// HttpHeaderReader.c : Reads HTTP header and counts them
//
#include "stdafx.h"
#include "stdio.h"
#include "string.h"
static int Hash (const char * pSrc, size_t len);
int main(int argc, char* argv[])
{
 static const char filename[] = "foo.txt";
 static const int MAX_COUNT = 255;
 static int frequency[MAX_COUNT] = {0};
 FILE* fd = fopen(filename, "r");
 if ( fd != NULL )
 {
  char line[MAX_COUNT + 1];
  while ( fgets(line, sizeof line, fd) != NULL )
  {
   char* p = strchr(line, ':');
   if (p != NULL)
   {
    frequency[Hash(_strlwr(line), (size_t)(p-line))%MAX_COUNT] += 1;
   }
  }
  fclose(fd);
  char header[MAX_COUNT + 1] = {0};
  printf( "Enter the header:\n " );
  scanf("%255s", header);
  header[MAX_COUNT] = '\0';
  printf( "%s occurs %d times.\n", header, frequency[Hash(_strlwr(header), strlen(_strlwr(header)))%MAX_COUNT] );
 }
 return 0;
}
static int Hash (const char * pSrc, size_t len)
{
  size_t res = 0;
  while (len--) res = (res << 1) ^ *pSrc++; 
  return (int)res;
}

content-length: 10
 len = 14, hash = 177
user-agent: test
 len = 10, hash = 107
content-length: 14
 len = 14, hash = 177
accept: comedy
 len = 6, hash = 16
content-length: 100
 len = 14, hash = 177
content-encoding: gzip
 len = 16, hash = 134
connection: close
 len = 10, hash = 50
user-agent: test
 len = 10, hash = 107
accept: flash
 len = 6, hash = 16
user-agent: test1
 len = 10, hash = 107
content-length: 20
 len = 14, hash = 177
user-agent: test2
 len = 10, hash = 107
user-agent: test3
 len = 10, hash = 107
accept: gzip
 len = 6, hash = 16
Enter the header:
 user-aGENT
user-agent occurs 5 times.