Optimize checksum calculation on verify

src/fileimpl.cpp

@@ -19,6 +19,7 @@
  *
  */
 
+#define CHUNK_SIZE 1024
 #include "fileimpl.h"
 #include <zim/error.h>
 #include "_dirent.h"
@@ -550,7 +551,7 @@ class Grouping
 
     struct zim_MD5_CTX md5ctx;
     zim_MD5Init(&md5ctx);
-
+    
     unsigned char ch[CHUNK_SIZE];
     offset_type checksumPos = header.getChecksumPos();
     offset_type toRead = checksumPos;
@@ -564,8 +565,12 @@ class Grouping
         zim_MD5Update(&md5ctx, ch, CHUNK_SIZE);
         toRead-=stream.gcount();
       }
-
-      // It reads the remaining amount of file when we reach the end of the file
+
+      // Previous read was good, so we have exited the previous `while` because
+      // `toRead<CHUNK_SIZE`. Let's try to read `toRead` chars and process them later.
+      // Else, the previous `while` exited because we didn't succeed to read
+      // `CHUNK_SIZE`, and we still have some data to process before changing part.
+      // It reads the remaining amount of part when we reach the end of the file
       if(stream.good()){
         stream.read(reinterpret_cast<char*>(ch),toRead);
       }