better and faster is_xschem_file detection

src/xschem.tcl

@@ -4123,6 +4123,7 @@ proc is_xschem_file {f} {
   set ret 0
   set score 0
   set instances 0
+  set symbol 0
   set nline 0
   set generator 0
   if {$a} {
@@ -4143,33 +4144,35 @@ proc is_xschem_file {f} {
         # close $fd
         # set fd [open "|$f"]
         set generator 1
+        break
         # continue
       }
+      if { [regexp {format *= *"@} $line] } { incr symbol; incr score}
+      if { [regexp {template *= *"} $line] } { incr symbol; incr score}
       if { [regexp {^[TKFGVSE] \{} $line] } { incr score }
       if { [regexp {^[BL] +[0-9]+ +[-0-9.eE]+ +[-0-9.eE]+ +[-0-9.eE]+ +[-0-9.eE]+ +\{} $line] } {incr score}
       if { [regexp {^N +[-0-9.eE]+ +[-0-9.eE]+ +[-0-9.eE]+ +[-0-9.eE]+ +\{} $line] } {incr score}
       if { [regexp {^C +\{[^{}]+\} +[-0-9.eE]+ +[-0-9.eE]+ +[0-3]+ +[0-3]+ +\{} $line] } {
-        set symname [regsub {([^{]+{)([^}]+)(}.*)} $line {\2}]
-        set type [xschem get_sym_type $symname]
-        # title/launcher instances in symbols are allowed, do not indicate it is a schematic.
-        if {![regexp "^(logo|launcher)$" $type]} {
-          incr instances
-          incr score
-        }
+        incr instances
+        incr score
+        if {[regexp {ipin\.sym|opin\.sym|iopin\.sym} $line]} {set symbol 0}
       }
+
       if { [regexp "^v\[ \t\]+\{xschem\[ \t\]+version\[ \t\]*=.*\[ \t\]+file_version\[ \t\]*=" $line] } {
         set ret 1
       }
+      if {$instances >= 4} {break}
+      if {$score > 2000} {break}  ;# give up on extremely big files
       incr nline
     } 
     if { $score > 4 }  { set ret 1} ;# Heuristic decision :-)
     if {$generator eq {1}} {
       set ret GENERATOR
     } elseif { $ret ne {0}} {
-      if { $instances} {
-        set ret SCHEMATIC
-      } else { 
+      if {$symbol && $instances < 4} {
         set ret SYMBOL
+      } else { 
+        set ret SCHEMATIC
       }
     }
     close $fd