Solution

src/main.leo

@@ -1,3 +1,69 @@
 program store.aleo {
+    mapping items: u8 => u64;
+    mapping prices: u8 => u64;
+
+    record Token {
+        owner: address,
+        amount: u64
+    }
+
+    record Item {
+        owner: address,
+        item: u8,
+        quantity: u64
+    }
+
+    // Address of store owner: aleo10ppqgwqzdr2h49z3w4faze74genk6zs0ye4tzemk3d8uxerfkqzspkzp7w
+    // Step 1: Mint tokens to certain address
+    transition mint(receiver: address, amount: u64) -> Token {
+        assert_eq(self.caller, aleo1fu0k2qfytzs5fhesgfgjuax6wsh9xx4ftpdapnhzrtruy0tx3urqx3p0ut);
+
+        return Token {
+            owner: receiver,
+            amount: amount
+        };
+    }
+
+    // Step 2: Add item
+    transition add_item(item: u8, quantity: u64, price: u64) {
+        assert_eq(self.caller, aleo1fu0k2qfytzs5fhesgfgjuax6wsh9xx4ftpdapnhzrtruy0tx3urqx3p0ut);
+
+        return then finalize(item, quantity, price);
+    }
+
+    // Increment items quantity, set item price
+    finalize add_item (item: u8, quantity: u64, price: u64) {
+        let item_count: u64 = Mapping::get_or_use(items, item, 0u64);
+        Mapping::set(items, item, item_count + quantity);
+
+        Mapping::set(prices, item, price);
+    }
 
+    transition buy(token: Token, item: u8, quantity: u64, bill_amount: u64) -> (Token, Item) {
+        let difference: u64 = token.amount - bill_amount;
+
+        let remaining: Token = Token {
+            owner: token.owner,
+            amount: difference
+        };
+
+        let item_record: Item = Item {
+            owner: self.caller,
+            item: item,
+            quantity: quantity
+        };
+
+        return (remaining, item_record) then finalize(item, quantity, bill_amount);
+    }
+
+    finalize buy (item: u8, quantity: u64, bill_amount: u64) {
+        let available_quantity: u64 = Mapping::get_or_use(items, item, 0u64);
+        assert(available_quantity >= quantity);
+
+        let item_price: u64 = Mapping::get_or_use(prices, item, 0u64);
+        assert_neq(item_price, 0u64);
+
+        let total_price: u64 = item_price * quantity;
+        assert_eq(total_price, bill_amount);
+    }
 }