cs784(prasad)l34adt1 specification and implementation of abstract data types

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Specification and Implementation of Abstract Data Types

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Data Abstraction

• Clients – Interested in WHAT services a module

provides, not HOW they are carried out. So, ignore details irrelevant to the overall behavior, for clarity.

• Implementors– Reserve the right to change the code, to

improve performance. So, ensure that clients do not make unwarranted assumptions.

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Specification of Data Types Type : Values + Operations

Specify

Syntax Semantics

Signature of Ops Meaning of Ops

Model-based Axiomatic(Algebraic)

Description in terms of Give axioms satisfied

standard “primitive” data types by the operations

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Syntax of LISP S-expr

• operations: nil, cons, car, cdr, null

• signatures: nil: S-expr

cons: S-expr S-expr S-expr

car: S-expr S-expr

cdr: S-expr S-expr

null: S-expr boolean

for every atom a: a : S-expr

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• Signature tells us how to form complex terms from primitive operations.

• Legalnil

null(cons(nil,nil))

cons(car(nil),nil)

• Illegalnil(cons)

null(null)

cons(nil)

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Semantics of +: What to expect?

+ : N x N N

1 + 2 = 3

zero + succ(succ(zero)) = succ(succ(zero))

x + 0 = x

2 * (3 + 4) = 2 * 7 = 14 = 6 + 8

x * ( y + z) = y * x + x * z

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Semantics of S-Expr : What to expect?

null(nil) = true

car(cons(nil,nil)) = nil

null(cdr(cons(nil,cons(nil,nil)))) = false

• for all E,F in S-Exprcar(cons(E,F)) = E

null(cons(E,F)) = false

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Formal Spec. of ADTs

Characteristics of an “Adequate” Specification– Completeness (No “undefinedness”)– Consistency/Soundness (No conflicting definitions)

• MinimalityMinimality

GOAL:

Learn to write sound and complete algebraic(axiomatic) specifications of ADTs

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Classification of Operations• Observers

– generate a value outside the type• E.g., null in ADT S-expr

• Constructors– required for representing values in the type

• E.g., nil, cons, atoms a in ADT S-expr

• Non-constructors– remaining operations

• E.g., car, cdr in ADT S-expr

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S-Expr in LISP a : S-Expr

nil : S-Exprcons : S-Expr x S-Expr S-Exprcar : S-Expr S-Expr cdr : S-Expr S-Exprnull : S-Expr boolean

Observers : null

Constructors : a, nil, cons

Non-constructors : car, cdr

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Algebraic Spec

• Write axioms (equations) that characterize the meaning of all the operations.

• Describe the meaning of the observers and the non-constructors on all possible constructor patterns.

• Note the use of typed variables to abbreviate the definition. (“Finite Spec.”)

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• for all S, T in S-expr

cdr(nil) = ?error? cdr(a) = ?error? cdr(cons(S,T)) = T car(nil) = ?error? car(a) = ?error?

car(cons(S,T)) = S null(nil) = true null(a) = false null(cons(S,T)) = false

• Omitting the equation for “nil” implies that implementations that differ in the interpretation of “nil” are all equally acceptableacceptable.

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S-Exprs• car(a)

• cons(a,nil)

• car(cons(a,nil)) =

• a

• cons( car(cons(a,nil)), cdr(cons(a,a)) ) =

• cons( a , a )

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• If car and cdr are also regarded as constructors (as they generate values in the type), then the spec. must consider other cases to guarantee completeness (or provide sufficient justification for their omission).

• for all S in S-expr: null(car(S)) = ...

null(cdr(S)) = ...

Motivation for Classification : Minimality

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ADT Table (symbol table/directory)

empty : Table

update : Key x Info x Table Table

lookUp: Key x Table nfo

lookUp(K,empty) = error

lookUp(K,update(Ki, I, T)) =

if K = Ki then I else lookUp(K,T)

(“last update overrides the others”)

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TableTables• empty• update(5, “abc”, empty)• update(10, “xyz”, update(5, “abc”, empty))• update(5, “xyz”, update(5, “abc”, empty))

(Search )

• lookup (5, update(5, “xyz”, update(5, “abc”, empty)) ) • lookup (5, update(5, “xyz”, update(5, “xyz”, empty)) )• lookup (5, update(5, “xyz”, empty) )• “xyz”

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Implementations– Array-based– Linear List - based– Tree - based

• Binary Search Trees, AVL Trees, B-Trees etc

– Hash Table - based

• These exhibit a common Table behavior, but differ in performance aspects (search time).

• Correctness of a program is assured even when the implementation is changed as long as the spec is satisfied.

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(cont’d)

• Accounts for various other differences (Data Invariants) in implementation such as

– Eliminating duplicates.– Retaining only the final binding.– Maintaining the records sorted on the key.– Maintaining the records sorted in terms of the

frequency of use (a la caching).

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A-list in LISP a : A

nil : A-listcons : A x A-list A-listcar : A-list A cdr : A-list A-listnull : A-list boolean

• Observers : null, car• Constructors : nil, cons• Non-constructors : cdr

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• for all L in A-list

cdr(cons(a,L)) = L

car(cons(a,L)) = a

null(nil) = true null(cons(a,L)) = false

• Consciously silent about nil-list.

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Natural Numberszero : succ : add : x iszero : boolean

observers : iszero

constructors : zero, succ

non-constructors : add

Each number has a unique representation in terms of its constructors.

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for all I,J in add(I,J) = ?

add(zero,I) = I

add(succ(J), I) = succ(add(J,I))

iszero(I) = ?

iszero(zero) = trueiszero(succ(I)) = false

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(cont’d)add(succ(succ(zero)), succ(zero))

= succ(succ(succ(zero)))

� The first rule eliminates add from an expression, while the second rule simplifies the first argument to add.

� Associativity, commutativity, and identity properties of add can be deduced from this definition through purely mechanical means.

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A-list Revisted

a : Anil : A-listlist : A A-listappend : A-list x A-list A-listnull : A-list boolean

• values – nil, list(a), append(nil, list(a)), ...

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Algebraic Spec

• constructors– nil, list, append

• observerisnull(nil) = trueisnull(list(a)) = falseisnull(append(L1,L2)) =

isnull(L1) /\ isnull(L2)

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• Problem : Same value has multiple representation in terms of constructors.

• Solution : Add axioms for constructors.

– Identity Ruleappend(L,nil) = L

append(nil,L) = L

– Associativity Rule append(append(L1,L2),L3)

=

append(L1, append(L2,L3))

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Intuitive understanding of constructors

• The constructor patterns correspond to distinct memory/data patterns required to store/represent values in the type.

• The constructor axioms can be viewed operationally as rewrite rules to simplify constructor patterns. Specifically, constructor axioms correspond to computations necessary for equality checking and aid in defining a normal form.

• Cf. == vs equal in Java

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Writing ADT Specs

• Idea: Specify “sufficient” axioms such that syntactically distinct terms (patterns) that denote the same value can be proven so.– Completeness

• Define non-constructors and observers on all possible constructor patterns

– Consistency• Check for conflicting reductions

• Note: A term essentially records the detailed history of construction of the value.

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General Strategy for ADT Specs

• Syntax– Specify signatures and classify operations.

• Constructors– Write axioms to ensure that two constructor

terms that represent the same value can be proven so.

• E.g., identity, associativity, commutativity rules.

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• Non-constructors– Provide axioms to collapse a non-constructor

term into a term involving only constructors.

• Observers– Define the meaning of an observer on all

constructor terms, checking for consistency.

Implementation of a type An interpretation of the operations of the ADT

that satisfies all the axioms.

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Declarative Specification

• Let *: N x N N denote integer multiplication. Equation: n * n = n

Solution: n = 0 \/ n = 1.

• Let f: N x N N denote a binary integer function. Equation: 0 f 0 = 0

Solution: f = “multiplication” \/

f = “addition” \/ f = “subtraction” \/ ...

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• for all n, m in N, s in SetSetdelete(n,empty) = empty

delete(n,insert(m,s)) =

if (n=m)

then delete(n,s) (invalid: s)

else insert(m,delete(n,s))

delete(5, insert(5,insert(5,empty)) ) {5,5}

== empty {}

=/= insert(5,empty)

[]

[5,5]

delete : SetSet

[5]

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• Previous axioms capture “remove all occurrences” semantics.

• For “remove last occurrence” semantics:

for all n, m in N, s in ListListdelete(n,empty) = emptydelete(n,insert(m,s)) = if (n=m) then s else insert(m,delete(n,s))

delete(5, insert(5,insert(5,empty)) ) [5,5]== insert(5,empty) [5]

delete : ListList

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• Previous axioms capture “remove all / last occurrences” semantics.

• For “remove first occurrence” semantics:

for all n, m in N, s in ListListdelete(n,empty) = emptydelete(n,insert(m,s)) = if (n=m) and not (n in s) then s else insert(m,delete(n,s))

delete(1, insert(1,insert(2,insert(1,insert(5,empty)))) ) [5,1,2,1]

== insert(1,insert(2,insert(5,empty))) [5,2,1]

delete : ListList

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size: List vs Set

• size(insert(m,l)) = 1 + size(l)– E.g., size([2,2,2]) = 1 + size([2,2])

• size(insert(m,s)) =

if (m in s) then size(s)

else 1 + size(s)– E.g., size({2,2,2}) = size({2,2})

= size ({2}) = 1

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Model-based vs Algebraic

• A model-based specification of a type satisfies the corresponding axiomatic specification. Hence, algebraic spec. is “more abstract” than the model-based spec.

• Algebraic spec captures the least common-denominator (behavior) of all possible implementations.

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Axiomatization: Algebraic Structures

• A set G with operation * forms a group if• Closure: a,b G implies a*b G.• Associativity: a,b,c G implies a*(b *c) = (a*b)*c.• Identity: There exists i G such that

i*a = a*i = a for all a G.• Inverses: For every a G there exists an element

~a G such that a * ~a = ~a * a = i.

• Examples:• (Integers, +), but not (N, +)• (Reals {0}, *), but not (Integers, *)• (Permutation functions, Function composition)

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Example car( cons( X, Y) ) = X

cdr( cons (X, Y) ) = Y

(define (cons x y) (lambda (m) (cond ((eq? m ’first) x) (eq? m ’second) y) ))) ; “closure”

(define (car z) (z ’first))(define (cdr z) (z ’second))

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Applications of ADT spec

• Least common denominator of all possible implementations.– Focus on the essential behavior.

• An implementation is a refinement of ADT spec.

– IMPL. = Behavior SPEC + Rep “impurities”– To prove equivalence of two implementations,

show that they satisfy the same spec.– In the context of OOP, a class implements an

ADT, and the spec. is a class invariant.

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(Cont’d)

• Indirectly, ADT spec. gives us the ability to vary or substitute an implementation. – E.g., In the context of interpreter/compiler, a

function definition and the corresponding calls (ADT FuncValues) together must achieve a fixed goal. However, there is freedom in the precise apportioning of workload between the two separate tasks:

• How to represent the function?

• How to carry out the call?

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(Cont’d)• ADT spec. are absolutely necessary to automate

formal reasoning about programs. Theorem provers such as Boyer-Moore prover (NQTHM), LARCH, PVS, HOL, etc routinely use such axiomatization of types.

• Provides a theory of equivalence of values that enables design of a suitable canonical form.

• Identity delete

• Associativity remove parenthesis

• Commutativity sort

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Spec vs ImplThe reason to focus on the behavioral aspects, ignoring

efficiency details initially, is that the notion of a “best

implementation” requires application specific issues and

trade-offs. In other words, the distribution of work among

the various operations is based on a chosen representation,

which in turn, is dictated by the pragmatics of an

application. However, in each potential implementation,

there is always some operations that will be efficient while

others will pay the price for this comfort.

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Ordered Integer Listsnull : oil booleannil : oil hd : oil inttl : oil oilins : int x oil oilorder : int_list oil

Constructors: nil, insNon-constructors: tl, orderObservers: null, hd

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• Problem:

– syntactically different, but semantically equivalent constructor terms

ins(2,ins(5,nil)) = ins(5,ins(2,nil))

ins(2,ins(2,nil)) = ins(2,nil)

– hd should return the smallest element.• It is not the case that for all I in int, L in oil,

hd(ins(I,L)) = I. • This holds iff I is the minimum in ins(I,L).

– Similarly for tl.

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Axioms for Constructors

• Idempotence– for all ordered integer lists L; for all I in int

ins(I, ins(I,L)) = ins(I,L)

• Commutativity– for all ordered integer lists L; for all I, J in int

ins(I, ins(J,L)) = ins(J, ins(I,L))

Completeness : Any permutation can be generated by exchanging adjacent elements.

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Axioms for Non-constructors

tl(nil) = errortl(ins(I,L)) = ?tl(ins(I,nil)) = nil

tl(ins(I,ins(J,L))) = I < J => ins( J, tl(ins(I,L)) ) I > J => ins( I, tl(ins(J,L)) )

I = J => tl( ins( I,L ) ) (cf. constructor axioms for duplicate elimination)

order(nil) = nil order(cons(I,L)) = ins(I,order(L))

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Axioms for Observers

hd(nil) = error hd(ins(I,nil)) = I

hd(ins(I,ins(J,L))) = I < J => hd( ins(I,L) ) I > J => hd( ins(J,L) )

I = J => hd( ins(I,L) )

null(nil) = true null(ins(I,L)) = false

Scheme Implementation

(define null null?)

(define nil ’())

(define ins cons)

(define (hd ol) *min* )

(define (tl ol) *list sans min* )

(define (order lis) *sorted list* )

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Possible Implementations• Representation Choice 1:

– List of integers with duplicates• ins is cons but hd and tl require linear-time search

• Representation Choice 2: – Sorted list of integers without duplicates

• ins requires search but hd and tl can be made more efficient

• Representation Choice 3: – Balanced-tree : Heap

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