Add core::fibbonacci::fibbonacci()

src/core/fibbonacci.rs

@@ -0,0 +1,29 @@
+pub fn fibbonacci(index: usize) -> usize {
+    if index == 0 {
+        0
+    } else if index < 3 {
+        1
+    } else {
+        let mut values = vec![1;index+1];
+        for i in 3..index+1 {
+            values[i] = values[i - 1] + values[i - 2];
+        }
+        values[index]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_first_few_values() {
+        let expected_values = vec![
+            0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181,
+            6765, 10946,
+        ];
+        for (i, &n) in expected_values.iter().enumerate() {
+            assert_eq!(fibbonacci(i), n);
+        }
+    }
+}

src/core/mod.rs

@@ -1 +1,3 @@
 pub mod rope;
+
+mod fibbonacci;