Optimize VersionHash() to not need temporary copy of input

Stop copying stuff, and just parse the bytes one by-one to the
newly created AddCRC16Byte. This improves the instruction count
for an update run from 720,850,121 to 455,801,749 according to
callgrind.

apt-pkg/contrib/crc-16.cc

@@ -64,6 +64,10 @@ static unsigned short const crc16_table[256] =
 
 /* Recompute the FCS with one more character appended. */
 #define CalcFCS(fcs, c) (((fcs) >> 8) ^ crc16_table[((fcs) ^ (c)) & 0xff])
+unsigned short AddCRC16Byte(unsigned short fcs, unsigned char byte)
+{
+   return CalcFCS(fcs, byte);
+}
 unsigned short AddCRC16(unsigned short fcs, void const *Buf,
 			unsigned long long len)
 {

apt-pkg/contrib/crc-16.h

@@ -13,6 +13,7 @@
 #include <apt-pkg/macros.h>
 
 #define INIT_FCS  0xffff
+unsigned short AddCRC16Byte(unsigned short fcs, unsigned char byte) APT_CONST;
 unsigned short AddCRC16(unsigned short fcs, void const *buf,
 			unsigned long long len) APT_PURE;
 

apt-pkg/deb/deblistparser.cc

@@ -353,7 +353,6 @@ unsigned short debListParser::VersionHash()
       pkgTagSection::Key::Breaks,
       pkgTagSection::Key::Replaces};
    unsigned long Result = INIT_FCS;
-   char S[1024];
    for (auto I : Sections)
    {
       const char *Start;
@@ -363,23 +362,16 @@ unsigned short debListParser::VersionHash()
       
       /* Strip out any spaces from the text, this undoes dpkgs reformatting
          of certain fields. dpkg also has the rather interesting notion of
-         reformatting depends operators < -> <= */
-      char *J = S;
-      for (; Start != End && (J - S) < sizeof(S); ++Start)
+         reformatting depends operators < -> <=, so we drop all = from the
+	 string to make that not matter. */
+      for (; Start != End; ++Start)
       {
-	 if (isspace_ascii(*Start) != 0)
+	 if (isspace_ascii(*Start) != 0 || *Start == '=')
 	    continue;
-	 *J++ = tolower_ascii_unsafe(*Start);
-
-	 /* Normalize <= to < and >= to >. This is the wrong way around, but
-	  * more efficient that the right way. And since we're only hashing
-	  * it does not matter which way we normalize. */
-	 if ((*Start == '<' || *Start == '>') && Start[1] == '=') {
-	    Start++;
-	 }
+	 Result = AddCRC16Byte(Result, tolower_ascii_unsafe(*Start));
       }
 
-      Result = AddCRC16(Result,S,J - S);
+
    }
    
    return Result;