added a simple boid program: will clean up later

.gitignore

@@ -2,4 +2,5 @@
 *.tmp
 c/scream
 c/timer
+c/boid
 c/a.out

Makefile

@@ -27,6 +27,7 @@ sh:
 mkc:
 	cc c/scream.c -o c/scream
 	cc c/timer.c -o c/timer
+	cc c/boid.c -o c/boid -lm -lX11
 c:
 	cp -f c/scream $(DESTDIR)$(PREFIX)/bin
 	cp -f c/timer $(DESTDIR)$(PREFIX)/bin

c/boid.c

@@ -0,0 +1,224 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <math.h>
+#include <time.h>
+
+#include <X11/Xlib.h>
+
+#define MIN(X, Y) (((X) < (Y)) ? (X) : (Y))
+
+const double sprite[][2] = {
+  {0, 1},
+  {-0.5, -1},
+  {0, -0.5},
+  {0.5, -1},
+};
+#define SIZE sizeof sprite / sizeof sprite[0]
+
+double buffer[SIZE][2];
+int boidcounter = 0;
+
+Display *d;
+Window w;
+XEvent e;
+int s;
+GC gc;
+
+typedef struct point {
+  double x;
+  double y;
+} point;
+
+typedef struct boid {
+  int id;
+  point *p;
+  double rot;
+  struct boid *next;
+} boid;
+
+void xinit(void) {
+  d = XOpenDisplay(NULL);
+  s = DefaultScreen(d);
+  w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 100, 100, 1,
+      BlackPixel(d, s), WhitePixel(d, s));
+  XSelectInput(d, w, ExposureMask | KeyPressMask | PointerMotionMask);
+  XMapWindow(d, w);
+  gc = XCreateGC(d, w, 0, NULL);
+}
+
+int sign(double x) {
+  return (x > 0) - (x < 0);
+}
+
+point *rotate(double x, double y, double rot) {
+  point *ret = malloc(sizeof ret);
+  double rad = rot * M_PI/180;
+
+  ret->x = x * cos(rad) - y * sin(rad);
+  ret->y = x * sin(rad) + y * cos(rad);
+
+  return ret;
+}
+
+double distance(point *a, point *b) {
+  return sqrt(pow(a->x - b->x, 2) + pow(a->y - b->y, 2));
+}
+
+void calculateRender(
+  double x,
+  double y,
+  double rot,
+  int scale
+) {
+  memcpy(&buffer, &sprite, SIZE);
+
+  for(int i = 0; i < SIZE; i++) {
+    point *p = rotate(sprite[i][0], sprite[i][1], rot);
+
+    buffer[i][0] = p->x * scale + x;
+    buffer[i][1] = p->y * scale + y;
+
+    free(p);
+  }
+}
+
+void renderBuffer(void) {
+  for(int i = 0; i < SIZE - 1; i++) {
+    XDrawLine(d, w, gc,
+        buffer[i][0], buffer[i][1],
+        buffer[i + 1][0], buffer[i + 1][1]);
+  }
+  XDrawLine(d, w, gc,
+      buffer[0][0], buffer[0][1],
+      buffer[SIZE - 1][0], buffer[SIZE - 1][1]);
+}
+
+void renderBoid(boid *boid) {
+  calculateRender(
+    boid->p->x,
+    boid->p->y,
+    boid->rot,
+    5
+  );
+  renderBuffer();
+}
+
+boid *mkBoid(double x, double y, double rot) {
+  boid *b = malloc(sizeof b);
+  point *p = malloc(sizeof p);
+  b->p = p;
+
+  b->p->x = x;
+  b->p->y = y;
+  b->rot = rot;
+  b->next = NULL;
+  b->id = boidcounter++;
+
+  return b;
+}
+
+void appendBoid(boid *destination, boid *source) {
+  destination->next = source;
+}
+
+double averageHeading(boid *b) {
+  boid *c = b;
+  double sum;
+  int count;
+  while(c) {
+    sum += c->rot;
+    count++;
+    c = c->next;
+  }
+  return sum / count;
+}
+
+void randomBoids(boid *b, int num) {
+  srand(time(0));
+  boid *ptr = b;
+  for(int i = 0; i < num; i++) {
+    int w = rand() % DisplayWidth(d, s) + 1;
+    int h = rand() % DisplayHeight(d, s) + 1;
+    int deg = rand() % 360;
+    appendBoid(ptr, mkBoid(w, h, deg));
+    ptr = ptr->next;
+  }
+}
+
+void updateBoid(boid *b, boid *chain, int id, double average) {
+  point *p = rotate(5, 0, b->rot + 90);
+  b->p->x = b->p->x + p->x;
+  b->p->y = b->p->y + p->y;
+  double toTurn = 0;
+
+  boid *c = chain;
+  while(c) {
+    if(c->id != id) {
+//      printf("boid %i is being pinged by boid %i\n", c->id, b->id);
+//      printf("boid %i is %f units away from boid %i\n", c->id,
+//        distance(c->p, b->p), b->id);
+      int dist = distance(c->p, b->p);
+      if(dist < 50) toTurn += dist / 75;
+    }
+    int w = DisplayWidth(d, s);
+    int h = DisplayHeight(d, s);
+    int centerw = w / 2 - b->p->x;
+    int centerh = h / 2 - b->p->y;
+    double deg = atan((double)centerh / (double)centerw) * 180/M_PI;
+//    printf("%f %i %i %i: degrees\n", deg, centerw, centerh, sign(deg));
+    toTurn += sign(deg) * 0.5;
+    toTurn += (average - c->rot) * 0.4;
+    if(b->p->x > w) b->p->x = 0;
+    else if(b->p->x < 0) b->p->x = w;
+    if(b->p->y > h) b->p->y = 0;
+    else if(b->p->y < 0) b->p->y = h;
+
+    toTurn = MIN(toTurn / 4, 30);
+
+    b->rot += toTurn;
+    c = c->next;
+  }
+
+  free(p);
+}
+
+int main() {
+  xinit();
+  boid *b = mkBoid(100, 100, 0);
+  randomBoids(b, 100);
+  while(1) {
+/*
+    while(XPending(d)) {
+      XNextEvent(d, &e);
+      if(e.type == MotionNotify) {
+        XClearWindow(d, w);
+        boid *ptr = b;
+        while(ptr) {
+          updateBoid(ptr, b, ptr->id, averageHeading(b));
+          renderBoid(ptr);
+          ptr = ptr->next;
+        }
+      }
+    }
+*/
+    XClearWindow(d, w);
+    boid *ptr = b;
+    while(ptr) {
+      updateBoid(ptr, b, ptr->id, averageHeading(b));
+      renderBoid(ptr);
+      XFlush(d);
+      ptr = ptr->next;
+    }
+    while(XPending(d)) {
+      XNextEvent(d, &e);
+      switch(e.type) {
+        case KeyPress:
+          break;
+      }
+    }
+    usleep(50000);
+  }
+  return 0;
+}