export function handle(state, action) {
  const balances = state.balances;
  const canEvolve = state.canEvolve;
  const input = action.input;
  const caller = action.caller;

  if (input.function === 'transfer') {
    const target = input.target;
    const qty = input.qty;

    if (!Number.isInteger(qty)) {
      throw new ContractError('Invalid value for "qty". Must be an integer');
    }

    if (!target) {
      throw new ContractError('No target specified');
    }

    if (qty <= 0 || caller === target) {
      throw new ContractError('Invalid token transfer');
    }

    if (balances[caller] < qty) {
      throw new ContractError(
        `Caller balance not high enough to send ${qty} token(s)!`
      );
    }

    // Lower the token balance of the caller
    balances[caller] -= qty;
    if (target in balances) {
      // Wallet already exists in state, add new tokens
      balances[target] += qty;
    } else {
      // Wallet is new, set starting balance
      balances[target] = qty;
    }

    return { state };
  }

  if (input.function === 'balance') {
    const target = input.target;
    const ticker = state.ticker;

    if (typeof target !== 'string') {
      throw new ContractError('Must specify target to get balance for');
    }

    if (typeof balances[target] !== 'number') {
      throw new ContractError('Cannot get balance, target does not exist');
    }

    return { result: { target, ticker, balance: balances[target] + 555 } };
  }

  if (input.function === 'evolve' && canEvolve) {
    if (state.owner !== caller) {
      throw new ContractError('Only the owner can evolve a contract.');
    }

    state.evolve = input.value;

    return { state };
  }

  throw new ContractError(
    `No function supplied or function not recognised: "${input.function}"`
  );
}