A start on VList.

vlist.py

@@ -0,0 +1,96 @@
+'''
+An exploration of Phil Bagwell's VList.
+
+A VList is modeled as a 5-tuple:
+    base: VList
+    offset: int (indexing base VList data)
+    size: int
+    last_used: to make a mutable int it's an int in a list
+               It's a count here rather than an offset!
+    data: a list of length length
+
+a Pointer to a VList is a pair of (VList and offset).
+
+'''
+
+def new_VList(base=(), offset=0, size=1):
+    assert size >= 1
+    return (base, offset, size, [0], [None] * length)
+
+
+# In Fig 2 a list has been created with the integers (8,7,6,5,4,3)
+
+o1 = ((), 0, 1, [1], [3])
+o2 = (o1, 0, 2, [2], [4, 5])
+o3 = (o2, 1, 4, [3], [6, 7, 8, None])
+
+p0 = (o3, 2)  # points to 8
+p1 = (o3, 0)  # points to 6
+
+# cons(9, p1) = o4
+
+o4 = (o3, 0, 1, [1], [9])
+p2 = (o4, 0)  # points to 9
+
+
+
+def cons(thing, vlist_ptr):
+    (base, offset, size, last_used_list, data), pointer_offset = vlist_ptr
+    [last_used] = last_used_list
+
+    '''
+
+    During the consing of (9) the pointer offset
+    is compared with the last used offset, LastUsed. If it is the same and less than
+    the block size then it is simply incremented, the new entry made and LastUsed
+    updated.
+
+    '''
+    if pointer_offset == last_used - 1 and last_used < size:
+        pointer_offset += 1
+        data[pointer_offset] = thing
+        last_used_list[0] = last_used + 1
+        return vlist_ptr[0], pointer_offset
+    '''
+
+    If on the other-hand the pointer offset is less than the LastUsed a cons is being applied
+    to the tail of a longer list, as is the case with the (9). In this case a new list block
+    must be allocated and its Base-Offset pointer set to the tail contained in the original
+    list. The offset part being set to the point in tail that must be extended. The new
+    entry can now be made and additional elements added.
+
+    '''
+
+    # Is this where we increase the size x 2?
+    # size <<= 1 ; l = [None] * size ; l[0] = thing
+    # return (vlist_ptr[0], pointer_offset, size, [1], l), 0
+
+    return (vlist_ptr[0], pointer_offset, 1, [1], [thing]), 0
+
+
+p3 = cons(10, p0)
+
+
+def head(vlist_ptr):
+    vlist, offset = vlist_ptr
+    if not vlist:
+        raise ValueError("empty list has no head!")
+    return vlist[-1][offset]
+
+
+def tail(vlist_ptr):
+    vlist, offset = vlist_ptr
+    offset -= 1
+    return vlist[:2] if offset < 0 else (vlist, offset)
+
+
+def iter_vlist(vlist_ptr):
+    while vlist_ptr[0]:
+        yield head(vlist_ptr)
+        vlist_ptr = tail(vlist_ptr)
+
+
+for i, p in enumerate((p0, p1, p2, p3)):
+    print(f'p{i}')
+    print(' '.join(map(str, iter_vlist(p))))
+    print()