From d9a3cac3515839556f36616a50c7a5f86beb0e80 Mon Sep 17 00:00:00 2001
From: Jules Doumeche <30329843+julio4@users.noreply.github.com>
Date: Thu, 5 Oct 2023 20:54:43 +0900
Subject: [PATCH] fix: #87 (#89)

---
 .../struct_as_mapping_key/src/contract.cairo     | 16 +++++-----------
 .../struct_as_mapping_key/src/test.cairo         |  1 -
 src/ch01-03-struct-mapping-key.md                |  2 +-
 3 files changed, 6 insertions(+), 13 deletions(-)

diff --git a/listings/ch01-advanced-concepts/struct_as_mapping_key/src/contract.cairo b/listings/ch01-advanced-concepts/struct_as_mapping_key/src/contract.cairo
index cf7b4460..217442da 100644
--- a/listings/ch01-advanced-concepts/struct_as_mapping_key/src/contract.cairo
+++ b/listings/ch01-advanced-concepts/struct_as_mapping_key/src/contract.cairo
@@ -1,13 +1,13 @@
 #[starknet::contract]
 mod PetRegistry {
-    use poseidon::poseidon_hash_span;
+    use hash::{HashStateTrait, Hash};
 
     #[storage]
     struct Storage {
-        registration_time: LegacyMap::<felt252, u64>,
+        registration_time: LegacyMap::<Pet, u64>,
     }
 
-    #[derive(Copy, Drop, Serde)]
+    #[derive(Copy, Drop, Serde, Hash)]
     struct Pet {
         name: felt252,
         age: u8,
@@ -18,17 +18,11 @@ mod PetRegistry {
     #[generate_trait]
     impl PetRegistry of PetRegistryTrait {
         fn register_pet(ref self: ContractState, key: Pet, timestamp: u64) {
-            let mut serialized: Array<felt252> = ArrayTrait::new();
-            Serde::<Pet>::serialize(@key, ref serialized);
-            let hashed_key = poseidon_hash_span(serialized.span());
-            self.registration_time.write(hashed_key, timestamp);
+            self.registration_time.write(key, timestamp);
         }
 
         fn get_registration_date(self: @ContractState, key: Pet) -> u64 {
-            let mut serialized: Array<felt252> = ArrayTrait::new();
-            Serde::<Pet>::serialize(@key, ref serialized);
-            let hashed_key = poseidon_hash_span(serialized.span());
-            self.registration_time.read(hashed_key)
+            self.registration_time.read(key)
         }
     }
 }
diff --git a/listings/ch01-advanced-concepts/struct_as_mapping_key/src/test.cairo b/listings/ch01-advanced-concepts/struct_as_mapping_key/src/test.cairo
index 8f4562e9..35ab1da0 100644
--- a/listings/ch01-advanced-concepts/struct_as_mapping_key/src/test.cairo
+++ b/listings/ch01-advanced-concepts/struct_as_mapping_key/src/test.cairo
@@ -25,7 +25,6 @@ mod tests {
         let pet = Pet { name: 'Cute Labrador', age: 5, owner: 'Louis' };
 
         // Store a pet.
-
         contract.register_pet(pet, 1234);
 
         // Read the array.
diff --git a/src/ch01-03-struct-mapping-key.md b/src/ch01-03-struct-mapping-key.md
index 974a160b..50e773dd 100644
--- a/src/ch01-03-struct-mapping-key.md
+++ b/src/ch01-03-struct-mapping-key.md
@@ -1,6 +1,6 @@
 # Structs as mapping keys
 
-It is currently impossible to use structs as mapping keys. However, a workaround consists in hashing the values of all the struct's fields, and use this hash as a key in the `LegacyMap` type.
+In order to use structs as mapping keys, you can use `#[derive(Hash)]` on the struct definition. This will automatically generate a hash function for the struct that can be used to represent the struct as a key in a `LegacyMap`.
 
 Consider the following example in which we would like to use an object of
 type `Pet` as a key in a `LegacyMap`. The `Pet` struct has three fields: `name`, `age` and `owner`. We consider that the combination of these three fields uniquely identifies a pet.