export const ACTIONS = {
     ADD_DIGIT: 'add-digit',
     CHOOSE_OPERATION: 'choose-operation',
     CLEAR: 'clear',
     EVALUATE: 'evaluate',
 };

 function reducer(state, { type, payload }) {
     switch (type) {
         case ACTIONS.ADD_DIGIT:
             if (state.overwrite) {
                 return {
                     ...state,
                     currentOperand: payload.digit,
                     overwrite: false,
                 };
             }

             if (payload.digit === '0' && state.currentOperand === '0') {
                 return state;
             }

             if (payload.digit === '.' && state.currentOperand.includes('.')) {
                 return state;
             }

             return { ...state, currentOperand: `${state.currentOperand || ''}${payload.digit}` };
         case ACTIONS.CHOOSE_OPERATION:
             if (state.currentOperand == null && state.previousOperand == null) {
                 return state;
             }

             if (state.currentOperand == null) {
                 return {
                     ...state,
                     operation: payload.operation,
                 };
             }

             if (state.previousOperand == null) {
                 return {
                     ...state,
                     operation: payload.operation,
                     previousOperand: state.currentOperand,
                     currentOperand: null,
                 };
             }

             return {
                 ...state,
                 previousOperand: evaluate(state),
                 operation: payload.operation,
                 currentOperand: null,
             };
         case ACTIONS.CLEAR:
             return {};
         case ACTIONS.EVALUATE:
             if (state.operation == null || state.currentOperand == null || state.previousOperand == null) {
                 return state;
             }

             return {
                 ...state,
                 overwrite: true,
                 previousOperand: null,
                 operation: null,
                 currentOperand: evaluate(state),
             };
         default:
             return state;
     }
 }