Move Along Points

// =====================================================================
//  Move Along Points
//  skimi3d script library
//  https://grid.skimi3d.com/scripts/move-along-points
//
//  Give it a list of places and it travels between them: once, round
//  and round, or there and back. Smoothly, at a speed you set in metres
//  a second, and carrying anybody sitting on it.
//
//  This is the one to reach for when Spin It is not enough, because
//  this really moves the object. A ferry, a lift, a patrolling airship,
//  a cloud crossing a valley, a door too big to hinge.
//
//  Plain LSL. It uses keyframed motion, which the region works out
//  properly rather than the viewers guessing at it.
// =====================================================================


// ---------------------------------------------------------------------
//  Settings
// ---------------------------------------------------------------------

// The places it visits, in order. By default these are read as steps
// away from wherever you rez the object, along the region's own axes:
// X is east, Y is north, Z is up. That way the same build works
// anywhere you drop it.
//
// The example below walks a ten by eight rectangle on the flat.
list POINTS = [
    < 0.0,  0.0, 0.0>,
    < 0.0, 10.0, 0.0>,
    < 8.0, 10.0, 0.0>,
    < 8.0,  0.0, 0.0>
];

// Read POINTS as real region coordinates instead of as steps from where
// it stands. Useful when you have copied positions out of the build
// tool, but the object then only works in that one spot.
integer ABSOLUTE = FALSE;

// How fast it travels, in metres a second. Walking pace is about 1.5,
// a running avatar about 5, a slow boat about 3.
float SPEED = 1.5;

// 0 = go once and stop
// 1 = round and round for ever
// 2 = there and back, for ever
integer MODE = 1;

// Seconds to wait at every place before moving on. 0.0 means it never
// stops. Anything under a tenth of a second is treated as no wait.
float WAIT_AT_EACH = 0.0;

// Turn so that it points the way it is going. Leave this off for a
// lift or a cloud; turn it on for a boat, a cart or a bird.
integer TURN_TO_FACE = FALSE;

// Which way the front of your build points, in its own body. The
// default is its X. If your boat sails sideways, this is the line.
vector NOSE = <1.0, 0.0, 0.0>;

// Should it set off as soon as it is rezzed?
integer START_MOVING = TRUE;

// Touch to stop it, touch again to send it on.
integer TOUCH_TO_TOGGLE = TRUE;

// Who may touch it. 0 = everyone, 1 = only the owner, 2 = only people
// wearing the same group tag.
integer WHO_MAY_TOUCH = 0;


// ---------------------------------------------------------------------
//  Everything below here is the machinery
// ---------------------------------------------------------------------

// Nothing shorter than this counts as a journey. Two points a
// millimetre apart would otherwise ask for a frame lasting no time at
// all, and the region has to divide by that.
float MIN_LEG = 0.01;

// The shortest a frame may last. Keyframed motion measures in whole
// milliseconds and a frame of nothing confuses it.
float MIN_TIME = 0.1;

integer gGoing = FALSE;


// Build the whole journey as a list of steps, because that is what
// keyframed motion wants: not "go to here" but "go this far from where
// you are".
list plan()
{
    vector   here = llGetPos();
    rotation face = llGetRot();

    // The settings first become real places, once, so that the rest of
    // this only ever subtracts one place from another.
    list places = [];
    integer n = llGetListLength(POINTS);
    integer i;

    for (i = 0; i < n; ++i)
    {
        vector p = llList2Vector(POINTS, i);
        if (!ABSOLUTE) p = here + p;
        places += [p];
    }

    // ROUND AND ROUND HAS TO COME HOME, AND THIS IS WHY.
    //
    // A loop does not replay the same places. It replays the same
    // STEPS, starting from wherever the last lap happened to end. If
    // the steps do not add up to nothing, the object is somewhere new
    // at the end of every lap, and it walks off across the region a
    // little further each time. It is a slow, baffling drift, and by
    // the time anybody notices, the thing is three regions away.
    //
    // So a loop gets a closing step home, added here rather than left
    // for you to remember.
    if (MODE == 1) places += [here];

    list frames = [];
    vector from = here;
    integer count = llGetListLength(places);

    for (i = 0; i < count; ++i)
    {
        vector to   = llList2Vector(places, i);
        vector step = to - from;
        float  len  = llVecMag(step);

        if (len >= MIN_LEG)
        {
            float t = len / SPEED;
            if (t < MIN_TIME) t = MIN_TIME;

            if (TURN_TO_FACE)
            {
                // Point the nose along this leg. The turn is spread
                // over the leg, so it swings round as it travels
                // rather than pivoting at the corner.
                rotation want = llRotBetween(llVecNorm(NOSE), llVecNorm(step));

                // Keyframes carry the turn from the last one, so what
                // goes in the list is the difference, not the heading.
                frames += [step, llRotInverse(face) * want, t];
                face = want;
            }
            else
            {
                frames += [step, ZERO_ROTATION, t];
            }

            from = to;
        }

        if (WAIT_AT_EACH >= MIN_TIME)
        {
            // A wait is a step of no distance that still takes time.
            frames += [ZERO_VECTOR, ZERO_ROTATION, WAIT_AT_EACH];
        }
    }

    return frames;
}


go(integer on)
{
    if (!on)
    {
        llSetKeyframedMotion([], [KFM_COMMAND, KFM_CMD_STOP]);
        gGoing = FALSE;
        return;
    }

    // Two things make keyframed motion do nothing at all, without a
    // word of complaint. Both are worth saying out loud, because
    // silence here costs an hour.
    if (llGetStatus(STATUS_PHYSICS))
    {
        llOwnerSay("I cannot move: this object is physical, and keyframed motion only works on things that are not. Turn Physical off in the build tool.");
        return;
    }
    if (llGetAttached())
    {
        llOwnerSay("I cannot move: keyframed motion does nothing while I am worn.");
        return;
    }

    list frames = plan();
    if (llGetListLength(frames) == 0)
    {
        llOwnerSay("I have nowhere to go: every place in POINTS is where I already am.");
        return;
    }

    integer mode = KFM_FORWARD;
    if (MODE == 1) mode = KFM_LOOP;
    if (MODE == 2) mode = KFM_PING_PONG;

    llSetKeyframedMotion(frames, [KFM_MODE, mode]);
    gGoing = TRUE;
}


integer mayTouch(key who)
{
    if (WHO_MAY_TOUCH == 1) return who == llGetOwner();
    if (WHO_MAY_TOUCH == 2) return llSameGroup(who);
    return TRUE;
}


default
{
    state_entry()
    {
        // The journey is worked out from where the object is standing
        // now, so it has to be planned after every move, not once and
        // for ever.
        go(START_MOVING);
    }

    touch_start(integer total)
    {
        if (!TOUCH_TO_TOGGLE) return;

        key who = llDetectedKey(0);
        if (!mayTouch(who))
        {
            llRegionSayTo(who, 0, "This one is not yours to stop.");
            return;
        }

        go(!gGoing);
    }

    on_rez(integer start)
    {
        // A rezzed copy stands somewhere new, so the plan it was
        // carrying points at the old place. Draw it again.
        llResetScript();
    }

    changed(integer what)
    {
        // After a region restart the object is back at its start but
        // the motion is not. Plan again from where it really is.
        if (what & CHANGED_REGION_START) llResetScript();
    }
}