You’ve already written extract-features, so next up is increment-count, which takes a word-feature and a message type and increments the appropriate slot of the feature. Since there’s no reason to think that the logic of incrementing these counts is going to change for different kinds of objects, you can write this as a regular function.7 Because you defined both ham-count and spam-count with an :accessor option, you can use **INCF** and the accessor functions created by **DEFCLASS** to increment the appropriate slot.

(defun increment-count (feature type)
  (ecase type
    (spam (incf (spam-count feature)))))

The **ECASE** construct is a variant of , both of which are similar to case statements in Algol-derived languages (renamed switch in C and its progeny). They both evaluate their first argument—the key form--and then find the clause whose first element—the key--is the same value according to **EQL**. In this case, that means the variable type is evaluated, yielding whatever value was passed as the second argument to increment-count.

the value of type will be the symbol ham, and the first branch of the **ECASE** will be evaluated and the feature’s ham count incremented. On the other hand, if it’s called like this:

(increment-count some-feature 'spam)

then the second branch will run, incrementing the spam count. Note that the symbols ham and are quoted when calling increment-count since otherwise they’d be evaluated as the names of variables. But they’re not quoted when they appear in **ECASE** since **ECASE** doesn’t evaluate the keys.8

To implement increment-total-count, you need to decide where to store the counts; for the moment, two more special variables, *total-spams* and *total-hams*, will do fine.

You should use **DEFVAR** to define these two variables for the same reason you used it with *feature-database*--they’ll hold data built up while you run the program that you don’t necessarily want to throw away just because you happen to reload your code during development. But you’ll want to reset those variables if you ever reset *feature-database*, so you should add a few lines to clear-database as shown here:

  (setf
   *feature-database* (make-hash-table :test #'equal)
   *total-hams* 0))