math: add fib, fib_seq functions (#34)

* math: add fib, fib_seq functions

* update: directory

* update: remove dot

DIRECTORY.md

@@ -8,6 +8,7 @@
   - [Median](math/median.jule)
   - [Min](math/min.jule)
   - [Sum](math/sum.jule)
+  - [Fib](math/fib.jule)
 
 ## Search
   - [Binary Search](search/binary_search.jule)

math/fib.jule

@@ -0,0 +1,21 @@
+// O(2^n)
+pub fn fib(i: int): int {
+    if i == 0 || i == 1 {
+        ret i
+    }
+
+    ret fib(i - 1) + fib(i - 2)
+}
+
+// O(n * 2^n)
+pub fn fib_seq(mut n: int): []int {
+    let mut s: []int
+
+    let mut i = n
+    for i > 0 {
+        s = append(s, fib(n - i + 1))
+        i--
+    }
+
+    ret s
+}

math/fib_test.jule

@@ -0,0 +1,9 @@
+#build test
+
+use std::testing::{T}
+
+#test
+fn test_fib(t: &T) {
+    t.assert(fib(6) == 8, "index 6 of fib. should be 8")
+    t.assert(fib_seq(6) == [1, 1, 2, 3, 5, 8], "a fib. sequence up to index 6 should be 1, 1, 2, 3, 5, 8")
+}