89 lines
3.6 KiB
Ruby
89 lines
3.6 KiB
Ruby
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# 1 - The time required to fetch a reference to an object from memory is a constant,
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# T_fetch, and the time required to store a reference to an object in memory is
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# a constant, T_store
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y = x
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# According to Axiom, the assignment statement has running time T_fetch + T_store.
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# That is, the time taken to fetch the object reference from variable x is
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# T_fetch and the time taken to store that object reference in the variable y is
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# T_store.
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y = 1
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# Also has running time T_fetch + T_store. To see why this should be the case,
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# consider that the constant '1' names a Fixnum object with value one. Therefore,
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# we can expect the cost of fetching the reference to the object named 1 is the
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# same as that of fetching a reference to any other object.
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# 2 - The times required to perform elementary arithmetic operations, such as
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# addition, subtraction, multiplication, division, and comparison, are all
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# constants. These times are denoted by T_+, T_-, T_/, T_*, and T_<, respectively.
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y = y + 1
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# We can determine time of a statement like is 2 * T_fetch + T_+ + T_store. This
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# is because we need to fetch two object references from the variables y and 1;
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# perform the addition giving a new object whose value is the sum; and, store
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# a reference to the new object in the variable y.
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# Ruby syntax provides an alternative way to express the same computation:
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y += 1
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# We shall assume that the alternative requires exactly the same running time as
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# the original statement.
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# 3 - The time required to call a method is a constant, T_call, and the time
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# required to return from a method is a constant, T_return
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# The rationale for making the overhead associated with parameter passing the
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# same as the time to store an object reference is that the passing of an
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# argument is conceptually the same as assignment of the actual parameter value
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# to the formal parameter of the method.
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y = f(x)
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# According to Axiom, the running time of the statement would be
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# T_fetch + 2 * T_store + T_call + T_f(x), where T_f(x) is the running time of
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# method f for input x. The first of the two stores is due to the passing of the
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# parameter x to the method f; the second arises from the assignment to the variable y.
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# 4 - The time required for the address calculation implied by an array
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# subscripting operation, e.g., a[i], is a constant, T_[]. This time does not
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# include the time to compute the subscript expression, nor does it include the
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# time to access the array element.
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y = a[i]
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# This is 3 * T_fetch Three operand fetches are required: the first to fetch a
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# reference to the array object a; the second to fetch a reference to the index
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# object i; and, the third to fetch a reference to the array element a[i].
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# 5 - The time required to create a new object instance of a class is a constant,
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# T_new. This time does not include any time taken to initialize the object.
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# By applying Axioms we can determine that the running time of the statement
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i = Fixnum.new(0)
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# is T_new + T_fetch + 2 * T_store + T_call + T_fixnum_init, where T_fixnum_init
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# is the running time of the initialize method of the class Fixnum.
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# Example - In this section we apply Axioms, the analysis of the running time of a
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# program to compute the following simple arithmetic series summation
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def sum(n)
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result = 0 # T_fetch + T_store
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i = 1 # T_fetch + T_store
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while i <= n # (2 * T_fetch + T_<) * (n - 1)
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result += i # (2 * T_fetch + T_+ + T_store) * n
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i += 1 # (2 * T_fetch + T_+ + T_store) * n
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end
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return result # T_return + T_fetch
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end
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# Total: (6 * T_fetch + 2 * T_store + T_< + 2 * T_+) * n + (5 * T_fetch + 2 * T_store + T_< + T_return)
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# T(n) = t1 * n + t2
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