/*

     File: Calculator.m

 Abstract: This file implements the Calculator class.

  Version: 1.2

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 */

#import "Calculator.h"

// These string constants contain the characters that the input: method accepts.

const NSString *Operators = @"+-*/";

const NSString *Equals    = @"=";

const NSString *Digits    = @"0123456789.";

const NSString *Period    = @".";

const NSString *Delete    = @"D";

const NSString *Clear     = @"C";

@interface Calculator ()

@property double operand;

@property (nonatomic, copy) NSString *operator;

// The calculator display (the value a harwdare-based calculator shows on its LCD screen).

@property (nonatomic, copy) NSMutableString *display;

@end

@implementation Calculator

#pragma mark Lifecycle

- init {

   if ((self = [super init])) {

      _display = [NSMutableString stringWithCapacity:20];

      _operator = nil;

   }

   return self;

}

#pragma mark Calculator Operation

/*

 * The input: method accepts the characters in the string constants

 * Operators, Equals, Digits, Period Delete, and Clear.

 *

 * The results of this method's computations are stored in _display.

 * This method uses operand and operator in its calculations.

 */

- (void) input:(NSString *) input_character {

   static BOOL last_character_is_operator = NO;

   BOOL bad_character;

   // Does input_character contain exactly one character?

   if (!(bad_character = !(input_character && [input_character length] == 1))) {

      // Is input_character in Digits?

      if ([Digits rangeOfString: input_character].length) {

         if (last_character_is_operator) {

            // Set the display to input_character.

            [self.display setString: input_character];

            last_character_is_operator = NO;

         }

         // Is input_character a digit, or is a period while a period has not been added to _display?

         else if (![input_character isEqualToString: (NSString *)Period] || [self.display rangeOfString: (NSString *)Period].location == NSNotFound) {

            // Add input_character to _display.

            [self.display appendString:input_character];

         }

      }

      // Is input_character in Operators or is it Equals?

      else if ([Operators rangeOfString:input_character].length || [input_character isEqualToString:(NSString *)Equals]) {

         if (!self.operator && ![input_character isEqualToString:(NSString *)Equals]) {

            // input_character is this calculation's operator.

            //

            // Save the operand and the operator.

            self.operand  = [[self displayValue] doubleValue];

            self.operator = input_character;

         }

         else {

            // input_character is in Operators or Equals.

            //

            // Perform the computation indicated by the saved operator between the saved operand and _display.

            // Place the result in _display.

            if (self.operator) {

               double operand2 = [[self displayValue] doubleValue];

               switch ([Operators rangeOfString: _operator].location) {

                  case 0:

                     self.operand = self.operand + operand2;

                     break;

                  case 1:

                     self.operand = self.operand - operand2;

                     break;

                  case 2:

                     self.operand = self.operand * operand2;

                     break;

                  case 3:

                     self.operand = self.operand / operand2;

                     break;

               }

               [self.display setString: [@(self.operand) stringValue]];

            }

            // Save the operation (if this is a chained computation).

            self.operator = ([input_character isEqualToString:(NSString *)Equals])? nil : input_character;

         }

         last_character_is_operator = YES;

      }

      // Is input_character Delete?

      else if ([input_character isEqualToString:(NSString *)Delete]) {

         // Remove the rightmost character from _display.

         NSInteger index_of_char_to_remove = [self.display length] - 1;

         if (index_of_char_to_remove >= 0) {

            [self.display deleteCharactersInRange:NSMakeRange(index_of_char_to_remove, 1)];

            last_character_is_operator = NO;

         }

      }

      // Is input_character Clear?

      else if ([input_character isEqualToString:(NSString *)Clear]) {

         // If there's something in _display, clear it.

         if ([self.display length]) {

            [self.display setString:[NSString string]];

         }

         // Otherwise, clear the saved operator.

         else {

            self.operator = nil;

         }

      }

      else {

         // input_character is an unexpected (invalid) character.

         bad_character = TRUE;

      }

   }

   if (bad_character) {

      // Raise exception for unexpected character.

      NSException *exception = [NSException exceptionWithName:NSInvalidArgumentException

                                                       reason:@"The input_character parameter contains an unexpected value."

                                                     userInfo:@{@"arg0": input_character}];

      [exception raise];

   }

}

/*

 * The displayValue method retuns a copy of display.

 */

- (NSString *) displayValue {

   if ([self.display length]) {

      return [self.display copy];

   }

   return @"0";

}

@end