Keyboard Interfacing using USB Host shield..Difficulty in understanding program!

can anybody explain me this code? i am getting output for this code which i downloaded from a website ( https://gist.github.com/davidbkemp/1602e1efd2ab7a6188c7/revisions ) But i am not getting how this code works?

#include <hidboot.h>
#include <usbhub.h>
// Satisfy IDE, which only needs to see the include statment in the ino.
#ifdef dobogusinclude
#include <spi4teensy3.h>
#endif

#define UHS_HID_BOOT_KEY_ESC        0x29
#define UHS_HID_BOOT_KEY_DEL        0x4C
#define UHS_HID_BOOT_KEY_BACKSPACE  0x2A
#define UHS_HID_BOOT_KEY_HOME       0x4A
#define UHS_HID_BOOT_KEY_PAGE_UP    0x4B
#define UHS_HID_BOOT_KEY_END        0x4D
#define UHS_HID_BOOT_KEY_PAGE_DN    0x4E
#define UHS_HID_BOOT_KEY_TAB        0x2B
#define UHS_HID_BOOT_KEY_CAPS       0x39


class KbdRptParser : public KeyboardReportParser
{
       void PrintKey(uint8_t mod, uint8_t key);
       uint8_t mapExtraAsciiKeys(uint8_t);

protected:
       virtual void OnControlKeysChanged(uint8_t before, uint8_t after);

 virtual void OnKeyDown  (uint8_t mod, uint8_t key);
 virtual void OnKeyUp  (uint8_t mod, uint8_t key);
 virtual void OnKeyPressed(uint8_t key);
};

void KbdRptParser::PrintKey(uint8_t m, uint8_t key)
{
   MODIFIERKEYS mod;
   *((uint8_t*)&mod) = m;
   Serial.print((mod.bmLeftCtrl   == 1) ? "C" : " ");
   Serial.print((mod.bmLeftShift  == 1) ? "S" : " ");
   Serial.print((mod.bmLeftAlt    == 1) ? "A" : " ");
   Serial.print((mod.bmLeftGUI    == 1) ? "G" : " ");

   Serial.print(" >");
   PrintHex<uint8_t>(key, 0x80);
   Serial.print("< ");

   Serial.print((mod.bmRightCtrl   == 1) ? "C" : " ");
   Serial.print((mod.bmRightShift  == 1) ? "S" : " ");
   Serial.print((mod.bmRightAlt    == 1) ? "A" : " ");
   Serial.println((mod.bmRightGUI    == 1) ? "G" : " ");
};

// Map some of the keys currently not handled by OemToAscii
uint8_t KbdRptParser::mapExtraAsciiKeys(uint8_t key) {

 switch(key) {
     case UHS_HID_BOOT_KEY_SPACE: return ' ';
     case UHS_HID_BOOT_KEY_ZERO2: return '0';
     case UHS_HID_BOOT_KEY_PERIOD: return '.';
     case UHS_HID_BOOT_KEY_ESC: return 0x1B;
     case UHS_HID_BOOT_KEY_DEL: return 0x7F;
     case UHS_HID_BOOT_KEY_BACKSPACE: return 0x08;
     case UHS_HID_BOOT_KEY_TAB: return 0x09;
 }

 return ( 0);
}


void KbdRptParser::OnKeyDown(uint8_t mod, uint8_t key)
{
   Serial.print("DN ");
   PrintKey(mod, key);
   uint8_t c = OemToAscii(mod, key);
   
   if (!c) {
     c = mapExtraAsciiKeys(key);
   }

   if (c) {
       OnKeyPressed(c);
   }

}

void KbdRptParser::OnControlKeysChanged(uint8_t before, uint8_t after) {

   MODIFIERKEYS beforeMod;
   *((uint8_t*)&beforeMod) = before;

   MODIFIERKEYS afterMod;
   *((uint8_t*)&afterMod) = after;

   if (beforeMod.bmLeftCtrl != afterMod.bmLeftCtrl) {
       Serial.println("LeftCtrl changed");
   }
   if (beforeMod.bmLeftShift != afterMod.bmLeftShift) {
       Serial.println("LeftShift changed");
   }
   if (beforeMod.bmLeftAlt != afterMod.bmLeftAlt) {
       Serial.println("LeftAlt changed");
   }
   if (beforeMod.bmLeftGUI != afterMod.bmLeftGUI) {
       Serial.println("LeftGUI changed");
   }

   if (beforeMod.bmRightCtrl != afterMod.bmRightCtrl) {
       Serial.println("RightCtrl changed");
   }
   if (beforeMod.bmRightShift != afterMod.bmRightShift) {
       Serial.println("RightShift changed");
   }
   if (beforeMod.bmRightAlt != afterMod.bmRightAlt) {
       Serial.println("RightAlt changed");
   }
   if (beforeMod.bmRightGUI != afterMod.bmRightGUI) {
       Serial.println("RightGUI changed");
   }

}

void KbdRptParser::OnKeyUp(uint8_t mod, uint8_t key)
{
   Serial.print("UP ");
   PrintKey(mod, key);
}

void KbdRptParser::OnKeyPressed(uint8_t key)
{
   Serial.print("ASCII: ");
   Serial.println(key, HEX);
};

USB     Usb;
//USBHub     Hub(&Usb);
HIDBoot<HID_PROTOCOL_KEYBOARD>    HidKeyboard(&Usb);

uint32_t next_time;

KbdRptParser Prs;

void setup()
{
   Serial.begin( 115200 );
   while (!Serial); // Wait for serial port to connect - used on Leonardo, Teensy and other boards with built-in USB CDC serial connection
   Serial.println("Start");

   if (Usb.Init() == -1)
       Serial.println("OSC did not start.");

   delay( 200 );

   next_time = millis() + 5000;

   HidKeyboard.SetReportParser(0, (HIDReportParser*)&Prs);
}

void loop()
{
   Usb.Task();
}

It would appear that you would like us to invest our time in assisting you with your project but you aren't prepared to invest a little of your time reading the post at the top of each section of the forum - 'How to use this forum - please read'.

The reason I know this is that you are requested to use code tags when posting code, and it carefully explains how to do this. However, you have either not read the post, or feel that it doesn't apply to you.