pic Specification

Concrete Syntax of pic

A pic program is a sequence of <element>s. The concrete syntax of the subset of pic you are to use for this assignment is given in the following grammar:

  <element> ::= <shape> <attributes> ;
            | <direction> ;

  <shape> ::= box | circle | move | arrow | line

  <direction> ::= right | left | up | down

  <attributes> ::= <string> <attributes>
                | <direction> <attributes>
                | <empty>

In this grammar, <string> represents a string constant contained in double quotes. The <attributes> can be a (possibly <empty>) list, where each elements of the list is either a <string> or a <direction>.

For example, Figure 1 shows the concrete syntax tree for this pic command, which appears in the diagram.pic program from lab:

  arrow "step 3" right down; 

Semantics of pic

Current position and direction

At any given point in a pic program, there is a current position (on the page) and a current direction (left, right, up, or down).

By themselves, the commands left; and right; and up; and down; just change the current direction.

Shapes: Drawing + updating position

The box and circle commands draw a box or circle, respectively, adjacent to the current position. Exactly how the shape is positioned depends on the current direction. If the current direction is “right”, then the middle of the left side of the box is placed at the current position (so that the box is to the right of the current position); similarly, if the current direction is “down” then the middle of the top edge of the box is placed at the current position (so that the box is below the current position). After the shape is drawn, the current position is changed to the opposite side of the shape.

The line and arrow commands draw a line or arrow, respectively, starting at the current position and extending (by default) in the current direction. The current position is then changed to the far end of the new line or arrow. For the arrow command , the “far end” is the tip of arrow’s triangle, and the line for the arrow ends at the base of the arrow’s triangle. The move command changes the current position just like line, though no line is actually drawn.

Attributes

Some commands can have a list of “attributes” that modify the command. In our limited subset of the language, the only attributes are strings or directions.

Shape commands (including the move) can have string attributes. The string attributes are drawn as text, one line per string, centered at the geometric center of the shape being drawn.

The line, move, and arrow commands additionally use their direction attributes to override the current direction, so the command move right is the same as the sequence right; move. More interestingly, multiple direction attributes are additive, thus, line right down draws a diagonal line right and down, while line right right draws a line twice as far to the right as normal.

If line, move, or arrow has at least one direction attribute, the current direction afterwards will be the last of the direction attributes (down and right respectively in the previous two examples).

If line, move, or arrow have any direction attributes, those attributes replace the existing direction: When computing the net result of the specified directions, pic ignores the existing direction and does not add it to the attributes.

pic ignores any direction attributes for box and circle.