Index: heromesh.h
==================================================================
--- heromesh.h
+++ heromesh.h
@@ -40,10 +40,11 @@
 extern xrm_quark optionquery[16];
 extern Uint32 generation_number;
 extern char main_options[128];
 
 unsigned char*read_lump(int sol,int lvl,long*sz,sqlite3_value**us);
+void write_lump(int sol,int lvl,long sz,const unsigned char*data);
 void set_cursor(int id);
 
 #define FIL_SOLUTION 1
 #define FIL_LEVEL 0
 #define LUMP_LEVEL_IDX (-1)

Index: main.c
==================================================================
--- main.c
+++ main.c
@@ -173,10 +173,26 @@
     if(us) *us=0;
   }
   sqlite3_reset(readusercachest);
   return buf;
 }
+
+void write_lump(int sol,int lvl,long sz,const unsigned char*data) {
+  // Writes a lump to the user cache.
+  // The actual Hamster archive files will be updated when the program terminates.
+  sqlite3_stmt*st;
+  int e;
+  if(e=sqlite3_prepare_v2(userdb,"INSERT INTO `USERCACHEDATA`(`FILE`,`LEVEL`,`NAME`,`DATA`) VALUES(?1,?2,CASE WHEN ?2 < 0 THEN ?3 ELSE ?2 || ?3 END,?4)"
+   " ON CONFLICT(`FILE`,`LEVEL`) DO UPDATE SET `DATA` = ?4;",-1,&st,0)) fatal("SQL error (%d): %s\n",e,sqlite3_errmsg(userdb));
+  sqlite3_bind_int64(st,1,sol?solutionuc:leveluc);
+  sqlite3_bind_int(st,2,lvl);
+  sqlite3_bind_text(st,3,lvl==LUMP_CLASS_DEF?"CLASS.DEF":lvl==LUMP_LEVEL_IDX?"LEVEL.IDX":sol?".SOL":".LVL",-1,SQLITE_STATIC);
+  sqlite3_bind_blob64(st,4,data,sz,0);
+  while((e=sqlite3_step(st))==SQLITE_ROW);
+  if(e!=SQLITE_DONE) fatal("SQL error (%d): %s\n",e,sqlite3_errmsg(userdb));
+  sqlite3_finalize(st);
+}
 
 static void init_usercache(void) {
   sqlite3_stmt*st;
   int z;
   sqlite3_int64 t1,t2;