day 1, part 2, simplified tokenize

day01-hard/main.scm

@@ -21,6 +21,8 @@
 (define (is-not value) (lambda (x) (not (= x value))))
 (define is-not-empty (lambda (x) (not (null? x))))
 
+(define (prepend first-value) (lambda (rest) (cons first-value rest)))
+
 (define Y (lambda (f) (f (lambda (x) ((Y f) x)))))
 (define Y2 (lambda (f) (f (lambda (x y) ((Y2 f) x y)))))
 
@@ -60,6 +62,10 @@
   '(1 2 3)
   ((first is-not-empty) '(() (1 2 3))))
 
+(define (coalesce-not-empty default) (lambda (value) ((first is-not-empty) (list value default))))
+
+(define (truthy-chaining f) (lambda (value) (if value (f value) #f)))
+
 (define (compose-two f g) (lambda (x) (f (g x))))
 
 (define compose (reduce-right compose-two id))
@@ -73,7 +79,6 @@
   (lambda (f) (lambda (n)
     (if (= n 0) '() (cons value (f (- n 1))))))))
 
-
 ;=============== starts-with ======================
 (define starts-with (Y2
   (lambda (f) (lambda (prefix values)
@@ -113,23 +118,17 @@
     (if
       (null? values)
       '(())
-      ((first is-not-empty) (list
-        (flat
-          ((filter id)
-            ((map
+      ((compose (list
+        (coalesce-not-empty (f (cdr values)))
+        flat
+        (filter id)
+        (map
           (lambda (token)
-                ((lambda (rest)
-                  (if
-                    rest
-                    ((map
-                      (lambda (rest-tokenized)
-                        (cons (car token) rest-tokenized)))
-                      (f (next rest values)))
-                    #f))
-                  (starts-with (cdr token) values))))
-            tokens)))
-        (f (cdr values))
-        '((77 78) (79 76)))))))))
+            ((truthy-chaining (compose (list
+              (map (prepend (car token)))
+              (lambda (rest) (f (next rest values))))))
+              (starts-with (cdr token) values))))))
+        tokens))))))
 
 (define (tokenize tokens)
   (tokenize-generic tokens (lambda (rest values) rest)))
@@ -151,6 +150,7 @@
 
 (define (tokenize-aoc tokens)
   (tokenize-generic tokens (lambda (rest values) (cdr values))))
+
 ;=============== solution ======================
 (define solution-tokens
   (list