Block 5100
( turtle.4th -- limited LOGO-style turtle graphics. Cookbook  )
( demo capsule. Built on capsules/fabric.4th's LINE and       )
( Q48.16 Q.SIN/Q.COS -- pure FORTH, no new C words. Load      )
( fabric.4th first: uses LINE, FB-WIDTH, FB-HEIGHT, PLOT,     )
( Q.SIN, Q.COS, Q.FROM-INT, Q.TO-INT, Q.*, Q.+.               )
1144 CONSTANT DEG2RAD  ( Q48.16 rad/deg, pi/180 ~= 0.0174533 )
VARIABLE TX  VARIABLE TY  VARIABLE THEAD
VARIABLE TPEN  VARIABLE TCOLOR

Block 5101
( HOME: turtle to screen center, heading 0 (east), pen down, )
( color white. Does not clear the screen -- see CS for that. )
: HOME ( -- )
  FB-WIDTH 2/ TX !
  FB-HEIGHT 2/ TY !
  0 THEAD !
  1 TPEN !
  16777215 TCOLOR ! ;

Block 5102
( PENUP / PENDOWN: whether FORWARD/BACK draw while moving.    )
( SETCOLOR: 24-bit 0xRRGGBB, matches PLOT's own format.       )
( SETXY: jump to (x y) without drawing, any pen state.        )
( SETHEADING: absolute heading in degrees, 0=east, CCW+.      )
: PENUP ( -- )      0 TPEN ! ;
: PENDOWN ( -- )    1 TPEN ! ;
: SETCOLOR ( color -- )  TCOLOR ! ;
: SETXY ( x y -- )  TY ! TX ! ;
: SETHEADING ( deg -- )  Q.FROM-INT DEG2RAD Q.* THEAD ! ;

Block 5103
( FORWARD / BACK: move n pixels along current heading        )
( (THEAD, Q48.16 radians), drawing a LINE from old to new     )
( position if pen is down. z is always 0 -- turtle graphics   )
( is flat; fabric.4th's LINE just carries z through for the   )
( 3D case it also supports.                                   )
VARIABLE TNX  VARIABLE TNY
: FORWARD ( n -- )
  DUP Q.FROM-INT THEAD @ Q.COS Q.* Q.TO-INT TX @ + TNX !
  Q.FROM-INT THEAD @ Q.SIN Q.* Q.TO-INT TY @ + TNY !
  TPEN @ IF
    TX @ TY @ 0 TNX @ TNY @ 0 TCOLOR @ LINE
  THEN
  TNX @ TX !  TNY @ TY ! ;
: BACK ( n -- )  NEGATE FORWARD ;

Block 5104
( LEFT / RIGHT: turn in place by n degrees, CCW / CW.         )
: LEFT ( deg -- )
  Q.FROM-INT DEG2RAD Q.* THEAD @ Q.+ THEAD ! ;
: RIGHT ( deg -- )  NEGATE LEFT ;

Block 5105
( CS: clear the framebuffer to black. framebuffer_words.c     )
( only exposes PLOT (no fill primitive), so this is a plain   )
( nested PLOT loop -- slow under TCG for a full screen, fine  )
( for a one-shot demo clear, not meant to run per-frame.      )
: CS ( -- )
  FB-HEIGHT 0 DO
    FB-WIDTH 0 DO
      I J 0 PLOT
    LOOP
  LOOP ;

Block 5106
( POLYGON: regular polygon, SIDES corners, LEN pixels per     )
( edge, drawn from the turtle's current position/heading      )
( (call HOME first for a clean start). Classic LOGO idiom:     )
( forward + turn 360/sides, repeated SIDES times.              )
VARIABLE PLEN
: POLYGON ( sides len -- )
  PLEN !
  DUP 0 DO
    PLEN @ FORWARD
    DUP 360 SWAP / LEFT
  LOOP
  DROP ;

Block 5107
( STAR: self-intersecting 5-pointed LOGO star -- forward +    )
( a 144-degree turn (not 360/5=72, a plain pentagon), 5x.      )
: STAR ( len -- )
  DUP 5 0 DO
    DUP FORWARD
    144 LEFT
  LOOP
  DROP ;

Block 5108
( TURTLE-DEMO: visual smoke test -- clear screen, a cyan      )
( hexagon, then a magenta star from the same start point.     )
( Run after loading: S" turtle.4th" EXEC  TURTLE-DEMO          )
: TURTLE-DEMO ( -- )
  CS
  HOME  65535 SETCOLOR     ( cyan  0x00FFFF )
  6 80 POLYGON
  HOME  16711935 SETCOLOR  ( magenta 0xFF00FF )
  100 STAR ;