variable-length-quantity: Add new exercise (#468)

exercises/variable-length-quantity/example.py

@@ -0,0 +1,34 @@
+EIGHTBITMASK = 0x80
+SEVENBITSMASK = 0x7f
+
+
+def encode_single(n):
+    bytes = [n & SEVENBITSMASK]
+    n >>= 7
+
+    while n > 0:
+        bytes.append(n & SEVENBITSMASK | EIGHTBITMASK)
+        n >>= 7
+
+    return bytes[::-1]
+
+
+def encode(numbers):
+    return sum((encode_single(n) for n in numbers), [])
+
+
+def decode(bytes):
+    values = []
+    n = 0
+
+    for i, byte in enumerate(bytes):
+        n <<= 7
+        n += (byte & SEVENBITSMASK)
+
+        if byte & EIGHTBITMASK == 0:
+            values.append(n)
+            n = 0
+        elif i == len(bytes) - 1:
+            raise ValueError('incomplete byte sequence')
+
+    return values