%title: Zero-cost
%author: Ivan Tham
%date: 2021-07-14
%usage: mdp slide.md

-> Zero-Cost Abstraction <-
===========================

Quite search on youtube and I found that there are quite a few talks on
this for the past month.

---

-> What is zero-cost abstractions? <-
=====================================

https://doc.rust-lang.org/book/ch00-00-introduction.html
https://prev.rust-lang.org/en-US/ (old website)
How it works? Any guess?

---

-> Abstraction? <-
==================

- high-level concepts
- classes, interfaces, traits
- iterator rather than imperative loop
- Example
```rust
let v1: Vec<i32> = vec![1, 2, 3];
let v2: Vec<_> = v1.iter().map(|x| x + 1).collect();
```
Let's talk about the second `Vec<_>` later.

---

-> Common misconception of zero-cost <-
=======================================

At least for me, at first I thought any code you write will magically become
faster due to "zero-cost", but that is not the case.

Let's go through it later.

-> Iterator basics <-
=====================

Syntactic sugar for `while i.next() { ... }`.
https://cheats.rs/#iterators

---

-> Abstractions, but zero-cost? <-
==================================

Let's see what it can do. An example is iterator.
https://doc.rust-lang.org/book/ch13-04-performance.html

From what I see online, abstractions usually have a runtime cost.

In rust, usually iterator perform as good as hand-rolled loops or faster.
Zero-cost abstractions convert runtime cost into compile-time cost.

---

-> Types and few other tricks <-
================================

Show few types in `std::iter`.
Iterator, different types for different iterator. Adapters `Map`, `Take`...
Traits help in this, different types have different traits.
- `DoubleEndedIterator`
- `ExactSizeIterator`
- `FusedIterator`
Speeding up an iterator is usually done by.
- `Iterator::size_hint`
- `TrustedLen` (nightly)
- `InPlaceIterable` (internal)
New: since rust 1.50 can use const-generic array indexing.
Can do `[1, 2, 3]` instead of `[1, 2, 3].iter()`. E.g. `[1].chain([2])`.

---

-> Other benefits of iterator <-
================================

- Expressiveness, able to collect into multiple types (`HashSet`, `HashMap`)
- Auto-vectorization, use SIMD for free (most cases due to types in place)
- Can switch to rayon easily to parallelize stuff (`iter` to `par_iter`)

---

-> Footguns <-
==============

`peekable` + `as_ref` when used with other combinators can easily be confused.

---

-> Go through few iterators in std <-
=====================================

Just for `Iterator`
- `first`, `last`, `nth`, `find`, `position`
- `step_by`, `max`, `min`
- `zip`, `enumerate`
- `map`, `filter`, `filter_map`, `fold`, `skip_while`, `take_while`, `skip`
- `try_*`
- `fold`, `reduce`
- `peekable`, `flatten`
- `inspect`, `by_ref`
- `partition`, `windows`
- `collect`
- `all`, `any`
- `sum`, `product`, `cycle`
- ...

---

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```