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As a ICT trainer I have used the following picture a lot of times:

This pictures, borrowed from IBM, is still actual. Today we don't speak about needs, features and use cases, but about big changes, features, requirements and user stories. But the concept is still the same. You have to break down a solution gradually and you have to keep track of your requirements. Sometimes a problem detected during the implementation can be traced back to a high level requirement that has to be adjusted.

The following picture shows how the requirements for the sudoku app have been refined during the development process. Note that most messages in the UML sequence diagrams have become operations in the UML Class diagram.

REQUIREMENTS

Use Case	Solve sudoku
Actor	Sudoku player who wants to learn and practice advanced solving techniques
Goal	Solve a difficult sudoku puzzle
Basic Flow	The actor loads a sudoku puzzle by the way of copy and paste The actor requests the app to apply basic sudoku logic The actor repeats step 2 until the app gets stuck The actor applies advanced sudoku techniques to break the puzzle open The actor repeats step 2, 3 and 4 until the puzzle has been solved The actor requests the app to check the validity of the solutiion The app confirms that the puzzle has been solved properly
Alternative Flows	Import puzzle from file Manually set givens and load

Additional requirements:

F Functionality

F001 Import/Load facilities, F002 Usage of colors, F003 Delete functionality

U Useability

U001 Undo/Redo possibility, U002 Reset possibility, U003 Hint/Autosolve possibility, U004 Desktop/Mobile devices

R Reliablility

R001 Autosave possibility, R002 24/7 availability

P Performance

P001 Response within 100ms

S Supportability

S001 Easy adaptable, S002 Easy extendable

ANALYSIS (click on a diagram to enlarge the picture)

			UC Solve sudoku
LoadSudoku		ApplyLogic		SetDigit	SetColor	CheckSolution
	AcceptSingles		EvaluateRules
	UpdateSiblings		EvaluateRule1	EvaluateRule2a	EvaluateRule2b	EvaluateRule3456
Hint	Autosolve

DESIGN

MVC

Autosave

Undo/Redo

IMPLEMENTATION

Code example:

Board.prototype.autoSolve = function (loc, digit) {// empty Location allowed
       var isSolved = false;
       if (!loc.isEmpty()) { // assign a digit to a location if provided
           var cell = this.getCell(loc);
           if (!cell.hasCandidate(digit))
               throw "Internal error.";
           cell.setDigit(digit);
           this.updateSiblings(loc, SibType.Row, digit);
           this.updateSiblings(loc, SibType.Col, digit);
           this.updateSiblings(loc, SibType.Box, digit);
       }
       while (true) {
           this.evaluateRules();
           if (!this.acceptSingles()) // keep doing deterministic answers
               break;
       }
       if (this.isSolved())
           isSolved = true;
       else {
            var locChoice = this.findCellWithFewestCandidates();
           if (!locChoice.isEmpty()) {
               var cell = this.getCell(locChoice);
               var Candidates = cell.getCandidatesArray();
               for (var i = 0; i < Candidates.length; i++) {
                   var val = Candidates[i];
                   var newBoard = this.clone();
                   if (newBoard.autoSolve(locChoice, val) == true) {
                       newBoard.copyTo(this);
                       isSolved = true;
                       break;
                   }
               }
           }
       }
       return isSolved
   };

As you can see this code results from the UML sequence diagram "AutoSolve" .

Requirements refinement