//pre-existing code before this
tethered = false; //attached to a ladder?
climb_speed = 2;

//in addition to pre-existing code:

if place_meeting(x, y, objLadder)
{ 
  if (keyboard_check(vk_up) or keyboard_check(vk_down))
    { tethered = true; } 
} else { tethered = false; }

// add constant force of gravity to our desired vert speed:

if not tethered{
  vert = vert + grav;
} else {
  // tethered climbing:
  if (keyboard_check(vk_up)) vert = -climb_speed;
  if (keyboard_check(vk_down)) vert = +climb_speed;
  if (keyboard_check(vk_nokey)) vert = 0;
}

grav = 1; //our gravity
horiz = 0; //our desired horizontal speed
vert = 0; //our desired vertical speed
move_speed = 5;
jump_speed = -12;
frict = 0.95;
facing = 1; //start facing right
jumped = false; //controls jump animations
status = ""; //status of platform "stick"
tethered = false; //attached to a ladder?
climb_speed = 2;

// left and right movement inputs AND animations:

if (keyboard_check(vk_right)){ horiz = move_speed; facing = 1; };
if (keyboard_check(vk_left)){ horiz = -move_speed; facing = -1; };
if (horiz != 0 and not jumped)
  { sprite_index = sprWalk; }
if (keyboard_check(vk_nokey)){
	horiz = horiz * frict; //friction
	if (abs(horiz) < 1 and abs(vert) < 1) { sprite_index = sprStill };
}; 

if place_meeting(x, y, objLadder)
{ 
  if (keyboard_check(vk_up) or keyboard_check(vk_down))
    { tethered = true; } 
} else { tethered = false; }

// add constant force of gravity to our desired vert speed:

if not tethered{
  vert = vert + grav;
} else {
  // tethered climbing:
  if (keyboard_check(vk_up)) vert = -climb_speed;
  if (keyboard_check(vk_down)) vert = +climb_speed;
  if (keyboard_check(vk_nokey)) vert = 0;
}


// only ALLOW jump IF the objBlock is 1 pixel BELOW the player,
// or in other words, if the player is EXACTLY standing on floor:

if (place_meeting(x, y+1, objBlock) and keyboard_check(vk_up))
  { vert = jump_speed; jumped = true; } 
  
// jump animations:

if (jumped) {
	// up down animations:
	if (vert > 0) { sprite_index = sprJumpUp; } 
    else { sprite_index = sprLanding; }
	// reset jump IF on ground again:
	if (place_meeting(x, y+1, objBlock)) { jumped = false; }
}

// check for a HORIZONTAL collision:

if (place_meeting(x+horiz, y, objBlock))
{
    // move AS CLOSE AS WE CAN to the HORIZONTAL collision object:
	// sign() can only return 1, -1, or 0.
	// sign(-7) == -1 (i.e. negative 1)
	// sign(7) == 1 (or +1 or positive 1)
	// sign(0) == 0
	
    while (!place_meeting(x+sign(horiz), y, objBlock))
    {
        x = x + sign(horiz); // move 1 pixel at a time towards object
    }
	
    horiz = 0; // no left right movement on HORIZONTAL collisions 
}

// we have worked out our horizontal speed, now APPLY the change:

x = x + horiz;

// check for a VERTICAL collision:

if (place_meeting(x, y+vert, objBlock))
{
    // move AS CLOSE AS WE CAN to the VERTICAL collision object:
	
    while (!place_meeting(x, y+sign(vert), objBlock))
    {
		// remember, sign() can only return 1, -1, or 0:
        y = y + sign(vert); // // move 1 pixel at a time towards object
    }
	
    vert = 0; // no up down movement on VERTICAL collisions 
}

// we have worked out our vertical speed, now APPLY the change:

y = y + vert;