day 8 part 2 solution

day08/sample3.in

@@ -0,0 +1,10 @@
+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)

day08/sol.tcl

@@ -41,28 +41,52 @@ proc ghost-solve {dirs graph} {
         }
         set end($node) [string match "*Z" $node]
     }
-    set node $start
-    set steps 0
     puts "start: $node"
-    while {1} {
-        set done 1
-        foreach n $node {
-            if {! $end($n)} {
-                set done 0
-                break
+    set lengths {}
+    foreach node $start {
+        array set seen {}
+        set node0 $node
+        set path $node
+        set steps 0
+        # we make a couple simplifying assumptions:
+        # first, that there is only one exit node on each cycle
+        # and two, that the exits do not line up before the first cycle
+        # 
+        # find how long it takes to get to the exit the first time
+        # then find the first cycle
+        set initial -1
+        set sanity -1
+        set cycle {}
+        while {1} {
+            set i [expr {$steps % [llength $dirs]}]
+            if {$end($node)} {
+                if {$initial < 0} {
+                    set initial $steps
+                    set sanity $i
+                } else {
+                    set cycle $steps
+                    if {$i != $sanity} { error "exit cycle desync" }
+                    break
+                }
             }
+            set d [expr {[lindex $dirs $i] eq "L" ? 0 : 1}]
+            set node [lindex $G($node) $d]
+            #lappend path $node
+            #puts "$d -> $node"
+            incr steps
         }
-        if {$done} break
-
-        set d [lindex $dirs [expr {$steps % [llength $dirs]}]]
-        set i [expr {$d eq "L" ? 0 : 1}]
-        set node [lmap n $node {lindex $G($n) $i}]
-        #puts "$d -> $node"
-        incr steps
+        set len [expr {$cycle - $initial}]
+        puts "$node0: found cycle of $len steps after $initial steps at node $node"
+        # assumption #3: len == initial
+        lappend lengths $len
     }
-    puts "$steps"
+
+    # i'm too tired to code up a LCM function in tcl
+    puts $lengths
+    puts [exec python -c "import numpy; print(numpy.lcm.reduce(\[[join $lengths ","]\]))"]
+    #puts "$steps"
 }
 
 set data [read-input stdin]
-#solve {*}$data
+solve {*}$data
 ghost-solve {*}$data