From 4251c81fdb2d2fed0ddb535fe1cc5d8629a70051 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Fri, 19 Dec 2014 23:47:35 -0500
Subject: [PATCH] Sketch of buffers

---
 ansi/buffer.rkt | 92 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 ansi/buffer.rkt

diff --git a/ansi/buffer.rkt b/ansi/buffer.rkt
new file mode 100644
index 0000000..77a6bb3
--- /dev/null
+++ b/ansi/buffer.rkt
@@ -0,0 +1,92 @@
+#lang racket/base
+
+(require "rope.rkt")
+(require "search.rkt")
+
+(struct buffer ([rope #:mutable]
+                [pos #:mutable]
+                ) #:transparent)
+
+(define (make-buffer #:initial-contents [initial-contents ""])
+  (buffer (string->rope initial-contents)
+          0))
+
+(define (buffer-size buf) (rope-size (buffer-rope buf)))
+
+(define (buffer-move-to! buf pos)
+  (set-buffer-pos! buf (max 0 (min (buffer-size buf) pos)))
+  (buffer-seek! buf pos))
+
+(define (buffer-seek! buf pos)
+  (buffer-lift0 rope-seek buf pos))
+
+(define (buffer-move-by! buf delta)
+  (buffer-move-to! buf (+ (buffer-pos buf) delta)))
+
+(define (buffer-mark! buf [mark 'mark] #:position [pos (buffer-pos buf)] #:value [value #t])
+  (buffer-lift0 replace-mark buf mark pos value))
+
+(define (buffer-search* buf start-pos forward? move? find-delta)
+  (define-values (l _marks r) (rope-split (buffer-rope buf) start-pos))
+  (define delta (find-delta (if forward? r l)))
+  (define new-pos (+ start-pos (cond [(not delta) 0] [forward? delta] [else (- delta)])))
+  (when delta
+    (if move?
+        (buffer-move-to! buf new-pos)
+        (buffer-seek! buf new-pos)))
+  new-pos)
+
+(define (buffer-search buf needle
+                       #:position [start-pos (buffer-pos buf)]
+                       #:forward? [forward? #t]
+                       #:move? [move? #t])
+  (buffer-search* buf start-pos forward? move?
+                  (lambda (piece) (search-rope needle piece #:forward? forward?))))
+
+(define (buffer-find buf delims
+                     #:position [start-pos (buffer-pos buf)]
+                     #:forward? [forward? #t]
+                     #:move? [move? #t])
+  (buffer-search* buf start-pos forward? move?
+                  (lambda (piece) (find-in-rope delims piece #:forward? forward?))))
+
+(define (buffer-lift0 f buf . args)
+  (define new-rope (apply f (buffer-rope buf) args))
+  (set-buffer-rope! buf new-rope)
+  buf)
+
+(define (buffer-lift1 f buf . args)
+  (define-values (result new-rope) (apply f (buffer-rope buf) args))
+  (set-buffer-rope! buf new-rope)
+  result)
+
+;; Finseth's book defines a C routine, Framer(), which is intended to
+;; ensure that the `top_of_window` mark, denoting the position where
+;; display should begin for the current window, is in a sane position.
+;; The mark is left alone unless the cursor is outside the currently
+;; displayed window, either above or below. If the mark needs to be
+;; moved, it is moved to a line such that the cursor, after redisplay,
+;; will end up at a configurable percentage of the way down the
+;; window.
+;;
+;; Mark Location Buffer -> Buffer
+;; Ensures the given mark is sanely positioned as a top-of-window mark
+;; with respect to the given cursor position.
+(define (frame-buffer! top-of-window-mark cursor-position window-height buf
+                       #:preferred-position-fraction [preferred-position-fraction 1/2])
+  (define old-top-of-window-pos (find-next-mark-pos (buffer-rope buf) top-of-window-mark))
+  (define preferred-distance-from-bottom (ceiling (* window-height (- 1 preferred-position-fraction))))
+  (let loop ((pos (buffer-find buf "\n" #:forward? #f #:move? #f))
+             (line-count 0)
+             (top-of-window-pos old-top-of-window-pos))
+    (define new-top-of-window-pos
+      (if (= line-count preferred-distance-from-bottom) pos top-of-window-pos))
+    (cond
+     [(<= pos old-top-of-window-pos)
+      buf]
+     [(= line-count window-height)
+      (buffer-mark! buf top-of-window-mark #:position new-top-of-window-pos)]
+     [else
+      (loop (buffer-find buf "\n" #:forward? #f #:move? #f #:position (- pos 1))
+            (+ line-count 1)
+            new-top-of-window-pos)])))