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; 
     } 
 }