adventofcode2023/day08/sol.tcl

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#!/usr/bin/env tclsh
source ../prelude.tcl
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proc read-input {input} {
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set dirs [split [gets $input] ""] ;# LR...
if {[gets $input] ne ""} { error "invalid input" }
set graph {}
while {[gets $input line] >= 0} {
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must_regexp {^([A-Z12]+) = \(([A-Z12]+), ([A-Z12]+)\)$} $line _ node left right
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lappend graph [list $node $left $right]
}
return [list $dirs $graph]
}
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proc solve {dirs graph} {
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foreach x $graph {
lassign $x node l r
set G($node) [list $l $r]
}
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if {![info exists G(ZZZ)]} { return }
#puts [array get G]
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set node AAA
set steps 0
while {$node ne "ZZZ"} {
set d [lindex $dirs [expr {$steps % [llength $dirs]}]]
set node [lindex $G($node) [expr {($d eq "L") ? 0 : 1}]]
puts "$d -> $node"
incr steps
}
puts "$steps"
}
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proc ghost-solve {dirs graph} {
set start {}
array set end {}
foreach x $graph {
lassign $x node l r
set G($node) [list $l $r]
if {[string match "*A" $node]} {
lappend start $node
}
set end($node) [string match "*Z" $node]
}
puts "start: $node"
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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
}
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}
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set d [expr {[lindex $dirs $i] eq "L" ? 0 : 1}]
set node [lindex $G($node) $d]
#lappend path $node
#puts "$d -> $node"
incr steps
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}
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set len [expr {$cycle - $initial}]
puts "$node0: found cycle of $len steps after $initial steps at node $node"
# assumption #3: len == initial
lappend lengths $len
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}
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# 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 "$steps"
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}
set data [read-input stdin]
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solve {*}$data
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ghost-solve {*}$data