Add Generate Permutations Of All Valid 9-ball Racks as a Math TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
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+_1808 TILs and counting..._
 
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@@ -60,6 +60,7 @@ If you've learned something here, support my efforts writing daily TILs by
 * [Linux](#linux)
 * [LLM](#llm)
 * [Mac](#mac)
+* [Math](#math)
 * [Mise](#mise)
 * [MongoDB](#mongodb)
 * [MySQL](#mysql)
@@ -771,6 +772,10 @@ If you've learned something here, support my efforts writing daily TILs by
 - [View All Windows Of The Current App](mac/view-all-windows-of-the-current-app.md)
 - [Write System Clipboard To A File](mac/write-system-clipboard-to-a-file.md)
 
+### Math
+
+- [Generate Permutations Of All Valid 9-ball Racks](math/generate-permutations-of-all-valid-9-ball-racks.md)
+
 ### Mise
 
 - [Create Umbrella Task For All Test Tasks](mise/create-umbrella-task-for-all-test-tasks.md)
@@ -1072,6 +1077,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Define Typed Class Interface With Protocol](python/define-typed-class-interface-with-protocol.md)
 - [Dunder Methods](python/dunder-methods.md)
 - [Easy Key-Value Aggregates With defaultdict](python/easy-key-value-aggregates-with-defaultdict.md)
+- [Enable Pyright Type Checking In Cursor](python/enable-pyright-type-checking-in-cursor.md)
 - [Get Absolute Seconds From `timedelta` Object](python/get-absolute-seconds-from-timedelta-object.md)
 - [Get Quotient And Remainder In One Operation](python/get-quotient-and-remainder-in-one-operation.md)
 - [Install With PIP For Specific Interpreter](python/install-with-pip-for-specific-interpreter.md)

math/generate-permutations-of-all-valid-9-ball-racks.md

@@ -0,0 +1,42 @@
+# Generate Permutations Of All Valid 9-ball Racks
+
+I wanted to produce a full listing of all valid rack arrangements for the game
+of [9-ball](https://en.wikipedia.org/wiki/Nine-ball). The constraints on how a
+9-ball rack can be arranged are, first, that the 1 ball must be placed at the
+head of the diamond and, second, that the 9 ball must be placed at the center of
+the diamond. After that, all other balls (2 through 8) can be placed in any
+arrangement.
+
+Because each of those seven remaining balls can be arranged in distinct
+orderings where each ball is placed once, this is a
+[_permutation_](https://en.wikipedia.org/wiki/Permutation) problem.
+
+> In elementary combinatorics, the k-permutations, or partial permutations, are
+> the ordered arrangements of k distinct elements selected from a set. When k is
+> equal to the size of the set, these are the permutations in the previous
+> sense.
+
+For this problem, the seven distinct elements can be arranged into `7!` (seven
+factorial) unique permutations. That is, 5040 permutations.
+
+I can use [Ruby's `Array#permutations`
+method](https://docs.ruby-lang.org/en/4.0/Array.html#method-i-permutation) to
+enumerate these 5040 permutations like so:
+
+```ruby
+[2,3,4,5,6,7,8].permutation.map do |perm|
+  [1, *perm[0..2], 9, *perm[3..7]]
+end.to_a
+=> [[1, 2, 3, 4, 9, 5, 6, 7, 8],
+ [1, 2, 3, 4, 9, 5, 6, 8, 7],
+ [1, 2, 3, 4, 9, 5, 7, 6, 8],
+ [1, 2, 3, 4, 9, 5, 7, 8, 6],
+ [1, 2, 3, 4, 9, 5, 8, 6, 7],
+ [1, 2, 3, 4, 9, 5, 8, 7, 6],
+ [1, 2, 3, 4, 9, 6, 5, 7, 8],
+ ...
+ [1, 8, 7, 6, 9, 5, 3, 2, 4],
+ [1, 8, 7, 6, 9, 5, 3, 4, 2],
+ [1, 8, 7, 6, 9, 5, 4, 2, 3],
+ [1, 8, 7, 6, 9, 5, 4, 3, 2]]
+```

python/enable-pyright-type-checking-in-cursor.md

@@ -0,0 +1,24 @@
+# Enable Pyright Type Checking In Cursor
+
+In most ways [Cursor](https://cursor.com/), a clone of VS Code, behaves like VS
+Code and uses the same extensions as VS Code. It even offers to clone all your
+existing extensions and setup from VS Code when you first install it.
+
+However, the Pyright type checking setup that I had in VS Code stopped working
+when I opened up the same Python project in Cursor. It seems that to get Pyright
+to reliably work with forks of VS Code, you need to use a compatible fork like
+[Based Pyright](https://docs.basedpyright.com/latest/).
+
+Once I installed _Based Pyright_ from the extension marketplace, I was able to
+enable it in `.vscode/settings.json`:
+
+```json
+{
+    ...,
+    "basedpyright.analysis.typeCheckingMode": "basic"
+}
+```
+
+I may have needed to restart Cursor at this point, I cannot remember exactly.
+However, once this setup was in place the helpful type checking errors started
+appearing as red squiggles.