First cut of 3.25

For now only lookup takes a list of indices.  Insert! is hardcoded to
two indices.

3_25.rkt

@@ -0,0 +1,52 @@
+#lang sicp
+
+(#%require racket/trace)
+
+(define (make-table)
+  (list '*table*))
+
+;;(define (find-record
+
+(define (assoc key records)
+  (cond ((null? records) false)
+        ((equal? key (caar records)) (car records))
+        (else (assoc key (cdr records)))))
+
+(define (table? l)
+  (pair? (cdr l)))
+
+(trace-define (lookup keys table)
+  (cond ((null? keys) (cdr table))
+        ((not (table? table)) false)
+        (else
+         (let ((subtable (assoc (car keys) (cdr table))))
+           (if subtable
+               (lookup (cdr keys) subtable)
+               false)))))
+
+(define (insert! key-1 key-2 value table)
+  (let ((subtable (assoc key-1 (cdr table))))
+    (if subtable
+        (let ((record (assoc key-2 (cdr subtable))))
+          (if record
+              (set-cdr! record value)
+              (set-cdr! subtable
+                        (cons (cons key-2 value)
+                              (cdr subtable)))))
+        (set-cdr! table
+                  (cons (list key-1
+                              (cons key-2 value))
+                        (cdr table)))))
+  'ok)
+
+(#%require (only racket/base module+))
+
+(module+ test
+  (#%require rackunit)
+
+  (test-begin
+    (define t (make-table))
+    (insert! 10 10 'a t)
+    (check-equal? (lookup '(10 10) t) 'a)
+    (check-equal? (lookup '(10 10 10) t) false))
+  )