Data commit

Task/Four-bit-adder/Python/four-bit-adder.py

@@ -0,0 +1,35 @@
+def xor(a, b): return (a and not b) or (b and not a)
+
+def ha(a, b): return xor(a, b), a and b     # sum, carry
+
+def fa(a, b, ci):
+    s0, c0 = ha(ci, a)
+    s1, c1 = ha(s0, b)
+    return s1, c0 or c1     # sum, carry
+
+def fa4(a, b):
+    width = 4
+    ci = [None] * width
+    co = [None] * width
+    s  = [None] * width
+    for i in range(width):
+        s[i], co[i] = fa(a[i], b[i], co[i-1] if i else 0)
+    return s, co[-1]
+
+def int2bus(n, width=4):
+    return [int(c) for c in "{0:0{1}b}".format(n, width)[::-1]]
+
+def bus2int(b):
+    return sum(1 << i for i, bit in enumerate(b) if bit)
+
+def test_fa4():
+    width = 4
+    tot = [None] * (width + 1)
+    for a in range(2**width):
+        for b in range(2**width):
+            tot[:width], tot[width] = fa4(int2bus(a), int2bus(b))
+            assert a + b == bus2int(tot), "totals don't match: %i + %i != %s" % (a, b, tot)
+
+
+if __name__ == '__main__':
+   test_fa4()