Adding Hash Solidity compatible example (#116)

* docs + contracts + test

* renaming + link

* fmt

* changes requested

* nit: fmt

Author: julienbrs

listings/ch02-advanced-concepts/hash_solidity_compatible/.gitignore

@@ -0,0 +1 @@
+target

listings/ch02-advanced-concepts/hash_solidity_compatible/Scarb.lock

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+# Code generated by scarb DO NOT EDIT.
+version = 1
+
+[[package]]
+name = "hash_solidity_compatible"
+version = "0.1.0"

listings/ch02-advanced-concepts/hash_solidity_compatible/Scarb.toml

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+[package]
+name = "hash_solidity_compatible"
+version = "0.1.0"
+
+[dependencies]
+starknet = ">=2.3.0"
+
+[[target.starknet-contract]]

listings/ch02-advanced-concepts/hash_solidity_compatible/src/contract.cairo

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+#[starknet::interface]
+trait ISolidityHashExample<TContractState> {
+    fn hash_data(ref self: TContractState, input_data: Span<u256>) -> u256;
+}
+
+
+#[starknet::contract]
+mod SolidityHashExample {
+    use keccak::{keccak_u256s_be_inputs};
+    use array::Span;
+
+    #[storage]
+    struct Storage {}
+
+    #[abi(embed_v0)]
+    impl SolidityHashExample of super::ISolidityHashExample<ContractState> {
+        fn hash_data(ref self: ContractState, input_data: Span<u256>) -> u256 {
+            let hashed = keccak_u256s_be_inputs(input_data);
+
+            // Split the hashed value into two 128-bit segments
+            let low: u128 = hashed.low;
+            let high: u128 = hashed.high;
+
+            // Reverse each 128-bit segment
+            let reversed_low = integer::u128_byte_reverse(low);
+            let reversed_high = integer::u128_byte_reverse(high);
+
+            // Reverse merge the reversed segments back into a u256 value
+            let compatible_hash = u256 { low: reversed_high, high: reversed_low };
+
+            compatible_hash
+        }
+    }
+}

listings/ch02-advanced-concepts/hash_solidity_compatible/src/lib.cairo

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+mod contract;
+
+#[cfg(test)]
+mod tests;

listings/ch02-advanced-concepts/hash_solidity_compatible/src/tests.cairo

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+mod tests {
+    use hash_solidity_compatible::{contract::{SolidityHashExample, ISolidityHashExample}};
+    use debug::PrintTrait;
+
+    use starknet::{
+        ContractAddress, get_contract_address, contract_address_const, call_contract_syscall,
+        testing::{set_contract_address}
+    };
+
+    fn setup() -> SolidityHashExample::ContractState {
+        let mut state = SolidityHashExample::contract_state_for_testing();
+        let contract_address = contract_address_const::<0x1>();
+        set_contract_address(contract_address);
+        state
+    }
+
+    #[test]
+    #[available_gas(2000000000)]
+    fn get_same_hash_solidity() {
+        let mut state = setup();
+        let mut array: Array<u256> = ArrayTrait::new();
+        array.append(1);
+
+        let hash_expected: u256 =
+            0xb10e2d527612073b26eecdfd717e6a320cf44b4afac2b0732d9fcbe2b7fa0cf6;
+        let hash_received: u256 = state.hash_data(array.span());
+
+        assert(hash_received == hash_expected, 'hash_received != hash_expected');
+    }
+}

src/SUMMARY.md

@@ -43,5 +43,6 @@ Summary
   - [Writing to any storage slot](./ch02/write_to_any_slot.md)
   - [Storing Arrays](./ch02/storing_arrays.md)
   - [Struct as mapping key](./ch02/struct-mapping-key.md)
+  - [Hash Solidity Compatible](./ch02/hash-solidity-compatible.md)
   - [Optimisations](./ch02/optimisations/optimisations.md)
     - [Storage Optimisations](./ch02/optimisations/store_using_packing.md)

src/ch02/hash-solidity-compatible.md

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+# Hash Solidity Compatible
+
+This contract demonstrates Keccak hashing in Cairo to match Solidity's keccak256. While both use Keccak, their endianness differs: Cairo is little-endian, Solidity big-endian. The contract achieves compatibility by hashing in big-endian using `keccak_u256s_be_inputs`, and reversing the bytes of the result with `u128_byte_reverse`.
+
+For example:
+
+```rust
+{{#include ../../listings/ch02-advanced-concepts/hash_solidity_compatible/src/contract.cairo}}
+```
+
+Play with the contract in [Remix](https://remix.ethereum.org/?#activate=Starknet&url=https://github.com/NethermindEth/StarknetByExample/blob/main/listings/ch02-advanced-concepts/hash_solidity_compatible/src/contract.cairo).