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+# 3x3 Sliding Puzzle Quine
+
+
+
+## Remarks
+
+This is a quine-like program.
+
+When run without arguments, it outputs `entry.rb` itself:
+
+```
+$ ruby entry.rb
+$ ruby entry.rb | ruby
+$ ruby entry.rb | ruby | ruby
+```
+
+It has been tested with the following environment:
+
+```
+$ ruby --version
+ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
+```
+
+
+
+## Description
+
+This program is not only a quine but also a 3x3 sliding puzzle.
+
+
+### How to Play
+
+You can move a tile adjacent to the empty slot using `h` (left), `j` (down), `k` (up), `l` (right).
+For example:
+
+```
+$ ruby entry.rb l
+```
+
+This command moves `8` to right.
+Of course, the output source code is still a quine. These two commands' output are identical:
+
+```
+$ ruby entry.rb l
+$ ruby entry.rb l | ruby
+```
+
+
+### How to Shuffle
+
+You can shuffle the board by passing a seed as an argument:
+
+```
+$ ruby entry.rb 2025
+```
+
+The output puzzle can be solved using the following move sequence:
+
+```
+$ ruby entry.rb 2025 | \
+    ruby - k | ruby - l | ruby - k | ruby - j | ruby - h | \
+    ruby - j | ruby - l | ruby - k | ruby - h | ruby - j | \
+    ruby - h | ruby - l | ruby - k | ruby - k | ruby - l | \
+    ruby - j | ruby - h | ruby - h | ruby - k | ruby - j | \
+    ruby - l | ruby - l | ruby - k | ruby - h | ruby - h > solved.rb
+$ diff entry.rb solved.rb
+```
+
+
+### Interactive mode
+
+In addition, it supports interactive mode. Simply run:
+
+```
+$ ruby entry.rb i
+```
+
+It will display the current board and ask for your next move.
+You can exit by typing `q` in interactive mode.
+
+
+
+## Internals
+
+* `entry.pretty.rb` is a human-readable version.
+* `$b` represents the board's current state.
+* `$s` is the program's own source code for quine generation. It contains no "padding" characters, such as unnecessary parentheses, semicolons, or comments.
+* Font data is encoded in the variables `n` and `m`.
+* The string "TRICK EPISODE I" shown in the empty slot is removed using `gsub(/[[:upper:]]/)`. This works because there are no uppercase letters in `$s` except for that.
+* The size of the `entry.rb` is 2025 byte.