Description

1. General matters
1.1. Aim. The purpose of the assignment is to:
• develop your problem solving skills;
• design and implement a solution to a problem in the form of medium sized Python program;
• in particular but no to only, parse text in files, make use of the dynamic programming algorithmic technique, implement class methods.
1.3. Assessment. The assignment is worth 13 marks, distributed as indicated in Part 3. It is going to be tested against a number of inputs. For each test, the automarking script will let your program run for 30 seconds. The outputs of your programs should be exactly as indicated.
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2. General presentation
This presentation refers to a number of text files that are provided together with this pdf. Consider the two text files file_1_1.txt and file_1_2.txt:
$ cat -n file_1_1.txt
1 A line to delete: 1
2 A line to delete: 2
3 A line that stays: 1
4 A line that stays: 2
5 A line to change: 1
6 A line that stays: 3
7 A line that stays: 4
8 A line that stays: 5
9 A line that stays: 6
10 A line to delete: 3
11 A line that stays: 7
12 A line that stays: 8
13 A line to change: 2
14 A line to change: 3
15 A line to change: 4
16 A line to change: 5
17 A line that stays: 9
$ cat -n file_1_2.txt
1 A line that stays: 1
2 A line to insert: 1
3 A line that stays: 2
4 A changed line: 1
5 A line that stays: 3
6 A line that stays: 4
7 A line to insert: 2
8 A line to insert: 3
9 A line that stays: 5 10 A line that stays: 6
11 A line that stays: 7
12 A line that stays: 8
13 A changed line: 2
14 A changed line: 3
15 A changed line: 4
16 A line that stays: 9
The Unix diff command can be applied to file_1_1.txt and file_1_2.txt:
$ diff file_1_1.txt file_1_2.txt
1,2d0
< A line to delete: 1
< A line to delete: 2
3a2
> A line to insert: 1
5c4
< A line to change: 1
—
> A changed line: 1
7a7,8
> A line to insert: 2
> A line to insert: 3
10d10
< A line to delete: 3
13,16c13,15
< A line to change: 2
< A line to change: 3
< A line to change: 4
< A line to change: 5
—
> A changed line: 2
> A changed line: 3
> A changed line: 4
Study this example to understand diff’s output.
• d is for delete, for commands of the form l_1dl_2 with l_1 and l_2 two line numbers, or of the form l_1,l_2dl_3 with l_1, l_2 and l_3 three line numbers.
• a is for add, for commands of the form l_1al_2 with l_1 and l_2 two line numbers, or of the form l_1al_2,l_3 with l_1, l_2 and l_3 three line numbers.
• c is for change, for commands of the form l_1cl_2 with l_1 and l_2 two line numbers, or of the form l_1,l_2cl_3 with l_1, l_2 and l_3 three line numbers, or of the form l_1cl_2,l_3 with l_1, l_2 and l_3 three line numbers, or of the form l_1,l_2cl_3,l_4 with l_1, l_2, l_3 and l_4 four line numbers.
diff identifies a longest common subsequence (LCS) of lines that are common to both files. It then computes the unique minimal set of commands that allows one to convert the first file into the second one (the ed editor can actually precisely do this). With the previous example, there is a unique LCS, consisting of the lines A line that stays: 1, A line that stays: 2, A line that stays: 3 and A line that stays: 4.
Still the LCS is not always unique. Consider for instance the two text files file_2_1.txt and file_2_2.txt:
$ cat -n file_2_1.txt
1 A line
$ cat -n file_2_2.txt
1 A line
2 A line
There are then 2 LCSs, as one can chose from file_2_2.txt either the first or the second line. Unix diff determines one LCS and yields the corresponding output:
$ diff file_2_1.txt file_2_2.txt
1a2
> A line
But another implementation could have output instead:
$ diff file_2_1.txt file_2_2.txt
0a1
> A line
For another example, consider the two text files file_3_1.txt and file_3_2.txt:
$ cat -n file_3_1.txt
1 Line 1
2 Line 2
3 A line to go
4 A line to go
5 A line to go
6 Line 3
7 Line 4
8 A line to go
$ cat -n file_3_2.txt
1 A line to come
2 A line to come
3 Line 1
4 Line 2
5 A line to come
6 A line to come
7 Line 1
8 Line 2
9 A line to come 10 A line to come
11 A line to come
12 Line 3
13 Line 4
There are then 3 LCSs, as the unique occurrences of Line 1 and Line 2 in the first file can be matched with the first occurrences of Line 1 and Line 2 in the second file, or with the first occurrence of Line 1 and the second occurrence of Line 2 in the second file, or with the second occurrences of Line 1 and Line 2 in the second file.
Unix diff determines one LCS and yields the corresponding output:
$ diff file_3_1.txt file_3_2.txt
0a1,2
> A line to come
> A line to come
3,5c5,11
< A line to go
< A line to go
< A line to go
—
> A line to come
> A line to come
> Line 1
> Line 2
> A line to come
> A line to come
> A line to come
8d13
< A line to go
But another implementation could have output instead either
0a1,2
> A line to come
> A line to come
1a4,7
> Line 2
> A line to come
> A line to come
> Line 1
3,5c9,11
< A line to go
< A line to go
< A line to go
—
> A line to come
> A line to come
> A line to come
8d13
< A line to go
or 0a1,6
> A line to come
> A line to come
> Line 1
> Line 2
> A line to come
> A line to come
3,5c9,11
< A line to go
< A line to go
< A line to go
—
> A line to come
> A line to come
> A line to come
8d13
< A line to go
One could want to output the LCS, common to both files, with … for longest nonempty sequences of lines that are not part of the LCS, w.r.t. the first file, or w.r.t. the second file.
For the Unix diff applied to file_1_1.txt and file_1_2.txt, that would be, w.r.t. file_1_1.txt:
…
A line that stays: 1
A line that stays: 2
…
A line that stays: 3
A line that stays: 4
A line that stays: 5
A line that stays: 6
…
A line that stays: 7
A line that stays: 8
…
A line that stays: 9
and w.r.t. file_1_2.txt: A line that stays: 1
…
A line that stays: 2
…
A line that stays: 3
A line that stays: 4
…
A line that stays: 5
A line that stays: 6
A line that stays: 7
A line that stays: 8
…
A line that stays: 9
For the Unix diff applied to file_2_1.txt and file_2_2.txt, that would be, w.r.t. file_2_1.txt: A line
and w.r.t. file_2_2.txt: A line …
For the Unix diff applied to file_3_1.txt and file_3_2.txt, that would be, w.r.t. file_3_1.txt:
Line 1
Line 2 …
Line 3
Line 4 …
and w.r.t. file_3_2.txt:
…
Line 1
Line 2 …
Line 3
Line 4
3. Task specifications
Write a program stored in a file named diff.py that implements three classes,
• DiffCommands,
• DiffCommandsError and
• OriginalNewFiles, the second one deriving from Exception.
DiffCommands does not provide any method in its public interface. It builds a DiffCommands object from a text file meant to store a plausible sequence of diff commands, one command per line, without any space on any line, and without any extra line. In case the text file does not satisfy those conditions, the __init__()
method of DiffCommands raises a DiffCommandsError error (1 mark) with as message
Cannot possibly be the commands for the diff of two files
Seven files of this type are provided (wrong_1.txt to wrong_7.txt). Three files from which you can build a DiffCommands object are provided:
• diff_1.txt, which stores the only possible sequence of diff commands for the sample files file_1_1.txt and file_1_2.txt;
• diff_2.txt, which stores one of the two possible sequences of diff commands for the sample files file_2_1.txt and file_2_2.txt;
• diff_3.txt, which stores one of the three possible sequences of diff commands for the sample files file_3_1.txt and file_3_2.txt.
DiffCommands implements the __str__() function (3 marks) so that print() can be used to output the diff commands of the object under consideration.
OriginalNewFiles provides a user interface with 5 methods:
• is_a_possible_diff() (2 marks)
• output_diff() (2 marks)
• output_unmodified_from_original() (1.5 marks)
• output_unmodified_from_new() (1.5 marks)
• get_all_diff_commands() (2 marks)
The following example of interaction shows how the methods are supposed to be called and what they are supposed to return. The only point which might not be obvious from that example is that in the sequence of diff commands output by the method
OriginalNewFiles.pair_of_files.get_all_diff_commands()
the diff commands are lexicographically ordered.
$ python3 …
>>> from diff import * >>> DiffCommands(‘wrong_1.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_2.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_3.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_4.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_5.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_6.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files >>> DiffCommands(‘wrong_7.txt’) …
diff.DiffCommandsError: Cannot possibly be the commands for the diff of two files
>>> DiffCommands(‘diff_1.txt’)
<diff.DiffCommands object at …>
>>> diff_1 = DiffCommands(‘diff_1.txt’)
>>> print(diff_1)
1,2d0
3a2
5c4
7a7,8
10d10
13,16c13,15
>>> diff_2 = DiffCommands(‘diff_2.txt’)
>>> print(diff_2)
1a2
>>> diff_3 = DiffCommands(‘diff_3.txt’)
>>> print(diff_3)
0a1,2
3,5c5,11
8d13
>>> pair_of_files = OriginalNewFiles(‘file_1_1.txt’, ‘file_1_2.txt’)

pair_of_files.is_a_possible_diff(diff_1)
True
>>> pair_of_files.is_a_possible_diff(diff_2)
False
>>> pair_of_files.is_a_possible_diff(diff_3)
False
>>> pair_of_files.output_diff(diff_1)
1,2d0
< A line to delete: 1
< A line to delete: 2
3a2> A line to insert: 1
5c4< A line to change: 1
> A changed line: 1
7a7,8> A line to insert: 2
> A line to insert: 3
< A line to delete: 3

—
10d10
13,16c13,15
< A line to change: 2
< A line to change: 3
< A line to change: 4
< A line to change: 5
—
> A changed line: 2
> A changed line: 3
> A changed line: 4
>>> pair_of_files.output_unmodified_from_original(diff_1) …
A line that stays: 1
A line that stays: 2
…
A line that stays: 3
A line that stays: 4
A line that stays: 5
A line that stays: 6
…
A line that stays: 7
A line that stays: 8
…
A line that stays: 9 pair_of_files.output_unmodified_from_new(diff_1) A line that stays: 1
…
A line that stays: 2
…
A line that stays: 3
A line that stays: 4
…
A line that stays: 5
A line that stays: 6
A line that stays: 7
A line that stays: 8
…
A line that stays: 9
>>> pair_of_files.get_all_diff_commands()
[<diff.DiffCommands object at …>]
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
1
>>> print(diffs[0])
1,2d0
3a2
5c4
7a7,8
10d10
13,16c13,15
>>> pair_of_files = OriginalNewFiles(‘file_1_2.txt’, ‘file_1_1.txt’)
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
1
>>> print(diffs[0])
0a1,2
2d3
4c5
7,8d7
10a10
13,15c13,16
>>> pair_of_files = OriginalNewFiles(‘file_1_1.txt’, ‘file_1_1.txt’)
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
1
>>> print(diffs[0])
>>> pair_of_files = OriginalNewFiles(‘file_2_1.txt’, ‘file_2_2.txt’) pair_of_files.is_a_possible_diff(diff_1)
False
>>> pair_of_files.is_a_possible_diff(diff_2)
True
>>> pair_of_files.is_a_possible_diff(diff_3)
False
>>> pair_of_files.output_diff(diff_2)
1a2
> A line
>>> pair_of_files.output_unmodified_from_original(diff_2)
A line
>>> pair_of_files.output_unmodified_from_new(diff_2)
A line …
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
2
>>> print(diffs[0])
0a1
>>> print(diffs[1])
1a2
>>> pair_of_files = OriginalNewFiles(‘file_2_2.txt’, ‘file_2_1.txt’)
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
2
>>> print(diffs[0])
1d0
>>> print(diffs[1])
2d1
>>> pair_of_files = OriginalNewFiles(‘file_3_1.txt’, ‘file_3_2.txt’)
>>> pair_of_files.is_a_possible_diff(diff_1)
False
>>> pair_of_files.is_a_possible_diff(diff_2)
False
>>> pair_of_files.is_a_possible_diff(diff_3) True
pair_of_files.output_diff(diff_3)
0a1,2
> A line to come
> A line to come
3,5c5,11
< A line to go
< A line to go
< A line to go
—
> A line to come
> A line to come
> Line 1
> Line 2
> A line to come
> A line to come
> A line to come
8d13
< A line to go
>>> pair_of_files.output_unmodified_from_original(diff_3)
Line 1
Line 2 …
Line 3
Line 4 …
>>> pair_of_files.output_unmodified_from_new(diff_3) …
Line 1
Line 2 …
Line 3
Line 4
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
3
>>> print(diffs[0])
0a1,2
1a4,7
3,5c9,11
8d13
>>> print(diffs[1])
0a1,2
3,5c5,11
8d13
>>> print(diffs[2])
0a1,6
3,5c9,11
8d13
>>> pair_of_files = OriginalNewFiles(‘file_3_2.txt’, ‘file_3_1.txt’)
>>> diffs = pair_of_files.get_all_diff_commands()
>>> len(diffs)
3
>>> print(diffs[0])
1,2d0

4,7d1
9,11c3,5
13a8
>>> print(diffs[1])
1,2d0
5,11c3,5
13a8
>>> print(diffs[2])
1,6d0
9,11c3,5
13a8