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+(import (Alpha Core))
+(import (Alpha String))
+(import (only (Alpha String) replace))
+(import (only (ice-9 ftw) scandir))
+(import (only (srfi srfi-1) remove))
+
+;; general functions
+
+(define (os.listdir path)
+  (scandir path))
+
+(define (os.path.expanduser path)
+  (let ([home (getenv "HOME")])
+    (replace path "~" home)))
+
+(define (rm ls item)
+  (remove (/. x (eq? x item)) ls))
+
+(define (set.difference s1 s2)
+  (cond [(null? s1) '()]
+        [(not (member? (first s1) s2))
+         (cons (first s1) (set.difference (rest s1) s2))]
+        [else
+         (set.difference (rest s1) s2)]))
+
+;; z program
+
+(define *zdir* (os.path.expanduser "~/test-z-wiki"))
+
+;; A node 'id' is a monotonically increasing number.
+
+;; It can be represented as a string, in which case it is encoded in
+;; base 36.
+
+(define (id->string id)
+  (number->string id 36))
+
+(define (string->id s)
+  (string->number s 36))
+
+;; It can be represented as a path, in which case it is
+;; `<*zdir*>/<string>.md'
+
+(define (path->id fname)
+  "Given a path, parse out the node id."
+  (-> fname
+      (replace ".md" "")
+      (replace *zdir* "")
+      (string->id)))
+
+(define (id->path id)
+  "Given an id, return the absolute path to the file."
+  (fmt "~a/~a.md"
+       *zdir*
+       (id->string id)))
+
+;; Manipulating nodes
+
+(define (list-nodes dir)
+  (->> dir
+       (os.listdir)
+       ;; remove '.' and '..'
+       ((/. s (set.difference s '("." ".."))))
+       (map path->id)))
+
+(define (latest-node ls)
+  (if (eq? ls '())
+      0
+      (apply max ls)))
+
+(define (make-node title)
+  (let ([id (inc (-> *zdir* list-nodes latest-node))])
+    (call-with-output-file (id->path id)
+      (fn [file]
+          (format file "title: ~a" title)
+          (newline file)))))