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"
@@ -550,28 +551,45 @@ class Grouping
 
     struct zim_MD5_CTX md5ctx;
     zim_MD5Init(&md5ctx);
-
+
+    unsigned char ch[CHUNK_SIZE];
     offset_type checksumPos = header.getChecksumPos();
-    offset_type currentPos = 0;
+    offset_type toRead = checksumPos;
+
     for(auto part = zimFile->begin();
         part != zimFile->end();
         part++) {
       std::ifstream stream(part->second->filename(), std::ios_base::in|std::ios_base::binary);
 
-      char ch;
-      for(/*NOTHING*/ ; currentPos < checksumPos && stream.get(ch).good(); currentPos++) {
-        zim_MD5Update(&md5ctx, reinterpret_cast<const uint8_t*>(&ch), 1);
+      while(toRead>=CHUNK_SIZE && stream.read(reinterpret_cast<char*>(ch),CHUNK_SIZE).good()) {
+        zim_MD5Update(&md5ctx, ch, CHUNK_SIZE);
+        toRead-=stream.gcount();
+      }
+
+      // 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);
       }
+
+      // It updates the checksum with the remaining amount of data when we
+      // reach the end of the file or part
+        zim_MD5Update(&md5ctx, ch, stream.gcount());
+        toRead-=stream.gcount();
+
       if (stream.bad()) {
         perror("error while reading file");
         return false;
       }
-      if (currentPos == checksumPos) {
+      if (!toRead) {
         break;
       }
     }
 
-    if (currentPos != checksumPos) {
+    if (toRead) {
       return false;
     }
 
@@ -583,7 +601,6 @@ class Grouping
     {
       return false;
     }
-
     return true;
   }