# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1481400834 -3600
# Node ID d75ff26f7c0aecf62eb0931261fde547cc105729
# Parent  2ab8235da0608202ee225b24dad1d23d30850fb8
Implemented the reduce function, adding some relevant tests.

diff -r 2ab8235da060 -r d75ff26f7c0a lib/__builtins__/iterable.py
--- a/lib/__builtins__/iterable.py	Sat Dec 10 19:05:06 2016 +0100
+++ b/lib/__builtins__/iterable.py	Sat Dec 10 21:13:54 2016 +0100
@@ -98,7 +98,32 @@
             lowest = arg
     return lowest
 
-def reduce(function, sequence, initial=None): pass
+_reduce_default = object()
+
+def reduce(function, sequence, initial=_reduce_default):
+
+    """
+    Using 'function', reduce the given 'sequence' to a single result.
+
+    With no 'initial' value specified, the first two elements in the 'sequence'
+    are used with the function to produce an initial result. With an initial
+    result available, a subsequent result is computed by using the initial
+    result and the next element in the sequence with the function.
+
+    All subsequent results are computed using the current result and the next
+    available element with the function. This continues for all remaining
+    elements until the end of the sequence is reached.
+    """
+
+    result = initial
+
+    for i in sequence:
+        if result is _reduce_default:
+            result = i
+        else:
+            result = function(result, i)
+
+    return result
 
 def reversed(sequence):
 
diff -r 2ab8235da060 -r d75ff26f7c0a tests/range.py
--- a/tests/range.py	Sat Dec 10 19:05:06 2016 +0100
+++ b/tests/range.py	Sat Dec 10 21:13:54 2016 +0100
@@ -1,27 +1,61 @@
+# Obtain a list computed by range.
+
 l = range(0, 10)
-print l                         # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
-print len(l)                    # 10
+print l                                     # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+print len(l)                                # 10
+
+# Test a descending xrange.
 
 x = xrange(0, -10, -2)
-print x                         # __builtins__.span.xrange(0, -10, -2)
-print len(x)                    # 5
+print x                                     # __builtins__.span.xrange(0, -10, -2)
+print len(x)                                # 5
 
 for i in x:
-    print i                     # 0
-                                # -2
-                                # -4
-                                # -6
-                                # -8
+    print i                                 # 0
+                                            # -2
+                                            # -4
+                                            # -6
+                                            # -8
+
+# Test an empty xrange.
 
 x = xrange(0, -10, 2)
-print x                         # __builtins__.span.xrange(0, 0, 2)
-print len(x)                    # 0
+print x                                     # __builtins__.span.xrange(0, 0, 2)
+print len(x)                                # 0
+
+# Test a simple ascending xrange.
 
 y = xrange(4, 8)
-print y                         # __builtins__.span.xrange(4, 8, 1)
-print enumerate(y)              # [(0, 4), (1, 5), (2, 6), (3, 7)]
-print sum(y)                    # 22
-print map(lambda x: x*2, y)     # [8, 10, 12, 14]
-print filter(lambda x: x%2, y)  # [5, 7]
-print max(y)                    # 7
-print min(y)                    # 4
+print y                                     # __builtins__.span.xrange(4, 8, 1)
+
+# Test enumerate and sum.
+
+print enumerate(y)                          # [(0, 4), (1, 5), (2, 6), (3, 7)]
+print sum(y)                                # 22
+
+# Test map and filter using lambdas.
+
+print map(lambda x: x*2, y)                 # [8, 10, 12, 14]
+print filter(lambda x: x%2, y)              # [5, 7]
+
+# Test the limits of the range.
+
+print max(y)                                # 7
+print min(y)                                # 4
+
+# Reproduce the sum function using reduce and a lambda.
+
+print reduce(lambda x, y: x+y, y)           # 22
+print reduce(lambda x, y: x+y, y, 0)        # 22
+
+# Test a single element range.
+
+single = xrange(3, 5, 2)
+print list(single)                          # [3]
+print reduce(lambda x, y: x+y, single)      # [3]
+
+# Test a double element range.
+
+double = xrange(3, 5, 1)
+print list(double)                          # [3, 4]
+print reduce(lambda x, y: x+y, double)      # [7]