From cced3571543ed3c629c7af7d18e0a9e7626220a0 Mon Sep 17 00:00:00 2001
From: Bruno Raoult <braoult@gmail.com>
Date: Sun, 9 Oct 2022 14:28:58 +0200
Subject: [PATCH] 2020 day 24 (bash) part 1.

---
 2020/day24/README   | 41 ++++++++++++++++++++++++++++++++++++++
 2020/day24/ex1.bash | 48 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100755 2020/day24/ex1.bash

diff --git a/2020/day24/README b/2020/day24/README
index ee5ba71..9f21f49 100644
--- a/2020/day24/README
+++ b/2020/day24/README
@@ -40,3 +40,44 @@ wseweeenwnesenwwwswnew
 In the above example, 10 tiles are flipped once (to black), and 5 more are flipped twice (to black, then back to white). After all of these instructions have been followed, a total of 10 tiles are black.
 
 Go through the renovation crew's list and determine which tiles they need to flip. After all of the instructions have been followed, how many tiles are left with the black side up?
+
+Your puzzle answer was 450.
+
+The first half of this puzzle is complete! It provides one gold star: *
+--- Part Two ---
+
+The tile floor in the lobby is meant to be a living art exhibit. Every day, the tiles are all flipped according to the following rules:
+
+    Any black tile with zero or more than 2 black tiles immediately adjacent to it is flipped to white.
+    Any white tile with exactly 2 black tiles immediately adjacent to it is flipped to black.
+
+Here, tiles immediately adjacent means the six tiles directly touching the tile in question.
+
+The rules are applied simultaneously to every tile; put another way, it is first determined which tiles need to be flipped, then they are all flipped at the same time.
+
+In the above example, the number of black tiles that are facing up after the given number of days has passed is as follows:
+
+Day 1: 15
+Day 2: 12
+Day 3: 25
+Day 4: 14
+Day 5: 23
+Day 6: 28
+Day 7: 41
+Day 8: 37
+Day 9: 49
+Day 10: 37
+
+Day 20: 132
+Day 30: 259
+Day 40: 406
+Day 50: 566
+Day 60: 788
+Day 70: 1106
+Day 80: 1373
+Day 90: 1844
+Day 100: 2208
+
+After executing this process a total of 100 times, there would be 2208 black tiles facing up.
+
+How many tiles will be black after 100 days?
diff --git a/2020/day24/ex1.bash b/2020/day24/ex1.bash
new file mode 100755
index 0000000..6155db2
--- /dev/null
+++ b/2020/day24/ex1.bash
@@ -0,0 +1,48 @@
+#!/bin/bash
+#
+# ex1.bash: Advent2020 game, day 24/game 1.
+
+CMD=${0##*/}
+shopt -s extglob
+set -o noglob
+
+declare -A plan
+declare -i x y
+
+while read -r line; do
+    x=0
+    y=0
+    for ((i=0; i<${#line}; ++i)); do
+        c=${line:i:1}
+        case "$c" in
+            e) ((++x))
+               ;;
+            w) ((--x))
+               ;;
+            s) ((--y, ++i))
+               c=${line:i:1}
+               ;;
+            n) ((++y, ++i))
+               c=${line:i:1}
+               ;;
+        esac
+        if [[ "$c" = e ]]; then
+            (( x++ ))
+        elif [[ "$c" = w ]]; then
+            (( x-- ))
+        else
+            printf "error c=%s\n" "$c"
+        fi
+    done
+    printf "new tile at (%d,%d): " "$x" "$y"
+    if [[ -v plan[$x,$y] ]]; then
+        printf "already set\n"
+        unset "plan[$x,$y]"
+    else
+        printf "new\n"
+        plan[$x,$y]=1
+    fi
+done
+res=${#plan[@]}
+printf "%s: res=%s\n" "$CMD" "$res"
+exit 0