166 lines
7.9 KiB
Python
166 lines
7.9 KiB
Python
'''
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An exploration of Phil Bagwell's VList.
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A VList is modeled as a 5-tuple:
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base: VList
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offset: int, indexing (from 0) the base VList data list.
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size: int, length of the data list.
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last_used: to make a mutable int it's an int in a list
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It's a count here rather than an offset!
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That is, it starts from 1, rather than 0.
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data: a list of length size.
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A pointer to a VList is a two-tuple of (VList, offset) where
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the offset is an index into the VList's data list.
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'''
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def cons(thing, vlist_ptr):
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if not vlist_ptr:
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return ((), 0, 1, [1], [thing]), 0
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vlist, pointer_offset = vlist_ptr
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base, _offset, size, last_used_list, data = vlist
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[last_used] = last_used_list # Note that we could do this pattern
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# matching directly in the assignment statement on the previous line,
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# however, we want to keep that length-one list "last_used_list"
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# around so we can mutate the last_used value of "vlist".
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'''
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During the consing of (9) the pointer offset is compared with the
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last used offset, LastUsed. If it is the same and less than the block
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size then it is simply incremented, the new entry made and LastUsed
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updated.
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'''
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if pointer_offset == last_used - 1 and last_used < size:
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pointer_offset += 1
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data[pointer_offset] = thing
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last_used_list[0] += 1
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return vlist, pointer_offset
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'''
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If on the other-hand the pointer offset is less than the LastUsed a
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cons is being applied to the tail of a longer list, as is the case
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with the (9). In this case a new list block must be allocated and its
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Base-Offset pointer set to the tail contained in the original list.
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The offset part being set to the point in tail that must be extended.
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The new entry can now be made and additional elements added.
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'''
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# Is this where we increase the size x 2?
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size <<= 1
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offset = 0
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data = [None] * size
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data[offset] = thing
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return (vlist, pointer_offset, size, [1], data), offset
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def head(vlist_ptr):
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if not vlist_ptr:
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raise ValueError("Empty list has no head!")
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vlist, offset = vlist_ptr
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return vlist[-1][offset]
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def tail(vlist_ptr):
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if not vlist_ptr:
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raise ValueError("Empty list has no tail!")
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vlist, offset = vlist_ptr
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offset -= 1
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return (vlist[:2] if vlist[0] else ()) if offset < 0 else (vlist, offset)
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def iter_vlist(vlist_ptr):
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while vlist_ptr:
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yield head(vlist_ptr)
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vlist_ptr = tail(vlist_ptr)
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def repr_vlist(vlist_ptr):
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return ' '.join(map(str, iter_vlist(vlist_ptr)))
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def pick(n, vlist_ptr):
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'''
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Consider starting with a list pointer in Fig 1 then to find the nth
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element subtract n from the pointer offset. If the result is positive
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then the element is in the first block of the list at the calculated
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offset from the base. If the result is negative then move to the next
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block using the Base-Offset pointer. Add the Previous pointer offset
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to the negative offset. While this remains negative keep moving onto
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the next block. When it finally becomes positive the position of the
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required element has been found
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'''
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assert n >= 0
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if not vlist_ptr:
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raise ValueError("Empty list!")
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vlist, pointer_offset = vlist_ptr
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q = pointer_offset - n
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while q < 0:
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if not vlist:
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raise ValueError(f'Pick index {n} greater than length of list.')
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vlist, offset = vlist[:2]
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q += offset + 1 # Offsets in the paper count from one, not zero?
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return vlist[-1][q]
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def length(vlist_ptr):
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if not vlist_ptr:
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return 0
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vlist, n = vlist_ptr
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while vlist:
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vlist, offset = vlist[:2]
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n += offset + 1
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return n
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p = ()
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p = cons(3, p)# ; print(p)
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p = cons(4, p)# ; print(p)
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p = cons(5, p)# ; print(p)
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p = cons(6, p)# ; print(p)
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p = cons(7, p)# ; print(p)
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p = cons(8, p)# ; print(p)
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print(repr_vlist(p))
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for n in range(length(p)):
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print(pick(n, p), end=' ')
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print()
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# There is no such thing as a vlist_ptr with a null () vlist. That's an invariant.
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## # In Fig 2 a list has been created with the integers (8,7,6,5,4,3)
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##
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## o1 = ((), 0, 1, [1], [3])
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## o2 = (o1, 0, 2, [2], [4, 5])
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## o3 = (o2, 1, 4, [3], [6, 7, 8, None])
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##
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## p0 = (o3, 2) # points to 8
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## p1 = (o3, 0) # points to 6
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##
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## # cons(9, p1) = o4
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##
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## o4 = (o3, 0, 1, [1], [9])
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## p2 = (o4, 0) # points to 9
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##
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##
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## p3 = cons(10, p0)
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##for i, p in enumerate((p0, p1, p2, p3)):
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## print(f'p{i}')
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## print(' '.join(map(str, iter_vlist(p))))
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## print()
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# https://pythontutor.com/visualize.html#code='''%0AAn%20exploration%20of%20Phil%20Bagwell's%20VList.%0A%0AA%20VList%20is%20modeled%20as%20a%205-tuple%3A%0A%20%20%20%20base%3A%20VList%0A%20%20%20%20offset%3A%20int,%20indexing%20%28from%200%29%20the%20base%20VList%20data%20list.%0A%20%20%20%20size%3A%20int,%20length%20of%20the%20data%20list.%0A%20%20%20%20last_used%3A%20to%20make%20a%20mutable%20int%20it's%20an%20int%20in%20a%20list%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20It's%20a%20count%20here%20rather%20than%20an%20offset!%0A%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20That%20is,%20it%20starts%20from%201,%20rather%20than%200.%0A%20%20%20%20data%3A%20a%20list%20of%20length%20size.%0A%0AA%20pointer%20to%20a%20VList%20is%20a%20two-tuple%20of%20%28VList,%20offset%29%20where%0Athe%20offset%20is%20an%20index%20into%20the%20VList's%20data%20list.%0A%0A'''%0A%0A%0Adef%20cons%28thing,%20vlist_ptr%29%3A%0A%20%20%20%20if%20not%20vlist_ptr%3A%0A%20%20%20%20%20%20%20%20return%20%28%28%29,%200,%201,%20%5B1%5D,%20%5Bthing%5D%29,%200%0A%0A%20%20%20%20%28base,%20_offset,%20size,%20last_used_list,%20data%29,%20pointer_offset%20%3D%20vlist_ptr%0A%20%20%20%20%5Blast_used%5D%20%3D%20last_used_list%0A%0A%20%20%20%20'''%0A%20%20%20%20During%20the%20consing%20of%20%289%29%20the%20pointer%20offset%20is%20compared%20with%20the%0A%20%20%20%20last%20used%20offset,%20LastUsed.%20If%20it%20is%20the%20same%20and%20less%20than%20the%20block%0A%20%20%20%20size%20then%20it%20is%20simply%20incremented,%20the%20new%20entry%20made%20and%20LastUsed%0A%20%20%20%20updated.%0A%20%20%20%20'''%0A%0A%20%20%20%20if%20pointer_offset%20%3D%3D%20last_used%20-%201%20and%20last_used%20%3C%20size%3A%0A%20%20%20%20%20%20%20%20pointer_offset%20%2B%3D%201%0A%20%20%20%20%20%20%20%20data%5Bpointer_offset%5D%20%3D%20thing%0A%20%20%20%20%20%20%20%20last_used_list%5B0%5D%20%3D%20last_used%20%2B%201%0A%20%20%20%20%20%20%20%20return%20vlist_ptr%5B0%5D,%20pointer_offset%0A%0A%20%20%20%20'''%0A%20%20%20%20If%20on%20the%20other-hand%20the%20pointer%20offset%20is%20less%20than%20the%20LastUsed%20a%0A%20%20%20%20cons%20is%20being%20applied%20to%20the%20tail%20of%20a%20longer%20list,%20as%20is%20the%20case%0A%20%20%20%20with%20the%20%289%29.%20In%20this%20case%20a%20new%20list%20block%20must%20be%20allocated%20and%20its%0A%20%20%20%20Base-Offset%20pointer%20set%20to%20the%20tail%20contained%20in%20the%20original%20list.%0A%20%20%20%20The%20offset%20part%20being%20set%20to%20the%20point%20in%20tail%20that%20must%20be%20extended.%0A%20%20%20%20The%20new%20entry%20can%20now%20be%20made%20and%20additional%20elements%20added.%0A%20%20%20%20'''%0A%0A%20%20%20%20%23%20Is%20this%20where%20we%20increase%20the%20size%20x%202%3F%0A%20%20%20%20size%20%3C%3C%3D%201%0A%20%20%20%20data%20%3D%20%5BNone%5D%20*%20size%0A%20%20%20%20data%5B0%5D%20%3D%20thing%0A%20%20%20%20return%20%28vlist_ptr%5B0%5D,%20pointer_offset,%20size,%20%5B1%5D,%20data%29,%200%0A%0A%0A%0Ap%20%3D%20%28%29%0Ap%20%3D%20cons%283,%20p%29%23%20%3B%20print%28p%29%0Ap%20%3D%20cons%284,%20p%29%23%20%3B%20print%28p%29%0Ap%20%3D%20cons%285,%20p%29%23%20%3B%20print%28p%29%0Ap%20%3D%20cons%286,%20p%29%23%20%3B%20print%28p%29%0Ap%20%3D%20cons%287,%20p%29%23%20%3B%20print%28p%29%0Ap%20%3D%20cons%288,%20p%29%23%20%3B%20print%28p%29%0A&cumulative=false&heapPrimitives=nevernest&mode=edit&origin=opt-frontend.js&py=3&rawInputLstJSON=%5B%5D&textReferences=false
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