a1k0n · October 22, 2015 01:02 · Oct 22, 2015
diff --git a/ylogo.c b/ylogo.c
@@ -0,0 +1,75 @@
+#include <math.h>
+#include <stdio.h>
+#include <unistd.h>
+
+// Each character encodes an angle of a plane we are checking
+const char plane_angles[] = "O:85!fI,wfO8!yZfO8!f*hXK3&fO;:O;#hP;\"i[";
+// and these encode an offset from the origin s.t. (x, y) dot (cos(a), sin(a)) < offset
+const char plane_offsets[] = "<[\\]O=IKNAL;KNRbF8EbGEROQ@BSXXtG!#t3!^";
+
+// this table encodes the offsets within the above tables of each polygon
+const char polygon_offsets[] = "8<AFJPTX";
+
+int main() {
+  float scale = 0, velocity = 0;
+
+  for(int frame = 0; frame < 40; frame++) {
+
+    if (frame != 0) {
+      // after the first frame, bring the cursor up 25 lines
+      puts("\x1b[25A");
+    }
+
+    // scale is controlled by a simple underdamped spring system -- velocity is
+    // proportional to displacement, with a damping factor (velocity/4)
+    scale += velocity;
+    velocity += (1 - scale) / 10 - velocity / 4;
+
+    // j loops over rows of the image
+    for(int j = 0; j < 72; j += 3) {
+      // and i loops over columns
+      for(int i = 0; i < 73; i++) {
+        float x = scale * (i - 27);  // x coordinate in object, controlled by scale and i
+
+        // now we make three vertical samples of our object and put them in the
+        // low bits of our accumulator c
+        int c = 0;
+        for(int n = 2; n >= 0; n--) {
+          float y = scale * (j+n-36);    // y coordinate in object
+          int plane_idx = 0;
+          int inside_ellipse = (136*x*x+84*y*y<92033);
+          c ^= inside_ellipse << n;
+
+          // this single loop runs over all polygons and all planes within each
+          // polygon. it really should be two loops, but this enables us to
+          // break out early without a goto, and this hews closer to the
+          // original obfuscated code.
+          for (int polygon = 0; polygon < 8;) {
+            // plane_end_idx is the last plane in our polygon
+            int plane_end_idx = polygon_offsets[polygon] - 50;
+            float angle = (plane_angles[plane_idx]-79)/14.64;
+            float plane_offset = plane_offsets[plane_idx++] / 1.16 - 68;
+
+            if (plane_offset > x*cos(angle) + y*sin(angle)) {
+              // we are outside the polygon
+              // advance to end of polygon and compute end of the next one
+              plane_idx = plane_end_idx;
+              polygon++;
+            } else if (plane_idx == plane_end_idx) {
+              // we are inside all planes of our polygon, so toggle our pixel
+              // and early-out
+              c ^= 1<<n;
+              break;
+            }
+          }
+        }
+
+        // finally, c indexes into a predefined list of characters based on the
+        // presence of the object in the three positions
+        putchar(" ''\".$u$"[c]);
+      }
+      putchar('\n');
+    }
+    usleep(50000);
+  }
+}