Uiua

Part 1 was easy, part 2 is …less so…

a hint that might help you

visualising the data reveals some interesting patterns. Follow link to do so.

Any way, here’s my Part 1 while I’m still thinking about this.

# AOC 2025 Day 09
# Experimental!
D ← &fras"AOC2025day09.txt"  # Drop your file here and edit name
/↥/×+1⌵⍉/-⍉₂⧅<2⋕°csv D       # Part 1 
∧⍜⊡⋅1⟜(˜↯0+1/↥)⍜⍉≡⍜⍆⊛⋕°csv D # Visualised data

Part 2

This is basically me thinking out loud, so it’s untidy, brutal, repetitive and slow. (20s in the pad) link if you care

# Experimental!
# AOC 2025 Day 09
D     ← "7,1\n11,1\n11,7\n9,7\n9,5\n2,5\n2,3\n7,3"
D     ← &fras"AOC2025day09.txt" # Drop your file here and edit name
Parse ← ⋕°csv
Part₁ ← /↥/×+1⌵⍉/-⍉₂⧅<2

Squeeze ← ⍜⍉≡⍜⍆⊛          # Squeeze (add a sort to inspect)
Draw    ← ∧⍜⊡⋅1⟜(˜↯0+1/↥) # Draw

HLines ← (
  Squeeze Parse D
  ⍆⊕(□⍆)⊛⊸≡⊢
  ∧◇(⍜⊡˜⍜(⊏|˙=)⊙˜∘⊃(⊢⊢|↘⊙⇡°⊟+0_1⊣⍉))
)

VLines ← (
  Squeeze Parse D
  ⍆⊕(□⍆)⊛⊸≡⊣
  ∧◇(⍜⊡˜⍜(⊏|˙=)⊙˜∘⊃(⊣⊣|↘⊙⇡°⊟+0_1⊢⍉))
)
DrawBorder ← ⍜⍉VLines HLines ˜↯0↯2+1/↥♭Squeeze Parse D # Draws full border

# DrawBorder Squeeze # running this shows these as key boundary points -> [219 121] [219 123]
# ⊏≡⌟˜⨂[[219 121] [219 123]]⊸Squeeze # which map to -> [[94985 48652][94985 50114]]
# leftmost -> only look left, rightmost-> only look right
# SO, now fill the shape to make this easier.
Fill ← (
  ⊙⊙1           # fill colour.
  Good ← =0⊙◌⊡⊢ # Needs filling. Simple edges-check.
  # Take first of list. If needs fill, do so and then add
  # its four neighbours into queue. Repeat until queue is empty.
  ⍢(⨬(↘1|◴⊂+A₂¤°⊂ ⊃(⋅∘|∘|⋅⋅⋅∘)◡(⍜(⊡|⋅∘)⊢))◡Good
  | >0⧻)
  ⋅⊙⋅
)
DrawBorder     # Comment out everything below here to view the boundary.
Fill [219_120] # Now fill that boundary ([2_1] for test)
Squeeze Parse D
# ([0 1] for test)
[219 123] #  [219 121] gave the wrong answer, so it's this.
≡⌞⊟       # couple this with every other point
# Extract the covered window of the array for each pair of corners.
# (Very probably doing this the hard way:-()
# Only keep those that are all 1.
▽⤚≡⌟(=1/×♭≡⌞⊏⊙⊏˜∘∩(↘⊙⇡°⊟+0_1⍆)°⊟⍉)
# Pick out our squeezed indices
⨂Squeeze Parse D
# Find out what the unsqueezed values were
˜⊏Parse D
# Find areas, return max.
/↥≡/×+1⌵≡/-