# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1481387758 -3600
# Node ID 853edfa770e31cfb5880c0f3e3d7597719bab967
# Parent  6d63ae9b8b7afcb265d0ed40aee620f664918112
Reorganised slice and xrange so that slices do not act as ranges, permitting the
start and end of a slice to be None.
Support reverse sequences in the __getslice__ sequence method.

diff -r 6d63ae9b8b7a -r 853edfa770e3 lib/__builtins__/sequence.py
--- a/lib/__builtins__/sequence.py	Sat Dec 10 17:32:16 2016 +0100
+++ b/lib/__builtins__/sequence.py	Sat Dec 10 17:35:58 2016 +0100
@@ -94,7 +94,10 @@
         # start index.
 
         if start is None:
-            start = 0
+            if step > 0:
+                start = 0
+            else:
+                start = length - 1
         else:
             start = _get_absolute_index(start, length)
 
@@ -102,7 +105,10 @@
         # otherwise normalising any end index.
 
         if end is None:
-            end = length
+            if step > 0:
+                end = length
+            else:
+                end = -1
         else:
             end = _get_absolute_index(end, length)
 
diff -r 6d63ae9b8b7a -r 853edfa770e3 lib/__builtins__/span.py
--- a/lib/__builtins__/span.py	Sat Dec 10 17:32:16 2016 +0100
+++ b/lib/__builtins__/span.py	Sat Dec 10 17:35:58 2016 +0100
@@ -21,15 +21,15 @@
 
 from __builtins__.sequence import _max, _min
 
-class xrange:
+class slice:
 
-    "Implementation of xrange."
+    "Implementation of slice."
 
     NO_END = object()
 
-    def __init__(self, start_or_end, end=NO_END, step=1):
+    def __init__(self, start_or_end=None, end=NO_END, step=1):
 
-        "Initialise the xrange with the given 'start_or_end', 'end' and 'step'."
+        "Initialise the slice with the given 'start_or_end', 'end' and 'step'."
 
         if end is xrange.NO_END:
             self.start = 0
@@ -38,8 +38,30 @@
             self.start = start_or_end
             self.end = end
 
+        if step == 0:
+            raise ValueError(self.step)
+
         self.step = step
 
+    def __str__(self):
+
+        "Return a string representation."
+
+        b = buffer([self.__name__, "(", self.start, ", ", self.end, ", ", self.step, ")"])
+        return str(b)
+
+    __repr__ = __str__
+
+class xrange(slice):
+
+    "Implementation of xrange."
+
+    def __init__(self, start_or_end, end=slice.NO_END, step=1):
+
+        "Initialise the xrange with the given 'start_or_end', 'end' and 'step'."
+
+        get_using(slice.__init__, self)(start_or_end, end, step)
+
         # Constrain the end according to the start and step.
 
         if step > 0:
@@ -51,15 +73,6 @@
 
         self.current = self.start
 
-    def __str__(self):
-
-        "Return a string representation."
-
-        b = buffer([self.__name__, "(", self.start, ", ", self.end, ", ", self.step, ")"])
-        return str(b)
-
-    __repr__ = __str__
-
     def __len__(self):
 
         "Return the length of the range."
@@ -83,16 +96,6 @@
         self.current += self.step
         return current
 
-class slice(xrange):
-
-    "Implementation of slice."
-
-    def __init__(self, start_or_end=None, end=xrange.NO_END, step=1):
-
-        "Initialise the slice with the given 'start_or_end', 'end' and 'step'."
-
-        get_using(xrange.__init__, self)(start_or_end, end, step)
-
 def range(start_or_end, end=None, step=1):
 
     "Implementation of range."
diff -r 6d63ae9b8b7a -r 853edfa770e3 tests/slice.py
--- a/tests/slice.py	Sat Dec 10 17:32:16 2016 +0100
+++ b/tests/slice.py	Sat Dec 10 17:35:58 2016 +0100
@@ -10,3 +10,4 @@
 print l[5:2:-1]             # [5, 4, 3]
 print l[1:9:2]              # [1, 3, 5, 7]
 print l[9:1:-2]             # [9, 7, 5, 3]
+print l[::-1]               # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]