Add reaction-diffusion in BQN, with video generation script

.gitignore

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 *\~
+out/

generate_animation.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+mkdir -p out
+bqn grayscott.bqn
+# first image is often bad
+rm out/step00000.pnm
+# convert all to PNG
+parallel --jobs 8 "pnmtopng {} > {.}.png" ::: out/*.pnm
+# generate video
+ffmpeg -y -framerate 30 -pattern_type glob -i 'out/*.png' -r 30 -vf scale=500:-1 -loop 0 out.mp4

grayscott.bqn

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+⟨PBM,PPM⟩←•Import"../bqn-graphics/pnm.bqn"
+⟨Inferno,Magma,Viridis⟩←•Import"../bqn-graphics/colormaps.bqn"
+
+# Parameters
+dt‿da‿db‿f‿k←⟨1.0,0.16,0.08,0.060,0.062⟩
+n←500  # image size
+m←10  # seed size at the center of the image
+
+# Discrete Laplacian
+Lapl←(+´∘∾1‿¯1(⌽⋈⌽˘)¨<)-4⊸×
+# Gray-Scott update
+UpdateA←{𝕨+dt×(da×Lapl 𝕨)+(-𝕨×𝕩⋆2)+f×1-𝕨}
+UpdateB←{𝕩+dt×(db×Lapl 𝕩)+(𝕨×𝕩⋆2)-𝕩×k+f}
+Update←(UpdateA⋈UpdateB)´
+
+# Initialization
+a←n‿n⥊1
+Mask←{𝕩‿𝕩↑(-𝕨‿𝕨+𝕩÷2)↑(2×𝕨‿𝕨)⥊1}
+b←(m Mask n)⌈0.1×n‿n•rand.Range 0
+
+# Run
+Step←{stepsize 𝕊 i‿a‿b:
+  •term.OutRaw"."
+  •term.Flush @
+  ap‿bp←Update⍟stepsize a‿b
+  ("out/step"∾(¯5↑∾•Fmt¨5-⊸↑i)∾".pnm")•file.Chars (PPM ⌊255×>Inferno¨1-a)∾@+10
+  (i+1)‿ap‿bp
+}
+
+50 Step•_while_(600⊸>∘⊑⊢) 0‿a‿b
+•Out""