add slice

src/util/slice.scad

@@ -0,0 +1,4 @@
+function slice(lt, begin, end) = 
+    let(ed = is_undef(end) ? len(lt) : end)
+    [for(i = begin; i < ed; i = i + 1) lt[i]];
+    

test/test_all.scad

@@ -67,6 +67,7 @@ include <test_path_extrude.scad>;
 include <util/test_sub_str.scad>;
 include <util/test_split_str.scad>;
 include <util/test_parse_number.scad>;
+include <util/test_slice.scad>;
 
 // Pixel
 include <pixel/test_px_line.scad>;

test/util/test_slice.scad

@@ -0,0 +1,15 @@
+module test_slice() {
+    echo("==== test_slice ====");
+
+    include <unittest.scad>;
+    include <util/slice.scad>;
+
+    lt = [for(c = "helloworld") c];
+    expected1 = [for(c = "hello") c];
+    expected2 = [for(c = "world") c];
+
+    assert(expected1 == slice(lt, 0, 5)); 
+    assert(expected2 == slice(lt, 5)); 
+}
+
+test_slice();