib(BigInteger a, BigInteger b, BigInteger p, BigInteger q, BigInteger n) {
+    return n.equals(ZERO) ? b :
+           !n.testBit(0) ? fib(a, b, p.pow(2).add(q.pow(2)), p.multiply(q.multiply(TWO)).add(q.pow(2)), n.divide(TWO)) :
+           fib(b.multiply(q).add(a.multiply(q)).add(a.multiply(p)), b.multiply(p).add(a.multiply(q)), p, q, n.subtract(ONE));
+  }
+}

kata/3-kyu/the-millionth-fibonacci-kata/test/FibonacciTest.java

@@ -0,0 +1,22 @@
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.math.BigInteger;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+class FibonacciTest {
+  @ParameterizedTest
+  @CsvSource(textBlock = """
+      0, 0
+      1, 1
+      2, 1
+      3, 2
+      4, 3
+      5, 5
+      -1, 1
+      -6, -8
+      """)
+  void sample(BigInteger input, BigInteger expected) {
+    assertEquals(expected, Fibonacci.fib(input));
+  }
+}