NEAR Core: Reference client for NEAR Protocol

use near_sdk::borsh::{self, BorshDeserialize, BorshSerialize};
use near_sdk::{env, near_bindgen, AccountId, Balance, Promise};
use std::collections::HashMap;

#[near_bindgen]
#[derive(BorshDeserialize, BorshSerialize)]
pub struct TokenContract {
    balances: HashMap<AccountId, Balance>,
    total_supply: Balance,
}

impl Default for TokenContract {
    fn default() -> Self {
        Self {
            balances: HashMap::new(),
            total_supply: 0,
        }
    }
}

#[near_bindgen]
impl TokenContract {
    pub fn transfer(&mut self, receiver_id: AccountId, amount: Balance) {
        let sender_id = env::predecessor_account_id();
        let sender_balance = self.balances.get(&sender_id).unwrap_or(&0);
        
        assert!(sender_balance >= &amount, "Insufficient balance");
        
        self.balances.insert(sender_id.clone(), sender_balance - amount);
        let receiver_balance = self.balances.get(&receiver_id).unwrap_or(&0);
        self.balances.insert(receiver_id, receiver_balance + amount);
    }
    
    pub fn get_balance(&self, account_id: AccountId) -> Balance {
        self.balances.get(&account_id).unwrap_or(&0).clone()
    }
}