feat: store using packing optimisation

src/SUMMARY.md

@@ -26,3 +26,7 @@ Summary
 
   - [Upgradeable Contract](./ch02-01-upgradeable_contract.md)
   - [Defi Vault](./ch02-02-simple_vault.md)
+
+- [Optimisations](./ch03-00-optimisations.md)
+
+  - [Storage Optimisations](./ch03-01-store_using_packing.md)

src/ch03-00-optimisations.md

@@ -0,0 +1,3 @@
+# Optimisations 
+
+A collection of optimisation patterns to save gas and steps.

src/ch03-01-store_using_packing.md

@@ -0,0 +1,32 @@
+# Storage optimisation 
+
+A smart contract has a limited amount of **storage slots**. Each slot can store a single `felt252` value.
+Writing to a storage slot has a cost, so we want to use as few storage slots as possible.
+
+In Cairo, every type is derived from the `felt252` type, which uses 252 bits to store a value.
+This design is quite simple, but it does have a drawback: it is not storage efficient. For example, if we want to store a `u8` value, we need to use an entire slot, even though we only need 8 bits.
+
+## Packing
+
+When storing multiple values, we can use a technique called **packing**. Packing is a technique that allows us to store multiple values in a single felt value. This is done by using the bits of the felt value to store multiple values.
+
+For example, if we want to store two `u8` values, we can use the first 8 bits of the felt value to store the first `u8` value, and the last 8 bits to store the second `u8` value. This way, we can store two `u8` values in a single felt value.
+
+Cairo provides a built-in store using packing that you can use with the `StorePacking` trait.
+
+```rust
+trait StorePacking<T, PackedT> {
+    fn pack(value: T) -> PackedT;
+    fn unpack(value: PackedT) -> T;
+}
+```
+
+This allows to store the type `T` by first packing it into the type `PackedT` with the `pack` function, and then storing the `PackedT` value with it's `Store` implementation. When reading the value, we first retrieve the `PackedT` value, and then unpack it into the type `T` using the `unpack` function.
+
+Here's an example of storing a `Time` struct with two `u8` values using the `StorePacking` trait:
+
+```rust
+{{#include ../listings/ch03-optimisations/store_using_packing/src/packing.cairo:Packing}}
+```
+
+Play with this contract in [Remix](https://remix.ethereum.org/?#activate=Starknet-cairo1-compiler&url=https://github.com/NethermindEth/StarknetByExample/blob/main/listings/ch03-optimisations/store_using_packing/src/packing.cairo).