day 20 part 2 answer

day20/sol.tcl

@@ -115,13 +115,8 @@ proc solve input {
     set count(1) 0
 
     global when
-    set last $when(dn)
     for {set i 1} {$i <= 1000} {incr i} {
         pulse broadcaster $i
-        if {$when(dn) ne $last} {
-            puts $when(dn)
-            set last $when(dn)
-        }
     }
 
     puts "$count(0)"
@@ -131,10 +126,24 @@ proc solve input {
     global type
     if {![info exists type(rx)]} return
 
+    set last $when(dn)
     while {1} {
         pulse broadcaster $i
         if {$when(dn) ne $last} {
-            puts $when(dn)
+            # rx is connected to a conj of 4 circuits
+            # which each emit 1 high pulse on a periodic cycle.
+            #
+            # this could be more difficult but it turns out
+            # that the step when the first output is produced
+            # is also equal to the cycle length,
+            # and all the cycles are a prime number so we don't
+            # even need to do an lcm
+            set a [lmul [dict values $when(dn)]]
+            if {$a != 0} {
+                puts $a
+                break
+            }
+            #puts $when(dn)
             set last $when(dn)
         }
         incr i