The C Interface

How would this translate into C? First some type definitions:

typedef ReturnCode int; /* 1 for success, zero or negative for failure */

typedef Position int; /* a position in the sequence */
/* the first item in the sequence is at position 0 */

typedef Item unsigned char; /* they are sequences of eight bit bytes */

typedef struct {
/* To be determined */
/* Whatever information we need for the data structures we choose */
} Sequence;

In this interface the only operations that change the Sequence are Insert and Delete.

• Sequence Empty( );
• ReturnCode Insert( Sequence *sequence, Position position, Item ch );
• ReturnCode Delete( Sequence *sequence, Position position );
• Item ItemAt( Sequence *sequence, Position position ); -- This does not actually require a pointer to a Sequence since no change to the sequence is being made but we expect that they will be large structures and should not be passing them around. I am ignoring error returns (e.g., position out of range) for the purposes of this discussion. These are easily added if desired.
• ReturnCode Close( Sequence *sequence );

Many variations are possible. The next few paragraphs discuss some of them.

Any practical interface would allow the sequence to be initialized with the contexts of a file. In theory this is just the Empty operation followed by an Insert operation for each character in the initializing file. Of course, this is too inefficient for a real text editor. Instead we would have a NewSequence operation:

• Sequence NewSequence( char * file_name ); -- The sequence is initialized with the contents of the file whose name is contained in `file_name'.

Usually the Delete operation will delete any logically contiguous subsequence

• ReturnCode Delete( Sequence *sequence, Position beginPosition, Position endPosition );

Sometimes the Insert operation will insert a subsequence instead of just a single character.

• ReturnCode Insert( Sequence *sequence, Position position, Sequence sequenceToInsert );

Sometimes Copy and Move are separate operations (instead of being composed of Inserts and Deletes).

• ReturnCode Copy( Sequence *sequence, Position fromBegin, Position fromEnd, Position toPosition );
• ReturnCode Move( Sequence *sequence, Position fromBegin, Position fromEnd, Position toPosition );

A Replace operation that subsumes Insert and Delete in another possibility.

• ReturnCode Replace( Sequence *sequence, Position fromBegin, Position fromEnd, Sequence sequenceToReplaceItWith );

Finally the ItemAt procedure could retrieve a subsequence.

• ReturnCode SequenceAt( Sequence *sequence, Position fromBegin, Position fromEnd, Sequence *returnedSequence );

These variations will not affect the basic character of the data structure used to implement the sequence or the comparisons between them that follow. Therefore I will assume the first interface (Empty, Insert, Delete, IntemAt, and Close).