Description
Little Quilt is a small language introduced by Ravi Sethi in his book ‘Programming Languages’. Here, a restricted version of Little Quilt is presented.
The language is defined by the following BNF grammar:
::= A | B | turn() | sew(,)
A and B represent the two primitive quilts. Each primitive quilt corresponds to a matricial arrangement of 2 × 2 characters. turn() and sew() are operations over quilts.
The instruction turn(x) turns the quilt x 90 degrees clockwise. The following table illustrates the primitive quilts as well as examples of the effect of the turn() operation:
A //
/+
turn(A) \
+\
turn(turn(A)) +/
//
turn(turn(turn(A))) +
\
B --
turn(B) ||
||
Accordingly, the instruction sew(x,y) sews quilt x to the left of quilt y. Both x and y must have the same height, otherwise an error will be generated. The following figure represents the result of sew(A,turn(B)):
//||
/+||
while the sew(turn(sew(B,turn(B))),A) generates an error message.
Your job is to build an interpreter of the Little Quilt language.
Input
The input file will be a text file containing different Little Quilt expressions, each one ended by a semicolon character (;). Space and new line characters must be ignored; this means that an expression may span several lines.
Output
The output file contains the quilts produced as a result of interpreting the input expressions.
Each quilt must be preceded by a line, left aligned, with the format
Quilt i:
where i is the quilt number, starting at 1. If the expression interpretation generates and error, the word
error
must be printed.
Sample Input
sew(turn(sew(B,turn(B))),
turn(sew(turn(B),B))) ;
sew(turn(sew(B,turn(B))),A);
sew(turn(sew(A,turn(A))),
turn(turn(
turn(sew(A,turn(A))))))
;
Sample Output
Quilt 1:
||--
||--
--||
--||
Quilt 2:
error
Quilt 3:
\//
+\/+
+/+
//\
