1 files changed, 74 insertions, 0 deletions
diff --git a/02/lib.scm b/02/lib.scm
new file mode 100644
index 0000000..0274dad
--- /dev/null
+++ b/02/lib.scm
@@ -0,0 +1,74 @@
+(library (lib)
+  (export solve-part1 solve-part2)
+  (import (chezscheme))
+
+  ; Split list at first delimiter into `prefix' and `suffix'
+  ; Return value is a pair like `((p r e f i x) s u f f i x)'
+  (define (list-split-left delimiter? xs)
+    (let loop ((prefix '())
+               (suffix xs))
+      (if (null? suffix)
+        (cons prefix suffix)
+        (let ((x (car suffix)))
+          (if (delimiter? x)
+              ; Found first delimiter, so return immediately
+              (cons prefix (cdr suffix))
+              ; `x' isn't a delimiter, so append it to `prefix'
+              (loop (append prefix (list x)) (cdr suffix)))))))
+
+  ; Split list at given delimiter into list of sublists
+  (define (list-split delimiter? xs)
+    (let loop ((pieces '())
+               (rest xs))
+      (if (null? rest)
+          ; Fix order and remove all empty sublists from output
+          ; (which are caused by consecutive delimiters in `xs')
+          (reverse (remp null? pieces))
+          ; Extract next piece from `rest' and prepend it to `pieces'
+          (let ((next (list-split-left delimiter? rest)))
+            (loop (cons (car next) pieces) (cdr next))))))
+
+  ; Split string at whitespace into list of words
+  (define (string-split str)
+    (map list->string (list-split char-whitespace? (string->list str))))
+
+  ; Partially applied divisibility check
+  (define (check-divisor? x)
+    (lambda (y)
+      (and (not (= y 0)) ; Don't try dividing by zero
+           (not (= x y)) ; Disallow trivial solution
+           (= (mod x y) 0))))
+
+  ; Find minimum and maximum in row, return difference
+  (define (find-range row)
+    (- (apply max row) (apply min row)))
+
+  ; Find `x' and `y' where `y' is divisor of `x', return quotient
+  (define (find-quotient row)
+    ; Note: we don't check if we've reached the end of `xs',
+    ; because we're guaranteed success before that happens.
+    ; We're also guaranteed only one non-trivial solution.
+    (let loop ((xs row))
+      (let* ((x (car xs))
+             (y (find (check-divisor? x) row)))
+        ; Is this the (`x',`y') combination we're looking for,
+        ; i.e. have we found a valid divisor `y' for this `x'?
+        (if y
+            (div x y)
+            (loop (cdr xs))))))
+
+  ; Add up result of `proc' for each `row' of numbers in `lines'
+  (define (solve-puzzle proc lines)
+    (fold-left
+      (lambda (accum row)
+        (+ accum (proc (map string->number row))))
+      0
+      (map string-split lines)))
+
+  (define (solve-part1 lines)
+    (solve-puzzle find-range lines))
+
+  (define (solve-part2 lines)
+    (solve-puzzle find-quotient lines))
+
+)