More factorial/fibonacci optimizing

benches/stock_functions.rs

@@ -1,16 +1,16 @@
 use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion};
-use rusty_numbers::{factorial, fibonacci, mem_fibonacci, mem_factorial};
+use rusty_numbers::{factorial, fibonacci, it_factorial, mem_fibonacci, rec_fibonacci};
 
 fn bench_factorial(c: &mut Criterion) {
     let mut group = c.benchmark_group("Factorials");
 
     for i in [0usize, 5, 10, 15, 20, 25, 30, 34].iter() {
-        group.bench_with_input(BenchmarkId::new("Recursive memoized", i), i, |b, i| {
-            b.iter(|| mem_factorial(*i))
-        });
         group.bench_with_input(BenchmarkId::new("Recursive naive", i), i, |b, i| {
             b.iter(|| factorial(*i))
         });
+        group.bench_with_input(BenchmarkId::new("Iterative", i), i, |b, i| {
+            b.iter(|| it_factorial(*i))
+        });
     }
     group.finish();
 }
@@ -27,6 +27,16 @@ fn bench_fibonacci(c: &mut Criterion) {
             b.iter(|| fibonacci(*i))
         });
     }
+
+    group.finish();
+
+    let mut group = c.benchmark_group("Recursive Fibonacci");
+    for i in [0usize, 5, 10, 15, 20, 25, 26, 27, 28, 29, 30].iter() {
+        group.bench_with_input(BenchmarkId::new("Naive Recursive", i), i, |b, i| {
+            b.iter(|| rec_fibonacci(*i))
+        });
+    }
+    group.finish();
 }
 
 criterion_group!(benches, bench_factorial, bench_fibonacci);

src/lib.rs

@@ -37,6 +37,11 @@ pub fn mem_fibonacci(n: usize) -> Option<u128> {
 /// Actual calculating function for `fibonacci`
 #[inline]
 fn _mem_fibonacci(n: usize, table: &mut [u128]) -> Option<u128> {
+    if n < 2 {
+        // The first values are predefined.
+        return Some(table[n]);
+    }
+
     if n > 186 {
         return None;
     }
@@ -51,8 +56,8 @@ fn _mem_fibonacci(n: usize, table: &mut [u128]) -> Option<u128> {
             table[n] = a + b;
 
             Some(table[n])
-        },
-        x => Some(x)
+        }
+        x => Some(x),
     }
 }
 
@@ -101,8 +106,7 @@ pub fn fibonacci(n: usize) -> Option<u128> {
     }
 }
 
-/// Calculate the value of a factorial,
-/// using a lookup table for better worst-case performance.
+/// Calculate the value of a factorial iteratively
 ///
 /// Can calculate up to 34! using native unsigned 128 bit integers.
 ///
@@ -117,40 +121,22 @@ pub fn fibonacci(n: usize) -> Option<u128> {
 /// # assert!(invalid.is_none());
 /// ```
 #[inline]
-pub fn mem_factorial(n: usize) -> Option<u128> {
-    let mut table = [0u128; 35];
-    table[0] = 1;
-    table[1] = 1;
-    table[2] = 2;
+pub fn it_factorial(n: usize) -> Option<u128> {
+    let mut total: u128 = 1;
 
+    match n {
+        0 | 1 => Some(1u128),
+        _ => {
+            for x in 1..=n {
+                total = total.checked_mul(x as u128)?;
+            }
 
-    _mem_factorial(n, &mut table)
-}
-
-/// Actual Calculation function for factoral
-#[inline]
-fn _mem_factorial(n: usize, table: &mut [u128]) -> Option<u128> {
-    if n > 34 {
-        return None
-    }
-
-    match table[n] {
-        // Empty spaces in the lookup array are zero-filled
-        0 => {
-            // The ? suffix passes along the Option value
-            // to be handled later
-            // See: https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator
-            let prev = _mem_factorial(n - 1, table)?;
-
-            table[n] = (n as u128) * prev;
-
-            Some(table[n])
-        },
-        x => Some(x),
+            Some(total)
+        }
     }
 }
 
-/// Calculate the value of a factorial,
+/// Calculate the value of a factorial recrursively
 ///
 /// Can calculate up to 34! using native unsigned 128 bit integers.
 ///
@@ -184,36 +170,71 @@ mod tests {
 
     #[test]
     fn test_factorial() {
-        assert_eq!(1, factorial(0).unwrap());
-        assert_eq!(1, factorial(1).unwrap());
-        assert_eq!(6, factorial(3).unwrap());
+        for pair in [[1usize, 0], [1, 1], [2, 2], [6, 3]].iter() {
+            assert_eq!(
+                Some(pair[0] as u128),
+                factorial(pair[1]),
+                "{}! should be {}",
+                pair[1],
+                pair[0]
+            );
+            assert_eq!(
+                Some(pair[0] as u128),
+                it_factorial(pair[1]),
+                "{}! should be {}",
+                pair[1],
+                pair[0]
+            );
+        }
 
+        // Verify each implementation returns the same results
         let res = factorial(34);
-        let res2 = mem_factorial(34);
+        let res2 = it_factorial(34);
         assert!(res.is_some());
-        assert_eq!(res.unwrap(), res2.unwrap());
+        assert!(res2.is_some());
+        assert_eq!(res, res2);
 
-        let res = factorial(35);
-        assert!(res.is_none());
+        // Bounds checks
+        assert!(factorial(35).is_none());
+        assert!(it_factorial(35).is_none());
     }
 
     #[test]
     fn test_fibonacci() {
-        assert_eq!(0, fibonacci(0).unwrap());
-        assert_eq!(1, fibonacci(1).unwrap());
-        assert_eq!(1, fibonacci(2).unwrap());
-
-        let res = fibonacci(20);
-        let res2 = rec_fibonacci(20);
-        assert_eq!(res, res2);
+        // Sanity checking
+        for pair in [[0usize, 0], [1, 1], [1, 2], [2, 3]].iter() {
+            assert_eq!(
+                Some(pair[0] as u128),
+                fibonacci(pair[1]),
+                "fibonacci({}) should be {}",
+                pair[1],
+                pair[0]
+            );
+            assert_eq!(
+                Some(pair[0] as u128),
+                mem_fibonacci(pair[1]),
+                "fibonacci({}) should be {}",
+                pair[1],
+                pair[0]
+            );
+            assert_eq!(
+                Some(pair[0] as u128),
+                rec_fibonacci(pair[1]),
+                "fibonacci({}) should be {}",
+                pair[1],
+                pair[0]
+            );
+        }
 
+        // Verify each implementation returns the same results
         let res = fibonacci(186);
         let res2 = mem_fibonacci(186);
         assert!(res.is_some());
         assert!(res2.is_some());
-        assert_eq!(res.unwrap(), res2.unwrap());
+        assert_eq!(res, res2);
 
-        let res = fibonacci(187);
-        assert!(res.is_none());
+        // Bounds checks
+        assert!(fibonacci(187).is_none());
+        assert!(mem_fibonacci(187).is_none());
     }
 }