From 17fba238d10778b04fb7335300398a0a25f00648 Mon Sep 17 00:00:00 2001
From: Allanis <allanis@saracraft.net>
Date: Wed, 7 Aug 2013 12:07:58 +0100
Subject: [PATCH] [Add] Innitial attempt at a proper facing system, primarily
 for missiles.

---
 src/collision.c | 107 +++++++++++++++++++++++++++++++++++++++++++-----
 src/collision.h |   5 +++
 2 files changed, 102 insertions(+), 10 deletions(-)

diff --git a/src/collision.c b/src/collision.c
index e369340..e5097a1 100644
--- a/src/collision.c
+++ b/src/collision.c
@@ -2,16 +2,20 @@
 #include "log.h"
 #include "collision.h"
 
-/* Collide sprite at (asx, asy) int 'at' at pos 'ap' with sprite at (bsx,bsy) */
-/*in 'bt' at 'bp' */
-/* at   - Texture a. */
-/* asx  - Position of x of sprite a. */
-/* asy  - Position of y of sprite a. */
-/* ap   - Position in space of sprite a. */
-/* bt   - Texture b. */
-/* bsx  - Position of x of sprite b. */
-/* bsy  - Position of y of sprite b. */
-/* bp   - Position in space of sprite b. */
+/**
+ * @brief Collide sprite at (asx, asy) in 'at' at pos 'ap' with sprite at (bsx,bsy)
+ * in 'bt' at 'bp'
+ *
+ * @param[in] at   - Texture a.
+ * @param[in] asx  - Position of x of sprite a.
+ * @param[in] asy  - Position of y of sprite a.
+ * @param[in] ap   - Position in space of sprite a.
+ * @param[in] bt   - Texture b.
+ * @param[in] bsx  - Position of x of sprite b.
+ * @param[in] bsy  - Position of y of sprite b.
+ * @param[in] bp   - Position in space of sprite b.
+ * @return 1 on collision, 0 else.
+ */
 int CollideSprite(const glTexture* at, const int asx, const int asy,
                   const Vec2* ap, const glTexture* bt,
                   const int bsx, const int bsy, const Vec2* bp) {
@@ -65,3 +69,86 @@ int CollideSprite(const glTexture* at, const int asx, const int asy,
   return 0;
 }
 
+/**
+ * @brief Calculate collision path corrections.
+ *
+ * @param[in] p     - Position of object that's trying to collide.
+ * @param[in] v     - Velocity of object that's trying to collide.
+ * @param[in] tp    - Position of the target to collide into.
+ * @param[in] tv    - Velocity of the target to collide into.
+ * @param[in] limit - Accuracy limit.
+ * @return Direction object will need to adjust velocity to for collision.
+ */
+/*
+ * Doesn't work as expected, needs to be fixed to allow missiles to use
+ * the physics engine instead of hacking their velocities.
+ */
+#if 0
+double CollidePath(const Vec2* p, Vec2* v,
+    const Vec2* tp, Vec2* tv, double limit) {
+
+  double mx, my;        /* Position modifiers. */
+  double t, tt;         /* Time to target. */
+  Vec2   test, ttest;   /* Test vector and target test vector. */
+  double offset;        /* Direction to face and error it produces. */
+  double mod, moddir;   /* Modifier for iteration. */
+  int i;
+
+  /* Test vector. */
+  vect_cset(&test, tp->x - p->x, tp->y - p->y); /* Start byt straight line. */
+  t = VMOD(test) / VMOD(*v);                    /* d=v*t ==> t=d/v. */
+
+  /* Target test vector. */
+  vectnull(&ttest); /* Starting from itself. */
+  tt = VMOD(ttest) / VMOD(*tv);
+
+  /* Special case object isn't moving. */
+  if(VMOD(*v) < 1e-6)
+    return VANGLE(test);
+
+  /* Target faster than object. */
+  if(VMOD(*v) < VMOD(*tv)) {
+    vect_reflect(&test, v, tv);
+    return VANGLE(test);
+  }
+
+  /* Loop until error is minimal. */
+  offset = tt - t;
+  moddir = 1.;    /* Start off by position increments. */
+  mod    = 10.;   /* Start off by a 10 second jump. */
+  i = 0;
+  while(FABS(offset) > limit) {
+    if(i > 100) break;
+
+    /* Calculate position modifiers. */
+    if(offset < -mod/2.)  /* tt>t ==> major overshot. */
+      moddir = -1.;
+    else if(offset < 0.) { /* tt>t ==> overshot. */
+      /* Invert direction and half. */
+      moddir = -1.;
+      mod = mod/2.;
+    }
+    else if(offset > mod) /* tt<t ==> undershot. */
+      moddir = 1.; /* Make sure it's positive. */
+    else if(offset > 0.) { /* tt<t ==> minor undershot. */
+      /* Positive and shrink step. */
+      moddir = 1.;
+      mod = FABS(mod)/2.;
+    }
+    mx = tv->x * mod * moddir;
+    my = tv->y * mod * moddir;
+
+    /* Increment test vectors. */
+    vect_cadd(&test, mx, my);
+    vect_cadd(&ttest, mx, my);
+
+    /* Compare results. */
+    t = VMOD(test) / VMOD(*v);
+    tt = VMOD(ttest) / VMOD(*tv);
+    offset = tt - t;
+
+    i++;
+  }
+  return VANGLE(test);
+}
+#endif
diff --git a/src/collision.h b/src/collision.h
index 30a71fe..02a5388 100644
--- a/src/collision.h
+++ b/src/collision.h
@@ -2,7 +2,12 @@
 #include "opengl.h"
 #include "physics.h"
 
+/* Return 1 if collision is detected. */
 int CollideSprite(const glTexture* at, const int asx, const int asy,
                   const Vec2* ap, const glTexture* bt,
                   const int bsx, const int bsy, const Vec2* bp);
 
+/* Get direction to face the target. */
+/*double CollidePath(const Vec2* p, Vec2* v,
+    const Vec2* tp, Vec2* tv, const double limit);*/
+