ABH: silent auto-ABH on the jump tick

- on landing with jump held, face opposite the horizontal velocity and zero
  forward/side movement for that tick; the HL2 jump speed-bound subtraction
  then accelerates along the velocity (the classic no-input ABH exploit)
- works standalone or with bhop; bhop's auto-strafe still applies in air
  when enabled
- rendered view is restored at FRAME_RENDER_START via the existing hook, so
  the camera does not flip around

1 files changed, 21 insertions, 3 deletions

diff --git a/src/hl2h.cpp b/src/hl2h.cpp
index 0491fd9..f3aa26f 100644
--- a/src/hl2h.cpp
+++ b/src/hl2h.cpp
@@ -210,10 +210,11 @@ static void do_move(int seq) {
   bool inwater = (flags & FL_INWATER) != 0;
 
   bool bhop_on = feature_active(FEAT_BHOP);
+  bool abh_on = feature_active(FEAT_ABH);
 
   static bool lastground = false;
 
-  if (bhop_on && (cmd->buttons & IN_JUMP)) {
+  if ((bhop_on || abh_on) && (cmd->buttons & IN_JUMP)) {
     if (inwater && (cmd->buttons & IN_DUCK)) {
       lastground = onground;
       return;
@@ -233,13 +234,30 @@ static void do_move(int seq) {
     if (!onground) {
       lastground = false;
       cmd->buttons &= ~IN_JUMP;
-      if (!inwater && bhop_autostrafe_enabled())
+      if (bhop_on && !inwater && bhop_autostrafe_enabled())
         apply_autostrafe(cmd, vel);
       note_modified(cmd, flags, vel);
     } else if (!lastground) {
       lastground = true;
-      if (!backmove)
+      if (abh_on) {
+        // ABH jump tick: face opposite the horizontal velocity, hold no
+        // directional keys. The jump speed-bound subtraction then flips into
+        // acceleration along the velocity. The rendered view is restored at
+        // FRAME_RENDER_START so the camera does not spin around.
+        float speed2d = sqrtf(vel[0] * vel[0] + vel[1] * vel[1]);
+        if (speed2d > 1.0f) {
+          float vyaw = atan2f(vel[1], vel[0]) * 180.0f / PI_F;
+          float fake = vyaw + 180.0f;
+          while (fake > 180.0f) fake -= 360.0f;
+          while (fake < -180.0f) fake += 360.0f;
+          cmd->viewangles[1] = fake;
+          cmd->forwardmove = 0.0f;
+          cmd->sidemove = 0.0f;
+          set_silent_restore(QAngle{g_cam_view.x, g_cam_view.y, g_cam_view.z});
+        }
+      } else if (!backmove) {
         cmd->forwardmove = -0.1f;
+      }
       note_modified(cmd, flags, vel);
     }
   }