From 76d31619cce7b5b9fff9951bdb4fc547bf99b934 Mon Sep 17 00:00:00 2001
From: Petra <petra@petras.space>
Date: Fri, 8 Dec 2023 19:09:33 +1300
Subject: [PATCH] 2023 Day 8 part 2

---
 2023/Makefile          |   2 +-
 2023/data/day08ex3.txt |  10 ++++
 2023/src/day08b.f90    | 124 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 135 insertions(+), 1 deletion(-)
 create mode 100644 2023/data/day08ex3.txt
 create mode 100644 2023/src/day08b.f90

diff --git a/2023/Makefile b/2023/Makefile
index d65d493..f84cb35 100644
--- a/2023/Makefile
+++ b/2023/Makefile
@@ -2,7 +2,7 @@ FC:=gfortran
 FFLAGS:=-Wall -Wno-maybe-uninitialized
 BIN:=./bin
 SRC:=./src
-BINS:=./bin/day01.bin ./bin/day01b.bin ./bin/day02.bin ./bin/day03.bin ./bin/day04.bin ./bin/day05.bin ./bin/day05b.bin ./bin/day06.bin ./bin/day07.bin ./bin/day07b.bin ./bin/day08.bin
+BINS:=./bin/day01.bin ./bin/day01b.bin ./bin/day02.bin ./bin/day03.bin ./bin/day04.bin ./bin/day05.bin ./bin/day05b.bin ./bin/day06.bin ./bin/day07.bin ./bin/day07b.bin ./bin/day08.bin ./bin/day08b.bin
 
 all: aoc19
 
diff --git a/2023/data/day08ex3.txt b/2023/data/day08ex3.txt
new file mode 100644
index 0000000..5b3fa58
--- /dev/null
+++ b/2023/data/day08ex3.txt
@@ -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)
diff --git a/2023/src/day08b.f90 b/2023/src/day08b.f90
new file mode 100644
index 0000000..7d44023
--- /dev/null
+++ b/2023/src/day08b.f90
@@ -0,0 +1,124 @@
+program day8b
+    implicit none
+    integer, parameter :: max_chars = 400
+    integer, parameter :: max_nodes = 1000
+    integer, parameter :: max_iter = 100000
+    character(200) :: fname
+    character(max_chars) :: fline
+    integer :: n_arguments
+    integer :: istat
+    character(max_chars) :: dir_ins
+    integer :: n_dir_ins
+    integer :: n_nodes
+    character(len=3) :: node
+    character(len=1) :: dir
+    character(len=3) :: nodes(1:max_nodes), lnodes(1:max_nodes), rnodes(1:max_nodes)
+    integer :: periods(max_nodes)
+    integer :: cursors(max_nodes)
+    integer :: n_cursors
+    integer :: i, ic, i2, LRcur, i3
+    logical :: found_z
+    integer, parameter :: b_kind = selected_int_kind(20)
+    integer(kind=b_kind) :: p_sum
+
+    n_arguments = command_argument_count()
+    if (n_arguments .eq. 1) then
+        call get_command_argument(1, fname)
+        print *, "File: ", trim(fname)
+        print *
+    else
+        print *, "Wrong number of arguments: ", n_arguments
+        stop
+    end if
+
+    open(10, file=fname)
+    read(10, "(a)") dir_ins
+    n_dir_ins = len_trim(dir_ins)
+    read(10, "(a)") fline
+    n_nodes = 0
+    do i=1,max_nodes
+        read(10, "(a)", iostat=istat) fline
+        if ((len_trim(fline) .eq. 0) .or. (is_iostat_end(istat))) then
+            exit
+        end if
+        read(fline(1:3), "(a)") nodes(i)
+        read(fline(8:10), "(a)") lnodes(i)
+        read(fline(13:15), "(a)") rnodes(i)
+        n_nodes = n_nodes + 1
+    end do
+    close(10)
+    cursors = -1
+    n_cursors = 0
+    do i=1, n_nodes
+        if (nodes(i)(3:3) .eq. "A") then
+            n_cursors = n_cursors + 1
+            cursors(n_cursors) = i
+        end if
+    end do
+    print *, "Number of cursors: ", n_cursors
+    periods = 0
+    do i=1,max_iter
+        LRcur = mod(i-1, n_dir_ins) + 1
+        dir = dir_ins(LRcur:LRcur)
+        do ic=1,n_cursors
+            if (dir .eq. "L") then
+                node = lnodes(cursors(ic))
+            else
+                node = rnodes(cursors(ic))
+            end if
+            do i2=1, n_nodes
+                if (nodes(i2) .eq. node) then
+                    cursors(ic) = i2
+                    exit
+                end if
+            end do
+        end do
+        found_z = .true.
+        do ic=1,n_cursors
+            if (nodes(cursors(ic))(3:3) .ne. "Z") then
+                found_z = .false.
+            else if (periods(ic) .eq. 0) then
+                periods(ic) = i
+            end if
+        end do
+        if (found_z) then
+            exit
+        end if
+    end do
+    if (found_z) then
+        print *, "Found nodes ending in Z after ", i, " iterations"
+    else
+        print *, "Did not find nodes ending in Z after ", max_iter, " iterations"
+        print *, "Periods:"
+        print "(1i9)", periods(1:n_cursors)
+        p_sum = 1
+        print *
+        do ic=1,n_cursors
+            p_sum = p_sum * periods(ic)
+        end do
+        print *, "Potential answer: ", p_sum
+        do i=1,n_cursors-1
+            do i2 = i+1,n_cursors
+                do i3=2,periods(i2)
+                    if (mod(periods(i), i3) .eq. 0 .and. mod(periods(i2), i3) .eq. 0) then
+                        do
+                            periods(i) = periods(i) / i3
+                            if (mod(periods(i), i3) .ne. 0) then
+                                exit
+                            end if
+                        end do
+                    end if
+                end do
+            end do
+        end do
+        print *, "Factors: "
+        print "(1i9)", periods(1:n_cursors)
+        p_sum = 1
+        print *
+        do ic=1,n_cursors
+            p_sum = p_sum * periods(ic)
+        end do
+        print *, "Potential answer: ", p_sum
+    end if
+
+end program day8b