Avoid copying back memory in FileChecksummer when data is valid

This may be less efficient than a circular buffer, but is much easier to implement

src/filechecksummer.cpp

@@ -135,14 +135,20 @@ bool FileCheckSummer::Jump(u64 distance)
 
 // Fill the buffer from disk
 
-bool FileCheckSummer::Fill(void)
+bool FileCheckSummer::Fill(bool longfill)
 {
   // Have we already reached the end of the file
   if (readoffset >= filesize)
     return true;
 
+  // Don't bother filling if we have enough data in the buffer
+  if (tailpointer >= &buffer[blocksize] && !longfill)
+    return true;
+
+  // Try reading at least one block of data
+  const char *target = tailpointer == buffer ? &buffer[blocksize] : &buffer[2*blocksize];
   // How much data can we read into the buffer
-  size_t want = (size_t)min(filesize-readoffset, (u64)(&buffer[2*blocksize]-tailpointer));
+  size_t want = (size_t)min(filesize-readoffset, (u64)(target-tailpointer));
 
   if (want > 0)
   {
@@ -156,7 +162,7 @@ bool FileCheckSummer::Fill(void)
   }
 
   // Did we fill the buffer
-  want = &buffer[2*blocksize] - tailpointer;
+  want = target - tailpointer;
   if (want > 0)
   {
     // Blank the rest of the buffer

src/filechecksummer.h

@@ -104,7 +104,8 @@ class FileCheckSummer
   void UpdateHashes(u64 offset, const void *buffer, size_t length);
 
   //// Fill the buffers with more data from disk
-  bool Fill(void);
+  // Set longfill = true to force fill the whole buffer
+  bool Fill(bool longfill = false);
 
 private:
   // private copy constructor to prevent any misuse.
@@ -157,6 +158,11 @@ inline bool FileCheckSummer::Step(void)
     return true;
   }
 
+  // Ensure we have enough data in the buffer
+  if (tailpointer <= inpointer)
+    if(!Fill(true))
+      return false;
+
   // Get the incoming and outgoing characters
   char inch = *inpointer++;
   char outch = *outpointer++;
@@ -171,13 +177,12 @@ inline bool FileCheckSummer::Step(void)
   assert(outpointer == &buffer[blocksize]);
 
   // Copy the data back to the beginning of the buffer
-  memmove(buffer, outpointer, (size_t)blocksize);
+  memcpy(buffer, outpointer, (size_t)blocksize);
   inpointer = outpointer;
   outpointer = buffer;
   tailpointer -= blocksize;
 
-  // Fill the rest of the buffer
-  return Fill();
+  return true;
 }