BUG: Fix binary_repr for negative numbers

Completely rewrote binary_repr method to use
the Python's built-in 'bin' function to generate
binary representations quickly.

Fixed the behaviour for negative numbers in which
insufficient widths resulted in outputs of all zero's
for the two's complement. Now, it will return the
two's complement with width equal to the minimum
number of bits needed to represent the complement.

Closes gh-7168.

1 files changed, 46 insertions, 44 deletions

diff --git a/numpy/core/numeric.py b/numpy/core/numeric.py
index 9ddc3c546..551d63a01 100644
--- a/numpy/core/numeric.py
+++ b/numpy/core/numeric.py
@@ -2090,31 +2090,6 @@ def isscalar(num):
     else:
         return type(num) in ScalarType
 
-_lkup = {
-    '0':'0000',
-    '1':'0001',
-    '2':'0010',
-    '3':'0011',
-    '4':'0100',
-    '5':'0101',
-    '6':'0110',
-    '7':'0111',
-    '8':'1000',
-    '9':'1001',
-    'a':'1010',
-    'b':'1011',
-    'c':'1100',
-    'd':'1101',
-    'e':'1110',
-    'f':'1111',
-    'A':'1010',
-    'B':'1011',
-    'C':'1100',
-    'D':'1101',
-    'E':'1110',
-    'F':'1111',
-    'L':''}
-
 def binary_repr(num, width=None):
     """
     Return the binary representation of the input number as a string.
@@ -2134,8 +2109,18 @@ def binary_repr(num, width=None):
     num : int
         Only an integer decimal number can be used.
     width : int, optional
-        The length of the returned string if `num` is positive, the length of
-        the two's complement if `num` is negative.
+        The length of the returned string if `num` is positive, or the length
+        of the two's complement if `num` is negative, provided that `width` is
+        at least a sufficient number of bits for `num` to be represented in the
+        designated form.
+
+        If the `width` value is insufficient, it will be ignored, and `num` will
+        be returned in binary(`num` > 0) or two's complement (`num` < 0) form
+        with its width equal to the minimum number of bits needed to represent
+        the number in the designated form. This behavior is deprecated and will
+        later raise an error.
+
+        .. deprecated:: 1.12.0
 
     Returns
     -------
@@ -2146,6 +2131,7 @@ def binary_repr(num, width=None):
     --------
     base_repr: Return a string representation of a number in the given base
                system.
+    bin: Python's built-in binary representation generator of an integer.
 
     Notes
     -----
@@ -2169,27 +2155,43 @@ def binary_repr(num, width=None):
     The two's complement is returned when the input number is negative and
     width is specified:
 
-    >>> np.binary_repr(-3, width=4)
-    '1101'
+    >>> np.binary_repr(-3, width=3)
+    '101'
+    >>> np.binary_repr(-3, width=5)
+    '11101'
 
     """
-    # ' <-- unbreak Emacs fontification
-    sign = ''
-    if num < 0:
+    def warn_if_insufficient(width, binwdith):
+        if width is not None and width < binwidth:
+            warnings.warn(
+                "Insufficient bit width provided. This behavior "
+                "will raise an error in the future.", DeprecationWarning
+            )
+
+    if num == 0:
+        return '0' * (width or 1)
+
+    elif num > 0:
+        binary = bin(num)[2:]
+        binwidth = len(binary)
+        outwidth = (binwidth if width is None
+                    else max(binwidth, width))
+        warn_if_insufficient(width, binwidth)
+        return binary.zfill(outwidth)
+
+    else:
         if width is None:
-            sign = '-'
-            num = -num
+            return '-' + bin(-num)[2:]
+
         else:
-            # replace num with its 2-complement
-            num = 2**width + num
-    elif num == 0:
-        return '0'*(width or 1)
-    ostr = hex(num)
-    bin = ''.join([_lkup[ch] for ch in ostr[2:]])
-    bin = bin.lstrip('0')
-    if width is not None:
-        bin = bin.zfill(width)
-    return sign + bin
+            poswidth = len(bin(-num)[2:])
+            twocomp = 2**(poswidth + 1) + num
+
+            binary = bin(twocomp)[2:]
+            binwidth = len(binary)
+            outwidth = max(binwidth, width)
+            warn_if_insufficient(width, binwidth)
+            return '1' * (outwidth - binwidth) + binary
 
 def base_repr(number, base=2, padding=0):
     """