buffer: add buffer-ref to obtain a single entry

lib/buffer.sld

@@ -18,7 +18,7 @@
 
           (edward util))
 
-  (export make-buffer buffer->list buffer-length buffer-empty?
+  (export make-buffer buffer->list buffer-length buffer-empty? buffer-ref
           buffer-append! buffer-remove! buffer-with-undo buffer-has-undo?
           buffer-undo! buffer-replace! buffer-join! buffer-move!)
 

lib/buffer/buffer.scm

@@ -14,6 +14,11 @@
 (define (make-buffer)
   (%make-buffer (flexvector) #f (make-stack)))
 
+;;> Returns the element at `index` in the `buffer`, starting at zero.
+
+(define (buffer-ref buffer index)
+  (flexvector-ref (buffer-lines buffer) index))
+
 ;;> Convert the line buffer to a list of lines. Additionally, this
 ;;> procedure accepts an optional `start` and `end` parameter. If
 ;;> these parameters are given the list only contains the elements

lib/buffer/srfi214-minimal.scm

@@ -36,6 +36,9 @@
 (define (flexvector)
   (%make-flexvector (make-vector 4) 0))
 
+(define (flexvector-ref fv index)
+  (vector-ref (vec fv) index))
+
 (define (flexvector-add-all! fv i xs)
   (let* ((len (flexvector-length fv))
          (xv (list->vector xs))