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In The Bulletin of the European Association for Theoretical Computer Science 76 (2002), 153-162.
Bead–Sort: A Natural Sorting Algorithm
Joshua J. Arulanandham, Cristian S. Calude, Michael J. Dinneen
Department of Computer Science
University of Auckland
Auckland, New Zealand
Email: hi josh@hotmail.com,{cristian,mjd}@cs.auckland.ac.nz
Abstract
Nature is not only a source of minerals and precious stones but is also a mine
of algorithms. By observing and studying natural phenomena, computer algorithms
can be extracted. In this note, a simple natural phenomenon is used to design a
sorting algorithm for positive integers, called here Bead–Sort. The algorithm’s run–
time complexity ranges from O(1) to O(S)(Sis the sum of the input integers)
depending on the user’s perspective. Finally, three possible implementations are
suggested.
1BeadsandRods
According to Dijkstra [3], “our systems are much more complicated than can be con-
sidered healthy, and are too messy and chaotic to be used in comfort and confidence.
. . . unmastered complexity is at the root of the misery”. One way to cope with this situa-
tion is to “go back to Nature” for resources and inspiration (see, for example, [4, 5, 2]), a
trend strongly advocated by Rozenberg (see his ideas in the inaugural column “Natural
Computing”, [6]; a recent account was presented in [7]).
In this note, a simple natural phenomenon is used to design a sorting algorithm for
positive integers, called here Bead–Sort. In what follows, we represent positive integers
by a set of beads (like those used in an Abacus) as illustrated below in Fig. 1.
beads
number 3 number 2
Fig. 1
Beads slide through rods asshowninFig.2.
4
3
34
2
3
4
2
(a) (b) (c) (d)
Fig. 2
Fig. 2 (a) shows the numbers 4 and 3 (represented by beads) attached to rods; beads
displayed in Fig. 2 (a) appear to be suspended in the air, just before they start sliding
down. Fig. 2 (b) shows the state of the frame (a frame is a structure with the rods
and beads) after the beads are ‘allowed’ to slide down. The row of beads representing
number 3 has ‘emerged’ on top of the number 4 (the ‘extra’ bead in number 4 has dropped
down one ‘level’). Fig. 2 (c) shows numbers of different sizes, suspended one over the
other (in a random order). We allow beads (representing numbers 3, 2, 4 and 2) to slide
down to obtain the same set of numbers, but in a sorted order again (see Fig. 2 (d)). In
this process, the smaller numbers emerge above the larger ones and this creates a natural
comparison (an online animation of the above process can be seen at [1]).
To study our main procedure, called Bead–Sort, we will adopt a few conventions.
Rods (vertical lines) are counted always from left to right and levels are counted from
bottom to top as shown in Fig. 3. A frame is a structure consisting of rods and beads.
..
1
2
3
n
...
.
.
.
Rods
.
Levels
123 m
Fig. 3
2 Algorithm and Complexity
In this section, we present the Bead–Sort algorithm and analyze its complexity in three
different ways.
2
Consider a set Aof npositive integers to be sorted and assume the biggest number
in Ais m. Then, the frame should have at least mrods and nlevels. The Bead–Sort
algorithm is the following:
The Bead–Sort Algorithm
For all a∈Adrop abeads (one bead per rod) along the rods, starting
from the 1st rod to the ath rod. Finally, the beads, seen level by level,
from the nth level to the first level, represent Ain ascending order.
The Algorithm Bead–Sort is correct. We prove the result by mathematical induction
on the number of rows of beads. We claim (*) that the set of positive integers represented
by the states of the frame before and after the beads are dropped is the same. Also we
claim (**) that the number of beads on each row i, after dropping, is at most the number
of beads on row i−1 (the row directly below it).
Consider a set of cardinality n= 1. Since there is no possibility for a bead to drop
the above two claims hold. Now assume that these two properties hold for an input set
whose cardinality is k. Suppose a row k+1 of m<mbeads is now dropped on top
of these krows. Since claim (**) holds there is an index j≤msuch that all m−j
beads in columns greater then jdrop down (from row k+1). If m−j= 0, we are done.
Otherwise, the values of row k+1 and row khave been swapped, with any excess beads
on row kready to drop further. Thus, claim (*) holds, after a series of at most kswaps1.
Claim (**) also holds since we repeat these swaps until the force of gravity cannot pull
down any more beads.
The time complexity of Bead–Sort can be evaluated at three different coarseness
of discretization: complexity (1) treats ‘dropping all beads together’ as a single (si-
multaneous) operation, complexity (2) treats ‘dropping the row of beads in the frame
(representing a number)’ as a distinct operation and complexity (3) treats ‘dropping each
and every bead’ as a separate operation.
According to the first measure of complexity, the ‘dropping’ works in one unit of time
and hence the complexity is O(1). For the second measure, there are ndistinct rows of
input beads in the frame and therefore the complexity is O(n). For the third measure,
the complexity is equal to the total number of beads dropped. Hence, the complexity is
O(S), where Sis the sum of the input integers.
Note that in each of the above cases, the number of levels each bead drops is not
taken into account. Some implementations might require that this factor is taken into
account, since a bead dropping down from a higher level could cost more.
3 An Analog Hardware Implementation
The presence or absence of a bead is represented by an analog voltage across electrical
resistors. A rod is represented by a series of electrical resistors of increasing value from
top to bottom (see Fig. 4).
1Note that, physically, there are no ‘swaps’, but, the proof assumes the occurrence of ‘pseudo–swaps’,
for convenience; the actual algorithm operates in one unit of time to do all these pseudo–swaps.
3
.
.
.
.
.
..
.
.
Rod m
.. .
V1 V2
R 1,n
R 1,1
R 1,2
R 2,n
R 2,2
R 2,1
Vm
R m,2
R m,n
R m,1
Rod 1 Rod 2
Level 1
Level 2
Level n
Fig. 4
If the voltage across a resistor is above a preset threshold–voltage t, then it indicates
the ‘presence of bead’. The values of resistances in series (representing a rod) are ar-
ranged in increasing order so that when a voltage is applied across them, there is more
‘concentration’ of charge (voltage–drop) in the bottom resistors than in the top ones; this
is the electrical equivalent of the effect of gravity (that attracts beads downward). Let
the voltages across the rods be V1,V2,...,Vm, respectively. Let the currents through the
rods be C1,C2,...,Cm, respectively. Each resistor is denoted by Ri,j where iis the rod
number and jthe level number; xi,j represents the voltage–drop across Ri,j . The resistors
lying on the same level (say, j) have the same value, i.e. R1,j =R2,j =... =Rm,j .
.
.
.
R 1,n
R 1,2
R 1,1
.
.
... .
.. .
.
.
.
R 2,1
R 2,2
R 2,n
Data Entry Device
Trim
Trim
Trim
Trim
Trim
Trim
R m,n
R m,1
Trim
Trim
Trim
R m,2
vn
v1
v2
Fig. 5
4
The voltage across each resistor is fed into a trimmer circuit (threshold unit) whose
output is given by x
i,j =1,ifxi,j ≥t,x
i,j = 0, otherwise. This is shown in Fig. 5, where
dotted lines/dashes do not indicate physical connections. Let v1,v2, ...,vndenote the
sums of individual voltages across resistors (after thresholding) in the 1st,2
nd, ...,nth
levels, respectively (note that in Fig. 5, the detailed circuitry for adding voltages is not
shown). Hence we have:
v1=x
1,1+x
2,1+...+x
m,1,
v2=x
1,2+x
2,2+...+x
m,2,
.
.
.
vn=x
1,n +x
2,n +...+x
m,n.
The data–entry device is used to enter data to be sorted. When an integer is keyed
in, the equivalent unary representation is generated (for instance, the number 3 is rep-
resented as three 1’s followed by 0’s). The output lines from the data–entry device are
attached to the rods consisting of series of resistors as shown in Fig. 5, the first unary
digit connected to the 1st rod, and so on. For every ‘1’ in the ith digit of the unary
representation, a voltage increment δv is applied to the corresponding ith rod (of resis-
tors), i.e. the voltage across the ith rod is increased by δv. This is the way a new bead
is introduced onto a rod.
Every time a new data is entered, the individual resistors would have a different
voltage level; and these voltage levels should reflect the presence/absence of a bead in the
frame at that point of time. Hence, the values of the resistors, the voltage increment δv
and the threshold–voltage tshould be designed accordingly. After the whole data has
been entered, the voltage vector v1,v2,...,vnwill contain the sorted list (in descending
order).
To get a concrete example, consider a simple analog resistor circuit (see Fig. 6) that
can sort 3 positive integers, the biggest of which is 3.
1
3
2
1
3
2
1
3
2
v3
v1
v2
Fig. 6
The circuit has three rods and three levels, 9 resistors on the whole. Let us show how it
can sort the data set {2,1,3}. The total resistance in each rod Ris 1Ω + 2Ω + 3Ω = 6Ω.
The threshold voltage tis 0.5 volt; the voltage increment δv for every new bead is 1
5
volt. The integers 2, 1, 3 are entered one by one; the corresponding changes in voltage
levels across individual resistors and the current after entering the data 2 (110 in unary),
1 (100), 3 (111) are shown in Fig. 7 (a), 7 (b) and 7 (c), respectively. The slanting
arrows represent the flow of current and the values of current are shown in amperes.
The figures written close to the individual resistors represent the voltage levels across
them (xi,j ); those within the brackets represent the voltage after thresholding (x
i,j ). The
corresponding state of the frame is shown together. Note that after the last integer has
been entered, v1,v2and v3represent the sorted list.
1 v
0.5
0.3
0.2
(0)
(0)
(1)
1 v
0.5
0.3
0.2
(0)
(0)
(1)
0 v
(0)
0.0
0.0
0.0
(0)
(0)
3 v 2 v 1 v
1.5
1.0
0.5
(1)
(1)
(1) 1.0
0.7
0.3
(0)
(1)
(1) 0.5
0.3
0.2
(0)
(0)
(1)
0.5
0.3
0.2
(0)
(0)
(1)
0 v
0.0
0.0
0.0
(0)
(0)
(0)
2 v
1.0
0.7
0.3
(0)
(1)
(1)
1 v
0
(b)
(a)
(c)
0
1
3
1
2
1
6
1
3
1
6
1
6
1
6
Fig. 7
The time complexity of sorting using the above implementation is due to two com-
ponents: data entry time and the actual sorting time (time taken for electrical charges
6
to settle down). In this implementation, data entry and sorting action alternate each
other; sorting does not wait till all data have been entered. Let t1be the average time
taken for entering a single data item and t2be the average charge settling–time. It takes
t1+t2units for every single data item in the set. The overall complexity is therefore
n×(t1+t2), i.e. O(n).
In the case of manual data entry, t2will be so small compared to t1such that the
actual sorting time (before the next data is entered) is negligible.
4 A Cellular Automaton Implementation
A cellular automaton (CA) is a suitable choice for simulating natural physical systems
because it is massively parallel, self-organizing and is driven by a set of simple, local
rules (see, for example [8]).
In its simplest form, a CA can be considered a homogeneous array of cells in one,
two or more dimensions. Each cell has a finite discrete state. Cells communicate with
a number of local neighbors and update synchronously according to deterministic rules.
A cell updates its state depending on its current state and its neighbor states.
When modeling physical systems using cellular automata, space is treated as having
finitely many locations per unit of volume. Each location is represented by a cell and a
state is associated with each cell.
To emulate Bead–Sort, we use a two dimensional CA as shown in Fig. 8.
0
0
0
0
0
0
0
000
0
0
0
0
0
0
00
0
0
0
000
1
1
1
1
1
1
1
1
1
1111
1
11
1
1
1
1
1
Fig. 8
Each cell has two discrete states. A cell in state ‘1’ represents the space occupied by a
bead and a cell in state ‘0’ represents a space without any bead. The local CA rule used
to ‘roll down’ a bead into its ‘empty’ lower–neighbor is “a state–1 cell and its state–0
(empty) lower–neighbor cell always swap their states”. Table 1 shows in detail how this
rule is applied. The local rules are applied until no change in the configuration can be
obtained by any further application. The snap-shots of the CA configurations that evolve
while sorting the data-set {1, 3, 2, 1}are shown in Fig. 8.
The number of sequential updates of CA configuration necessary for sorting a partic-
ular set of data would depend on the ‘degree of disorder’ (entropy) associated with the
initial configuration (state) of the frame; this is the extent to which the initial state of
the frame ‘deviates’ from the state of a sorted frame (showing the same set of data in
sorted form). In the worst case, there will not be more than n−1 sequential updates of
CA configuration (note that state–changes in a CA are simultaneous) which leads to a
worst–case complexity of O(n).
7
Present State Upper–neighbour(x)Lower–neighbour(x)Next State
of Cell xof Cell x
0 0 0 0
0 0 1 0
0 1 0 1
0 1 1 1
1 0 0 0
1 0 1 1
1 1 0 0
1 1 1 1
Table 1
5 A Digital Hardware Implementation
We now discuss a digital implementation: the presence and absence of a bead is repre-
sented by the states ‘1’ and ‘0’ of a cell (flip–flop). A rod is represented by an array
of cells. Fig. 9 shows how the frame can be described with a two–dimensional array of
cells cell[i, j ]; cell[i, j]=1,ifthereisabead in the ith rod at the jth level, cell[i, j ]=0,
otherwise.
1
2
3
4
j
i (rods)
(cells)
flip-flops
0
0
0
0
00 0
0
0
0
0
234
0
1
(levels)
1
1
1
1
1
1
1
1
1
1
11
Fig. 9
There are several possible approaches to make beads ‘go down’ one of which is discussed
here. The input data (in unary form) is entered one–by–one into a data entry register as
shown in Fig. 10. Let the register be represented by a Boolean vector Bead[i]; Bead[i]
= 1 indicates that a bead is to be ‘dropped’ along the ith rod. All 1–bit cells from the
register are to be shifted ‘downward’ into the two dimensional array of cells cell[i, j]
(representing a frame) beneath, before the next data is entered. Thus, after the last data
is entered, the whole set would already be sorted in the array of cells.
A simple logic circuit shown in Fig. 10 is connected to each and every cell in the two
dimensional array of cells (the frame); when Bead[i] = 1, the circuit helps to locate that
cell in the ith rod which should ‘receive’ the bead. The circuit attached to that cell alone
‘triggers’ and makes it flip to a 1–state. Thus, shifting the bead downward is quite fast;
the 1–bit states in the register (the beads) do not have to ‘propagate’ cell–by–cell along
the corresponding rods.
8
0
3
0
0
00
4
0
1
0
1
0
0
0
00
0
0
0
0
0
0
0
0
0
00
0
0
0
0
0
00
0
0
0
00
0
0
2
0
j
0
0
0
0
(levels)
i (rods)
Data Entry Register
4
3
Bead[i]
Cell[i, j-1]
Cell[i, j] Cell[i, j]
2
1 1
1
1
1
1
1
1
11
Fig. 10
The circuit attached to each and every cell cell[i, j] enforces the following logic: Set
cell[i, j ](makecell[i, j] = 1) if (a) it is currently OFF, i.e. cell[i, j] = 0 and (b) the cell
in the data–entry register representing the ith rod is ON, i.e. Bead[i] = 1 (there is a
bead up ready to ‘drop down’ along ith rod) and (c) its immediate lower neighbour cell
(cell in the next lower level) is ON, i.e. cell[i, j −1] = 1 (which means the bead falling
down can not go any further). Note that cell[i, 0] is assumed to be 1. Thus, after the
last data is entered, the frame cell[i, j ] will have the whole set of data in a sorted form.
The time complexity of sorting using the above implementation is due to two com-
ponents: data entry time and the actual sorting time (time taken for the cells to switch
states). In this implementation also, data entry and sorting alternate each other; sorting
does not wait till all data are entered. Let t1be the average time taken for entering a
single data item and t2the average time taken for the cells to switch states. It takes
t1+t2units for every single data item in the set. The overall complexity is therefore
n×(t1+t2), i.e. O(n).
The observation made at the end of section 3 is equally valid here.
6 Conclusions
A way to design a natural algorithm is to discover an analogous natural property in
a physical system, and then to compute by “hitching a ride” on that inherent natural
property. In our case, “beads fall down in the sorted order”. Bead–Sort, the emergent
sorting algorithm, is simple, parallel and low cost. Some possible implementations of
Bead–Sort have been discussed. The analog implementation is the most natural imple-
mentation; electrical charges behave in the same manner as beads react to gravity. The
cellular automaton implementation is simple and very easy to understand. The digital
design is easy to implement and is cost effective.
References
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ral algorithms/josh/beadsort.ppt.
9
[2]C.S.Calude,G.P˘aun, Monica T˘at˘arˆam. A glimpse into natural computing,
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[3] E. W. Dijkstra. The end of computing science? Comm. ACM 44, 3 (2001), 92.
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10
