/*    counter.c --
 *    "Access Counter"
 *    Written by J. Patrick Van Metre for
 *    World Wide Web: Beyond the Basics
 *    CS 6204 at Virginia Polytechnic Institute and State University
 *
 *    This CGI application returns the location of a GIF image which
 *      can be displayed to represent a single digit in the count of
 *      accesses to a WWW page.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/file.h>

#define STRING_LENGTH 1024
#define SEPARATOR '&'
#define DATABASE "/home/vanmetre/cgi-db/counter/counter.db"
#define PICTURE_LOC "http://csgrad.cs.vt.edu/~vanmetre/numbers/"
#define LOCK_FILE "/home/vanmetre/cgi-db/counter/LOCK"

/* GetParam -- Accepts a query string in Query an searches it
 *             for a term named Name.
 *             Returns the value assigned to Name in Dest.
 */

void GetParam(char *Dest, const char *Name, const char *Query)
{
  int Pos=0, copy, QueryLen;

  QueryLen = strlen(Query);

  while (Pos < QueryLen)  {                          /* Iterate through */
    if (!strncmp(Query+Pos, Name, strlen(Name)))  {  /*  query looking  */
      Pos += strlen(Name)+1;                         /*  for Name; if   */
      copy = 0;                                      /*  found, copy to */
      while ((Query[Pos] != SEPARATOR) && (Pos < QueryLen))     /* Dest */
        Dest[copy++] = Query[Pos++];
      Dest[copy] = 0;
      return;
      }  /* if */
     else  {                    /* If not found, move along to the next */
      Pos += strlen(Name)+1;               /*  name-value pair in query */
      while ((Query[Pos] != SEPARATOR) && (Pos < QueryLen))
        Pos++;
      Pos++;
      }  /* else */
    }  /* while */

  }  /* GetParam */


void main()
{
  char Query[STRING_LENGTH];
  char ReqDigit[STRING_LENGTH];
  char Location[STRING_LENGTH];
  char Temp[STRING_LENGTH];
  char TempLocation[STRING_LENGTH];
  char NewDBName[STRING_LENGTH];
  char Command[STRING_LENGTH];
  int NumAccess, Value, Work, CurrDigit, Digit;
  int TempNumAccess;
  int Found = 0;
  FILE *OldDB, *NewDB;
  int lock_fd;

/* Remember the query string and the URL of the page being counted */

  strcpy(Query, getenv("QUERY_STRING"));
  strcpy(Location, getenv("HTTP_REFERER"));

/* Retrieve the value for "digit=" from the query string */

  GetParam(ReqDigit, "digit", Query);
  Digit = atoi(ReqDigit);

/* Get a lock on the counter database file */

  lock_fd = open(LOCK_FILE, O_RDONLY | O_CREAT, 256+128+32+16);
  if (lock_fd < 0)  {
    printf("<H1>Error opening lock file</H1>\n");
    exit(1);  }
  flock(lock_fd, LOCK_EX);

/* Open the database file for reading */

  OldDB = fopen(DATABASE, "r");
  if (OldDB == NULL)  {
    printf("Content-type: text/html\n\n");
    printf("Couldn't open db for reading\n");
    exit(1);
    }

/* We only update a location's counter when the ones digit is accessed */

  if (Digit == 0)  {

/* And when we update, we write a new database file, then rename it */

    sprintf(NewDBName, "%s.new", DATABASE);

    NewDB = fopen(NewDBName, "w");
    if (NewDB == NULL)  {
      printf("Content-type: text/html\n\n");
      printf("Couldn't open db for writing\n");
      exit(1);
      }

    }

/* Read through the database file looking for the accessed URL */

  while (!feof(OldDB))  {
    fgets(Temp, STRING_LENGTH, OldDB);         /* Read one line at a time */
    if (!feof(OldDB))  {            /* Make sure we haven't read past EOF */
      sscanf(Temp, "%d %s", &TempNumAccess, TempLocation);
      if (!strcmp(TempLocation, Location))  {   /* If the accessed URL is */
        NumAccess = TempNumAccess;            /*  already in the database */
        Found = 1;                            /*  increment the number of */
        NumAccess++;                                          /* accesses */
        if (Digit == 0)
          fprintf(NewDB, "%d %s\n", NumAccess, Location);
        }
       else  {
        if (Digit == 0)
          fputs(Temp, NewDB);
        }
      }
    }

  fclose(OldDB);

  if (Digit == 0)  {

    fclose(NewDB);

    sprintf(Command, "cp -f %s %s", NewDBName, DATABASE);
    system(Command);

    }

/* If the URL isn't in the database, then just stick it on the end */

  if (!Found)  {
    OldDB = fopen(DATABASE, "a");
    if (OldDB == NULL)  {
      printf("Content-type: text/html\n\n");
      exit(1);
      }

    NumAccess = 1;
    fprintf(OldDB, "%d %s\n", 1, Location);
    fclose(OldDB);
    }

/* Unlock the database file */

  flock(lock_fd, LOCK_UN);
  close(lock_fd);

  Value = 0;
  CurrDigit = 0;
  Work = NumAccess;

/* Isolate the digit we want */

  while (Work != 0)  {
    if (CurrDigit == Digit)
      Value = Work % 10;
    CurrDigit++;
    Work /= 10;
    }

/* Refer the accessed URL to the location of the digit image */
/*   by putting it in the header output                      */

  printf("Location: %s%d.gif\n\n", PICTURE_LOC, Value);

}