Chapter 2 exercises: memoize fib and timeit

chapter-2/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "chapter-2"
+version = "0.1.0"
+

chapter-2/Cargo.toml

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+[package]
+name = "chapter-2"
+version = "0.1.0"
+authors = ["Vladan Popovic <vladanovic@gmail.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

chapter-2/src/main.rs

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+use std::collections::HashMap;
+use std::hash::Hash;
+use std::time::Instant;
+
+fn memoize<A: Copy + Hash + Eq, B: Clone, F>(f: F, mut store: HashMap<A, B>) -> impl FnMut(A) -> B
+where
+    F: Fn(A) -> B,
+{
+    move |x: A| {
+        let res = store.entry(x).or_insert_with(|| f(x));
+        res.clone()
+    }
+}
+
+fn fib(n: u64) -> u64 {
+    match n {
+        0 => 1,
+        1 => 1,
+        _ => fib(n - 1) + fib(n - 2),
+    }
+}
+
+fn fibt(n: u64) -> u64 {
+    fn fib_tailrec(nt: u64, ct: u64, previous: u64, current: u64) -> u64 {
+        match ct {
+            1 => fib_tailrec(nt, 2, 1, 1),
+            _ => {
+                if ct != nt {
+                    fib_tailrec(nt, ct + 1, current, previous + current)
+                } else {
+                    current
+                }
+            }
+        }
+    }
+    fib_tailrec(n + 1, 1, 0, 1)
+}
+
+// mut f because of the mutable hash in memoize and hence the return FnMut
+fn timeit<T, F>(mut f: F) -> (T, u128)
+where
+    F: FnMut() -> T,
+{
+    let now = Instant::now();
+    let res = f();
+    (res, now.elapsed().as_micros())
+}
+
+fn main() {
+    let mut mem_fib = memoize(fib, HashMap::new());
+    let mut mem_fibt = memoize(fibt, HashMap::new());
+    let (res, duration) = timeit(|| mem_fib(40));
+    println!(
+        "Finished regular fib(40) = {} in {} microseconds",
+        res, duration
+    );
+    let (res, duration) = timeit(|| mem_fib(40));
+    println!(
+        "Finished memoized fib(40) = {} in {} microseconds",
+        res, duration
+    );
+    let (res, duration) = timeit(|| mem_fibt(40));
+    println!(
+        "Finished regular tailrec fib(40) = {} in {} microseconds",
+        res, duration
+    );
+    let (res, duration) = timeit(|| mem_fibt(40));
+    println!(
+        "Finished memoized tailrec fib(40) = {} in {} microseconds",
+        res, duration
+    );
+}