add footprints2.scad

src/turtle/_impl/_footprints2.scad

@@ -0,0 +1,9 @@
+use <../turtle2d.scad>;
+
+function _footprints2(cmds, t, leng, i = 0) =
+    i == leng ? [] :
+        let(
+            nxt = turtle2d(cmds[i][0], t, cmds[i][1]),
+            pts = _footprints2(cmds, nxt, leng, i + 1)
+        )
+        cmds[i][0] != "forward" ? pts : concat([turtle2d("pt", nxt)], pts);

src/turtle/footprints2.scad

@@ -0,0 +1,56 @@
+use <turtle2d.scad>;
+use <_impl/_footprints2.scad>;
+
+/*
+    cmds: "turn", "forward"
+
+demo code
+```
+use <turtle/footprints2.scad>;
+use <hull_polyline2d.scad>;
+ 
+function arc_cmds(radius, angle, steps) = 
+    let(
+        fa = angle / steps,
+        ta = fa / 2,
+        leng = sin(ta) * radius * 2
+    )
+    concat(
+        [["turn", ta]],
+        [
+            for(i = [0:steps - 2])
+            each [["forward", leng], ["turn", fa]]
+        ],
+        [["forward", leng], ["turn", ta]]
+    );
+    
+poly = footprints2(
+    concat(
+        [
+            ["forward", 10],
+            ["turn", 90],
+            ["forward", 10] 
+        ], 
+        arc_cmds(5, 180, 12),
+        [
+            ["turn", -90],
+            ["forward", 10],
+            ["turn", 90],
+            ["forward", 10],
+            ["turn", 90],
+            ["forward", 10]
+        ]
+    ),
+    start = [10, 10],
+    angle = 90
+);
+
+hull_polyline2d(poly, width = 1);
+```
+*/
+function footprints2(cmds, start = [0, 0], angle = 0) = 
+    let(
+        t = turtle2d("create", start[0], start[1], angle),
+        leng = len(cmds)
+    )
+    concat([turtle2d("pt", t)], _footprints2(cmds, t, leng));