Use Mapping in ERC-1155 Example (#1057)

* Switch out `BTreeMap` with `Mapping`

* Use new `Mapping::clear_entry` API

* Document panic behaviour in `perform_transfer`

* Use `initialize_contract` in constructor

* Pass tuples by reference when inserting into map

* RustFmt

* Pass tuple by reference when querying storage

* Add note about proper initialization

* Fix typo

Co-authored-by: Michael Müller <michi@parity.io>

Co-authored-by: Michael Müller <michi@parity.io>

examples/erc1155/lib.rs

@@ -187,10 +187,13 @@ pub trait Erc1155TokenReceiver {
 mod erc1155 {
     use super::*;
 
-    use ink_prelude::collections::BTreeMap;
-    use ink_storage::traits::{
-        PackedLayout,
-        SpreadLayout,
+    use ink_storage::{
+        lazy::Mapping,
+        traits::{
+            PackedLayout,
+            SpreadAllocate,
+            SpreadLayout,
+        },
     };
 
     /// Indicate that a token transfer has occured.
@@ -236,6 +239,7 @@ mod erc1155 {
         PartialOrd,
         Eq,
         PartialEq,
+        Default,
         PackedLayout,
         SpreadLayout,
         scale::Encode,
@@ -249,12 +253,12 @@ mod erc1155 {
 
     /// An ERC-1155 contract.
     #[ink(storage)]
-    #[derive(Default)]
+    #[derive(Default, SpreadAllocate)]
     pub struct Contract {
         /// Tracks the balances of accounts across the different tokens that they might be holding.
-        balances: BTreeMap<(AccountId, TokenId), Balance>,
+        balances: Mapping<(AccountId, TokenId), Balance>,
         /// Which accounts (called operators) have been approved to spend funds on behalf of an owner.
-        approvals: BTreeMap<Approval, ()>,
+        approvals: Mapping<Approval, ()>,
         /// A unique identifier for the tokens which have been minted (and are therefore supported)
         /// by this contract.
         token_id_nonce: TokenId,
@@ -264,7 +268,11 @@ mod erc1155 {
         /// Initialize a default instance of this ERC-1155 implementation.
         #[ink(constructor)]
         pub fn new() -> Self {
-            Default::default()
+            // This call is required in order to correctly initialize the
+            // `Mapping`s of our contract.
+            //
+            // Not that `token_id_nonce` will be initialized to its `Default` value.
+            ink_lang::codegen::initialize_contract(|_| {})
         }
 
         /// Create the initial supply for a token.
@@ -281,7 +289,7 @@ mod erc1155 {
 
             // Given that TokenId is a `u128` the likelihood of this overflowing is pretty slim.
             self.token_id_nonce += 1;
-            self.balances.insert((caller, self.token_id_nonce), value);
+            self.balances.insert(&(caller, self.token_id_nonce), &value);
 
             // Emit transfer event but with mint semantics
             self.env().emit_event(TransferSingle {
@@ -308,7 +316,7 @@ mod erc1155 {
             ensure!(token_id <= self.token_id_nonce, Error::UnexistentToken);
 
             let caller = self.env().caller();
-            self.balances.insert((caller, token_id), value);
+            self.balances.insert(&(caller, token_id), &value);
 
             // Emit transfer event but with mint semantics
             self.env().emit_event(TransferSingle {
@@ -326,21 +334,27 @@ mod erc1155 {
         //
         // Should not be used directly since it's missing certain checks which are important to the
         // ERC-1155 standard (it is expected that the caller has already performed these).
+        //
+        // # Panics
+        //
+        // If `from` does not hold any `token_id` tokens.
         fn perform_transfer(
             &mut self,
             from: AccountId,
             to: AccountId,
             token_id: TokenId,
             value: Balance,
         ) {
-            self.balances
-                .entry((from, token_id))
-                .and_modify(|b| *b -= value);
+            let mut sender_balance = self
+                .balances
+                .get(&(from, token_id))
+                .expect("Caller should have ensured that `from` holds `token_id`.");
+            sender_balance -= value;
+            self.balances.insert(&(from, token_id), &sender_balance);
 
-            self.balances
-                .entry((to, token_id))
-                .and_modify(|b| *b += value)
-                .or_insert(value);
+            let mut recipient_balance = self.balances.get(&(to, token_id)).unwrap_or(0);
+            recipient_balance += value;
+            self.balances.insert(&(to, token_id), &recipient_balance);
 
             let caller = self.env().caller();
             self.env().emit_event(TransferSingle {
@@ -507,7 +521,7 @@ mod erc1155 {
 
         #[ink(message)]
         fn balance_of(&self, owner: AccountId, token_id: TokenId) -> Balance {
-            *self.balances.get(&(owner, token_id)).unwrap_or(&0)
+            self.balances.get(&(owner, token_id)).unwrap_or(0)
         }
 
         #[ink(message)]
@@ -541,7 +555,7 @@ mod erc1155 {
             };
 
             if approved {
-                self.approvals.insert(approval, ());
+                self.approvals.insert(&approval, &());
             } else {
                 self.approvals.remove(&approval);
             }
@@ -653,9 +667,9 @@ mod erc1155 {
 
         fn init_contract() -> Contract {
             let mut erc = Contract::new();
-            erc.balances.insert((alice(), 1), 10);
-            erc.balances.insert((alice(), 2), 20);
-            erc.balances.insert((bob(), 1), 10);
+            erc.balances.insert((alice(), 1), &10);
+            erc.balances.insert((alice(), 2), &20);
+            erc.balances.insert((bob(), 1), &10);
 
             erc
         }