KWIC作為一個早年間在ACM的Paper提出的一個問題,被全世界各個大學的軟件設計課程奉為課堂講義或者作業的經典。(From Wiki,FYI,D. L. Parnas uses a KWIC Index as an example on how to perform modular design in his paper "On the Criteria To Be Used in Decomposing Systems into Modules" - Available as ACM Classic Paper)
問題陳述:KWIC(Key Word In Context),Parnas (1972)
KWIC索引系統接受一些行,每行有若干字,每個字由若干字符組成;每行都可以循環移位,亦即重復地把第一個字刪除,然后接到行末; KWIC把所有行的各種移位情況按照字母表順序輸出

•
目的:
考察不同的體系結構對變化的適應能力(modifiability)
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評價准則
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處理算法的改變:例如,行的移位可在每行讀入后、在所有行讀入后、或當排序要求一組移位的行時執行;
–
數據表示的改變:例如,行、字、字符可以不同的方式存儲;類似地,循環移位后的行可以顯式或隱式存儲(索引和偏移量);
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系統功能的增強:例如,限制以某些“修飾詞”(a, an, and等)打頭的移位結果;支持交互,允許用戶從原始輸入表中刪除一些行等;
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效率:時間和空間;
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復用:構件被復用的潛力。
Solution 1
:
Main Program/Subroutine with Shared Data
Main Program/Subroutine with Shared Data
Elements of Main/Subroutine Architectural style are:
- Components: Functions
- Interactions: Function calls
- Pattern: Main function controls the calling sequence
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Decompose the overall processing into a sequence of
processing steps.
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Read lines; Make shifts; Alphabetize; Print results
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Each step transforms the data completely.
每一步完全轉換數據
每一步完全轉換數據
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Intermediate data stored in shared memory.
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Arrays of characters with indexes
帶索引的字符數組
帶索引的字符數組
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Relies on sequential processing
串行處理
串行處理

Solution 1
:
Modularization
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Module 1: Input
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Reads data lines and stores them in
“
core
”
.
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Storage format: 4 chars/machine word; array of pointers to
start of each line.
–
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Module 2: Circular Shift
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Called after Input is done.
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Reads line storage to produce new array of pairs: (index of
1st char of each circular shift, index of original line)
–
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Module 3: Alphabetize
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Called after Circular Shift.
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Reads the two arrays and produces new index.
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Module 4: Output
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Called after alphabetization and prints nicely formatted
output of shifts
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Reads arrays produced by Modules 1 & 3
–
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Module 5: Master Control
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Handles sequencing of the first 4 modules
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Handles errors
Properties of Solution 1
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Batch sequential processing.
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Uses shared data to get good performance.
用共享數據獲得性能
用共享數據獲得性能
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Processing phases handled by control module.
–
So has some characteristics of main program – subroutine
organization.
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Depends critically on single thread of control.
•
Shared data structures exposed as inter-module
knowledge.
共享數據的結構是所有模塊必須知道的
共享數據的結構是所有模塊必須知道的
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Design of these structures must be worked out before work
can begin on those modules.
數據結構的設計必須在其他模塊設計開始之前進行
數據結構的設計必須在其他模塊設計開始之前進行
方案優缺點:
+系統自然分解,符合人的處理習慣
+數據共享,處理效率高
+
–難以適應數據存儲格式和整體處理算法的變化——爲什麽?
–系統構件難以支持復用——爲什麽?
仔細參考: