Christopher Allen

By the beginning of summer vacations, my niece Thao was pretty comfortable programming in Blockly Games. So for the next couple of months I spent one day a week teaching her more advanced...

Let's start with a list:

'(a b c d e f g h i j)

We'll add some elements:

'(a b c d e f g h i j k l m n o)

Delete some:

'(a b c d e i j k l m n o)

Add some more:

'(x y z a b c d e i j k l m n o)

Maybe delete one:

'(x y z b c d e i j k l m n o)

We want to compare the original sequence with the end sequence and
summarize the difference with a set of "indels", which specify the
indexes at where elements were inserted or deleted:

(diff '(a b c d e f g h i j) '(x y z b c d e i j k l m n o))

(#S(INDEL
    :LEFT-SUBSEQ-START 0         ;; 'a'
    :LEFT-SUBSEQ-LIMIT 1
    :RIGHT-SUBSEQ-START 0        ;; 'x y z'
    :RIGHT-SUBSEQ-LIMIT 3)
 #S(INDEL
    :LEFT-SUBSEQ-START 5         ;; 'f g h'
    :LEFT-SUBSEQ-LIMIT 8
    :RIGHT-SUBSEQ-START 7        ;; nothing
    :RIGHT-SUBSEQ-LIMIT 7)
 #S(INDEL
    :LEFT-SUBSEQ-START 10        ;; nothing
    :LEFT-SUBSEQ-LIMIT 10
    :RIGHT-SUBSEQ-START 9        ;; 'k l m n o'
    :RIGHT-SUBSEQ-LIMIT 14))

Exercise 6 (hard): Implement this.

For an extreme challenge, implement this in a way that is not hopelessly inefficient.