slice can accept negative indices

docs/scarpet/language/Containers.md

@@ -170,14 +170,21 @@ split('\\.','foo.bar')  => [foo, bar]
 ### `slice(expr, from, to?)`
 
 extracts a substring, or sublist (based on the type of the result of the expression under expr with 
-starting index of `from`, and ending at `to` if provided, or the end, if omitted
+starting index of `from`, and ending at `to` if provided, or the end, if omitted. Can use negative indices to 
+indicate counting form the back of the list, so `-1 <=> length(_)`.
+
+Special case is made for iterators (`range`, `rect` etc), which does require non-negative indices (negative `from` is treated as
+`0`, and negative `to` as `inf`), but allows retrieving parts of the sequence and ignore
+other parts. In that case consecutive calls to `slice` will refer to index `0` the current iteration position since iterators
+cannot go back nor track where they are in the sequence (see examples).
 
 <pre>
-slice(l(0,1,2,3,4,5), 1, 3)  => [1, 2, 3]
+slice([0,1,2,3,4,5], 1, 3)  => [1, 2, 3]
 slice('foobar', 0, 1)  => 'f'
 slice('foobar', 3)  => 'bar'
 slice(range(10), 3, 5)  => [3, 4, 5]
 slice(range(10), 5)  => [5, 6, 7, 8, 9]
+r = range(100); [slice(r, 5, 7), slice(r, 1, 3)]  => [[5, 6], [8, 9]]
 </pre>
 
 ### `sort(list), sort(values ...)`

src/main/java/carpet/script/value/ListValue.java

@@ -289,27 +289,22 @@ public class ListValue extends AbstractListValue implements ContainerValueInterf
             items.add(v);
     }
 
-    public void addAtIndex(int index, List<Value> subList)
+    /**
+     * Finds a proper list index >=0 and < len that correspont to the rolling index value of idx
+     *
+     * @param idx
+     * @param len
+     * @return
+     */
+    public static int normalizeIndex(long idx, int len)
     {
-        int numitems = items.size();
-        long range = abs(index)/numitems;
-        index += (range+2)*numitems;
-        index = index % numitems;
-        for (Value v: subList)
-        {
-            if (index < numitems)
-            {
-                items.set(index, v);
-            }
-            else
-            {
-                items.add(v);
-            }
-            index++;
-        }
+        if (idx >=0 && idx < len) return (int)idx;
+        long range = abs(idx)/len;
+        idx += (range+2)*len;
+        idx = idx % len;
+        return (int)idx;
     }
 
-
     public static class ListConstructorValue extends ListValue
     {
         public ListConstructorValue(Collection<? extends Value> list)
@@ -339,15 +334,15 @@ public class ListValue extends AbstractListValue implements ContainerValueInterf
     }
 
     @Override
-    public Value slice(long from, long to)
+    public Value slice(long fromDesc, long toDesc)
     {
         List<Value> items = getItems();
         int size = items.size();
-        if (to < 0 || to > size) to = size;
-        if (from < 0 || from > size) from = size;
+        int from = normalizeIndex(fromDesc, size);
+        int to = normalizeIndex(toDesc, size);
         if (from > to)
             return ListValue.of();
-        return new ListValue((Collection<? extends Value>) getItems().subList((int)from, (int) to));
+        return new ListValue((Collection<? extends Value>) getItems().subList(from, to));
     }
 
     @Override
@@ -395,10 +390,8 @@ public class ListValue extends AbstractListValue implements ContainerValueInterf
                 return false;
             int index = (int)((NumericValue) ind).getLong();
             if (index < 0)
-            {
-                long range = abs(index) / numitems;
-                index += (range + 2) * numitems;
-                index = index % numitems;
+            {// only for values < 0
+                index = normalizeIndex(index, numitems);
             }
             if (replace)
             {
@@ -424,11 +417,7 @@ public class ListValue extends AbstractListValue implements ContainerValueInterf
     public Value get(Value value)
     {
         long index = NumericValue.asNumber(value, "'address' to a list index").getLong();
-        int numitems = items.size();
-        long range = abs(index)/numitems;
-        index += (range+2)*numitems;
-        index = index % numitems;
-        return items.get((int)index);
+        return items.get(normalizeIndex(index, items.size()));
     }
 
     @Override
@@ -443,11 +432,7 @@ public class ListValue extends AbstractListValue implements ContainerValueInterf
     {
         if (!(where instanceof NumericValue) || items.isEmpty()) return false;
         long index = ((NumericValue) where).getLong();
-        int numitems = items.size();
-        long range = abs(index)/numitems;
-        index += (range+2)*numitems;
-        index = index % numitems;
-        items.remove((int)index);
+        items.remove(normalizeIndex(index, items.size()));
         return true;
     }
 

src/main/java/carpet/script/value/StringValue.java

@@ -5,6 +5,8 @@ import net.minecraft.nbt.Tag;
 
 public class StringValue extends Value
 {
+    public static Value EMPTY = StringValue.of("");
+
     private String str;
 
     @Override

src/main/java/carpet/script/value/Value.java

@@ -178,16 +178,14 @@ public abstract class Value implements Comparable<Value>, Cloneable
         return getString().length();
     }
 
-    public Value slice(long from, long to)
+    public Value slice(long fromDesc, long toDesc)
     {
         String value = this.getString();
         int size = value.length();
-        if (to > size) to = -1;
-        if (from < 0) from = 0;
-        if (from > size) from = size;
-        if (to>=0)
-            return new StringValue(value.substring((int)from, (int)to));
-        return new StringValue(value.substring((int)from));
+        int from = ListValue.normalizeIndex(fromDesc, size);
+        int to = ListValue.normalizeIndex(toDesc, size);
+        if (from > to) return StringValue.EMPTY;
+        return new StringValue(value.substring(from, to));
     }
     public double readDoubleNumber()
     {