From 18bacdfb012042bb31f73bf86b6b0f4af3ba05bb Mon Sep 17 00:00:00 2001
From: arthomnix <arthomnix@arthomnix.dev>
Date: Tue, 9 Dec 2025 15:02:06 +0000
Subject: [PATCH] Day 9

---
 README.md     |  2 +-
 d09/README.md | 16 ++++++++++++----
 d09/day9.pi   | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 59 insertions(+), 5 deletions(-)
 create mode 100644 d09/day9.pi

diff --git a/README.md b/README.md
index 9c7d33c..a5511ab 100644
--- a/README.md
+++ b/README.md
@@ -20,7 +20,7 @@ _in which I attempt to use a different language every day_
 6. LaTeX
 7. sed
 8. Noulith
-9. ??
+9. Picat
 10. ??
 11. ??
 12. ??
\ No newline at end of file
diff --git a/d09/README.md b/d09/README.md
index 6ad4b2c..67c8e80 100644
--- a/d09/README.md
+++ b/d09/README.md
@@ -2,10 +2,18 @@
 
 |                   |                   |
 |-------------------|-------------------|
-| Language          | ??                |
-| Input Method      | ??                |
-| Output Method     | ??                |
+| Language          | Picat             |
+| Input Method      | File              |
+| Output Method     | stdout            |
 
 ## Running
+Obtain Picat from [here](https://picat-lang.org/download.html) and add it to your PATH,
+then
+```sh
+picat day9.pi <path to input file>
+```
 
-## Notes
\ No newline at end of file
+## Notes
+This solution is based on constraint programming, which is what Picat was designed for.
+This is of course nowhere near as fast as a bespoke optimised solution; this code takes
+~15 minutes to solve both parts on my laptop.
\ No newline at end of file
diff --git a/d09/day9.pi b/d09/day9.pi
new file mode 100644
index 0000000..f456573
--- /dev/null
+++ b/d09/day9.pi
@@ -0,0 +1,46 @@
+import util.
+import cp.
+
+read_input(Path, Points) =>
+    Lines = read_file_lines(Path),
+    Points = new_array(Lines.length),
+    I = 1,
+    foreach (Line in Lines)
+        Points[I] = Line.split(",").map(to_integer).to_array,
+        I := I + 1
+    end.    
+
+area(Ax, Ay, Bx, By, Area) =>
+    Area #= (abs(Ax - Bx) + 1) * (abs(Ay - By) + 1).
+
+part_one(Points) = Area =>
+    area(Ax, Ay, Bx, By, Area),
+    matrix_element(Points, I, 1, Ax),
+    matrix_element(Points, I, 2, Ay),
+    matrix_element(Points, J, 1, Bx),
+    matrix_element(Points, J, 2, By),
+    solve([$max(Area)], [I, J]).
+
+pairs(L) = [{L[I], L[(I mod L.len) + 1]} : I in 1..L.len].
+
+part_two(Points) = Area =>
+    area(Ax, Ay, Bx, By, Area),
+    matrix_element(Points, I, 1, Ax),
+    matrix_element(Points, I, 2, Ay),
+    matrix_element(Points, J, 1, Bx),
+    matrix_element(Points, J, 2, By),
+    foreach ({{Fx, Fy}, {Sx, Sy}} in Points.pairs)
+        (Fx #<= min(Ax, Bx) #/\ Sx #<= min(Ax, Bx))
+            #\/ (Fx #>= max(Ax, Bx) #/\ Sx #>= max(Ax, Bx))
+            #\/ (Fy #<= min(Ay, By) #/\ Sy #<= min(Ay, By))
+            #\/ (Fy #>= max(Ay, By) #/\ Sy #>= max(Ay, By))
+    end,
+    solve([$max(Area)], [I, J]).
+
+main =>
+    println("Please provide input filename as a command-line argument.").
+
+main(Args) =>
+    read_input(Args[1], Points),
+    println(part_one(Points)),
+    println(part_two(Points)).
-- 
2.51.2