/***
   EEPROM Update method
  
   Stores values read from analog input 0 into the EEPROM.
   These values will stay in the EEPROM when the board is
   turned off and may be retrieved later by another sketch.
  
   If a value has not changed in the EEPROM, it is not overwritten
   which would reduce the life span of the EEPROM unnecessarily.
   
   Released using MIT licence.
 ***/

#include <EEPROM.h>

/** the current address in the EEPROM (i.e. which byte we're going to write to next) **/
int addr = 0;

void setup(){ /** EMpty setup **/ }

void loop()
{
  /***
    need to divide by 4 because analog inputs range from
    0 to 1023 and each byte of the EEPROM can only hold a
    value from 0 to 255.
  ***/
  int val = analogRead(0) / 4;

  /***
    Update the particular EEPROM cell.
    these values will remain there when the board is
    turned off.
  ***/
  EEPROM.update(addr, val);
  
  /***
    The function EEPROM.update(addr, val) is equivalent to the following:
  
    if( EEPROM.read(addr) != val ){
      EEPROM.write(addr, val);
    }
  ***/
  

  /***
    Advance to the next address, when at the end restart at the beginning.    
    
    Larger AVR processors have larger EEPROM sizes, E.g:
    - Arduno Duemilanove: 512b EEPROM storage.
    - Arduino Uno:        1kb EEPROM storage.
    - Arduino Mega:       4kb EEPROM storage.
    
    Rather than hard-coding the length, you should use the pre-provided length function.
    This will make your code portable to all AVR processors.    
  ***/
  addr = addr + 1;
  if(addr == EEPROM.length())
    addr = 0;
    
  /***
    As the EEPROM sizes are powers of two, wrapping (preventing overflow) of an 
    EEPROM address is also doable by a bitwise and of the length - 1.
    
    ++addr &= EEPROM.length() - 1;
  ***/

  delay(100);
}