Replace most trivial uses of slice() with faster list_head() and list_tail()

arrays.scad

@@ -142,18 +142,59 @@ function last(list) =
     list[len(list)-1];
 
 
-// Function: delete_last()
+// Function: list_head()
 // Usage:
-//   list = delete_last(list);
+//   list = list_head(list,<to>);
 // Topics: List Handling
-// See Also: select(), slice(), subindex(), last()
+// See Also: select(), slice(), list_tail(), last()
 // Description:
-//   Returns a list with all but the last entry from the input list.  If input is empty, returns empty list.
-// Example:
-//   nlist = delete_last(["foo", "bar", "baz"]);  // Returns: ["foo", "bar"]
-function delete_last(list) =
+//   Returns the head of the given list, from the first item up until the `to` index, inclusive.
+//   If the `to` index is negative, then the length of the list is added to it, such that
+//   `-1` is the last list item.  `-2` is the second from last.  `-3` is third from last, etc.
+//   If the list is shorter than the given index, then the full list is returned.
+// Arguments:
+//   list = The list to get the head of.
+//   to = The last index to include.  If negative, adds the list length to it.  ie: -1 is the last list item.
+// Examples:
+//   hlist = list_head(["foo", "bar", "baz"]);  // Returns: ["foo", "bar"]
+//   hlist = list_head(["foo", "bar", "baz"], -3); // Returns: ["foo"]
+//   hlist = list_head(["foo", "bar", "baz"], 2);  // Returns: ["foo","bar"]
+//   hlist = list_head(["foo", "bar", "baz"], -5); // Returns: []
+//   hlist = list_head(["foo", "bar", "baz"], 5);  // Returns: ["foo","bar","baz"]
+function list_head(list, to=-2) =
    assert(is_list(list))
-   list==[] ? [] : slice(list,0,-2);
+   assert(is_finite(to))
+   to<0? [for (i=[0:1:len(list)+to]) list[i]] :
+   to<len(list)? [for (i=[0:1:to]) list[i]] :
+   list;
+
+
+// Function: list_tail()
+// Usage:
+//   list = list_tail(list,<from>);
+// Topics: List Handling
+// See Also: select(), slice(), list_tail(), last()
+// Description:
+//   Returns the tail of the given list, from the `from` index up until the end of the list, inclusive.
+//   If the `from` index is negative, then the length of the list is added to it, such that
+//   `-1` is the last list item.  `-2` is the second from last.  `-3` is third from last, etc.
+//   If you want it to return the last three items of the list, use `from=-3`.
+// Arguments:
+//   list = The list to get the tail of.
+//   from = The first index to include.  If negative, adds the list length to it.  ie: -1 is the last list item.
+// Examples:
+//   tlist = list_tail(["foo", "bar", "baz"]);  // Returns: ["bar", "baz"]
+//   tlist = list_tail(["foo", "bar", "baz"], -1); // Returns: ["baz"]
+//   tlist = list_tail(["foo", "bar", "baz"], 2);  // Returns: ["baz"]
+//   tlist = list_tail(["foo", "bar", "baz"], -5); // Returns: ["foo","bar","baz"]
+//   tlist = list_tail(["foo", "bar", "baz"], 5);  // Returns: []
+function list_tail(list, from=1) =
+   assert(is_list(list))
+   assert(is_finite(from))
+   from>=0? [for (i=[from:1:len(list)-1]) list[i]] :
+   let(from = from + len(list))
+   from>=0? [for (i=[from:1:len(list)-1]) list[i]] :
+   list;
 
 
 // Function: force_list()