tasks a-s

Task/Brownian-tree/D/brownian-tree.d

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+import core.stdc.stdio, std.random, grayscale_image;
+
+enum uint side = 600; // Square world side.
+enum uint num_particles = 1_000;
+static assert(side > 2 && num_particles < (side ^^ 2 * 0.7));
+
+void main() {
+    auto rng = Xorshift(unpredictableSeed);
+    ubyte[side][side] W; // World.
+    W[side / 2][side / 2] = 1; // Set tree root.
+
+    foreach (immutable _; 0 .. num_particles) {
+        // Random initial particle position.
+        OVER: uint x, y;
+        do {
+            x = uniform(1, side - 1, rng);
+            y = uniform(1, side - 1, rng);
+        } while (W[y][x]); // Assure the chosen cell is empty.
+
+        while (W[y-1][x-1] + W[y-1][x] + W[y-1][x+1] +
+               W[y][x-1]               + W[y][x+1] +
+               W[y+1][x-1] + W[y+1][x] + W[y+1][x+1] == 0) {
+            // Randomly choose a direction (Moore neighborhood).
+            uint dxy = uniform(0, 8, rng);
+            if (dxy > 3) dxy++; // To avoid the center.
+            x += (dxy % 3) - 1;
+            y += (dxy / 3) - 1;
+            if (x < 1 || x >= side - 1 || y < 1 || y >= side - 1)
+                goto OVER;
+        }
+
+        W[y][x] = 1; // Touched, set the cell.
+    }
+
+    ubyte[] data = (&W[0][0])[0 .. side ^^ 2]; // Flat view.
+    data[] += 255;
+    Image!ubyte.fromData(data, side,side).savePGM("brownian_tree.pgm");
+}