From 8d0782d3631cf94b5a6033140df61559ab0d672e Mon Sep 17 00:00:00 2001
From: Xu Shaohua <shaohua@biofan.org>
Date: Mon, 24 Jul 2023 10:43:02 +0800
Subject: [PATCH] math: Tuning fib(), remove cache vector

---
 math/benches/fibonacci.rs |  4 ++--
 math/src/fibonacci.rs     | 30 +++++++++++++-----------------
 2 files changed, 15 insertions(+), 19 deletions(-)

diff --git a/math/benches/fibonacci.rs b/math/benches/fibonacci.rs
index a2db1aaf..fe60bb73 100644
--- a/math/benches/fibonacci.rs
+++ b/math/benches/fibonacci.rs
@@ -2,11 +2,11 @@
 // Use of this source is governed by General Public License that can be found
 // in the LICENSE file.
 
-use criterion::{criterion_group, criterion_main, Criterion};
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use math::fibonacci::fib;
 
 pub fn fib_bench(c: &mut Criterion) {
-    c.bench_function("fib 80", |b| b.iter(|| fib(80)));
+    c.bench_function("math::fib-80", |b| b.iter(|| fib(black_box(80))));
 }
 
 criterion_group!(benches, fib_bench);
diff --git a/math/src/fibonacci.rs b/math/src/fibonacci.rs
index db9edb75..344a4879 100644
--- a/math/src/fibonacci.rs
+++ b/math/src/fibonacci.rs
@@ -18,24 +18,20 @@ pub const MAX_INDEX: u8 = 93;
 /// # Errors
 /// Returns error if `num` is not a possitive integer.
 pub fn fib(num: u8) -> Result<u64, FibError> {
-    if num > MAX_INDEX {
-        return Err(FibError::IllegalIndex);
+    let mut a = 0;
+    let mut b = 1;
+    match num {
+        0 => Ok(0),
+        num if num > MAX_INDEX => Err(FibError::IllegalIndex),
+        _ => {
+            for _ in 1..num {
+                let c = a + b;
+                a = b;
+                b = c;
+            }
+            Ok(b)
+        }
     }
-    if num == 0 {
-        return Ok(0);
-    }
-    if num == 1 {
-        return Ok(1);
-    }
-
-    let num = num as usize;
-    let mut buf: Vec<u64> = vec![0; num + 1];
-    buf[0] = 0;
-    buf[1] = 1;
-    for i in 2..=num {
-        buf[i] = buf[i - 1] + buf[i - 2];
-    }
-    Ok(buf[num])
 }
 
 #[cfg(test)]