From 41ecae1291f54718cb4049154cd7f398bfc2a870 Mon Sep 17 00:00:00 2001
From: Juergen Stuber <juergen@jstuber.net>
Date: Wed, 15 Apr 2026 12:08:33 +0200
Subject: [PATCH] Add new app phase-change2

---
 src/bin/phase-change2/main.rs | 167 ++++++++++++++++++++++++++++++++++
 1 file changed, 167 insertions(+)
 create mode 100644 src/bin/phase-change2/main.rs

diff --git a/src/bin/phase-change2/main.rs b/src/bin/phase-change2/main.rs
new file mode 100644
index 0000000..3292f4f
--- /dev/null
+++ b/src/bin/phase-change2/main.rs
@@ -0,0 +1,167 @@
+use std::env::args;
+use std::io::stdout;
+use std::io::Write;
+use std::thread::sleep;
+use std::time::Duration;
+
+use rand::rng;
+use rand::Rng;
+
+use lowdim::bb2d;
+use lowdim::p2d;
+use lowdim::Array2d;
+use lowdim::BBox2d;
+use lowdim::Point2d;
+
+use pixelfoo_apps::color::Color;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum Square {
+    Empty,
+    Full,
+}
+
+#[derive(Clone, Debug)]
+struct Board(Array2d<i64, Square>);
+impl Board {
+    pub fn new(bbox: BBox2d, mut f: impl FnMut() -> Square) -> Board {
+        Board(Array2d::with(bbox, |_p| f()))
+    }
+    fn bbox(&self) -> BBox2d {
+        self.0.bbox()
+    }
+    fn get(&self, pos: Point2d) -> Square {
+        self.0[pos]
+    }
+    fn set(&mut self, pos: Point2d, sq: Square) {
+        self.0[pos] = sq;
+    }
+}
+
+fn send<T: Write>(w: &mut T, board: &Board) -> std::io::Result<()> {
+    for y in board.bbox().y_range() {
+        for x in board.bbox().x_range() {
+            let square = board.0[p2d(x, y)];
+            let c = if (x + y) % 2 == 0 {
+                match square {
+                    Square::Empty => Color::new(0, 0, 128),
+                    Square::Full => Color::new(220, 180, 80),
+                }
+            } else {
+                match square {
+                    Square::Empty => Color::new(0, 128, 0),
+                    Square::Full => Color::new(220, 80, 180),
+                }
+            };
+            w.write_all(&c.rgb())?;
+        }
+    }
+    w.flush()
+}
+
+const DEFAULT_ARG: usize = 60;
+
+fn main() -> std::io::Result<()> {
+    let args = args().collect::<Vec<_>>();
+    eprintln!("executing {}", args[0]);
+
+    let x_size = args[1].parse::<i64>().unwrap();
+    let y_size = args[2].parse::<i64>().unwrap();
+    let arg = if let Some(s) = args.get(3) {
+        s.parse::<usize>().unwrap_or(DEFAULT_ARG)
+    } else {
+        DEFAULT_ARG
+    };
+    eprintln!("screen size {}x{}, arg {}", x_size, y_size, arg);
+
+    let mut rng = rng();
+    let bbox = bb2d(0..x_size, 0..y_size);
+
+    let p_full = 0.5;
+    let mut board = Board::new(bbox, || {
+        let p = rng.random::<f64>();
+        if p < p_full {
+            Square::Full
+        } else {
+            Square::Empty
+        }
+    });
+
+    let t_frame = 0.040; // s
+    let delay = Duration::new(0, (1_000_000_000.0 * t_frame) as u32);
+
+    let actions_per_frame = 200;
+
+    // Temperature
+    let temp_min = 0.0;
+    let temp_max = 1.5;
+
+    // Period for temperature cycle in seconds.
+    let t_period = arg as f64;
+
+    // Temperature change per frame.
+    let d_temp = 2.0 * (temp_max - temp_min) / (t_period / t_frame);
+
+    let mut temp = temp_max;
+    let mut heating = false;
+    loop {
+        if heating && temp > temp_max {
+            heating = false;
+        } else if !heating && temp < temp_min {
+            heating = true;
+        }
+
+        if heating {
+            temp += d_temp;
+        } else {
+            temp -= d_temp;
+        }
+
+        for _ in 0..actions_per_frame {
+            let x0 = rng.random_range(bbox.x_range());
+            let y0 = rng.random_range(bbox.y_range());
+            let pos0 = p2d(x0, y0);
+
+            let x1 = rng.random_range(bbox.x_range());
+            let y1 = rng.random_range(bbox.y_range());
+            let pos1 = p2d(x1, y1);
+
+            let sq0 = board.get(pos0);
+            let sq1 = board.get(pos1);
+            if sq0 != sq1 {
+                let count0 = pos0
+                    .neighbors_l1()
+                    .filter(|&np| bbox.contains(&np) && board.get(np) == Square::Full)
+                    .count() as i8;
+                let count1 = pos1
+                    .neighbors_l1()
+                    .filter(|&np| bbox.contains(&np) && board.get(np) == Square::Full)
+                    .count() as i8;
+
+                // The energy change if we swap `sq0` and `sq1`.
+                // For each pair of orthogonally adjacent full squares
+                // we count `-1` for the total energy.
+                let delta = -if sq0 == Square::Full {
+                    count1 - count0
+                } else {
+                    count0 - count1
+                } as f64;
+
+                let q = (delta / temp).exp();
+                let p = q / (1.0 + q);
+
+                if rng.random::<f64>() < p {
+                    // Actually do the swap.
+                    board.set(pos1, sq0);
+                    board.set(pos0, sq1);
+                }
+            }
+        }
+
+        let mut buf = Vec::with_capacity(bbox.area() * 3);
+        send(&mut buf, &board)?;
+        stdout().write_all(&buf)?;
+        stdout().flush()?;
+        sleep(delay);
+    }
+}