Add commands next-buffer / previous-buffer (#20)

* Bind `next-buffer` to `C-x C-right` and `previous-buffer` to `C-x left` just like GNU Emacs
* next-buffer & previous-buffer skip buffers with BF_SYSTEM flag.
* next-buffer & previous-buffer command is ignored from minibuffer and popups.
* fix compilation errors on linux

TODO.md

@@ -62,8 +62,6 @@
   an EditBuffer could also have an embedded QECursor with no EditState
 * use hash tables for command and variable names
 * sort key binding tables?
-* `next-buffer` command on C-x C-right
-* `previous-buffer` command on C-x C-left
 * `save-some-buffers` command on C-x s
 * standardize key prefixes: C-, M-, S- and possibly others
 * use C-M- prefix instead of M-C- in source and documentation but support

qe.c

@@ -7422,6 +7422,67 @@ void do_switch_to_buffer(EditState *s, const char *bufname)
         switch_to_buffer(s, b);
 }
 
+/* Find next non-system buffer from the specified buffer. If the specified
+   buffer is the last one, returns the first one. If only one non-system buffer
+   exists currently, return itself. */
+static EditBuffer *next_non_system_buffer(EditState *s, EditBuffer *b) {
+    EditBuffer *nb = b == NULL ? NULL : b->next;
+    if (nb == NULL) {
+        nb = s->qe_state->first_buffer;
+    }
+    /* qemacs guarantees at least one non-system buffer (i.e. `*scratch*`) exists */
+    while (nb->flags & BF_SYSTEM) {
+        nb = nb->next;
+        if (nb == NULL) {
+            nb = s->qe_state->first_buffer;
+        }
+    }
+    return nb;
+}
+
+void do_next_buffer(EditState *s)
+{
+    EditBuffer *b;
+
+    /* ignore command from the minibuffer and popups */
+    if (s->flags & (WF_POPUP | WF_MINIBUF))
+        return;
+
+    b = next_non_system_buffer(s, s->b);
+    switch_to_buffer(s, b);
+}
+
+void do_previous_buffer(EditState *s)
+{
+    // TODO(chqrlie): simplify this by counting non system buffers
+    //   also handle universal argument
+    EditBuffer *orig, *curr, *curr_next;
+
+    /* ignore command from the minibuffer and popups */
+    if (s->flags & (WF_POPUP | WF_MINIBUF))
+        return;
+
+    /* what we want to compare & find are non-system buffers, so need to ignore
+       all non-system buffers; or it may fall into an infinite loop. */
+
+    /* original buffer is immutable */
+    orig = s->b;
+    if (orig->flags & BF_SYSTEM) {
+        orig = next_non_system_buffer(s, orig);
+    }
+    /* find from first buffer*/
+    curr = s->qe_state->first_buffer;
+    if (curr->flags & BF_SYSTEM) {
+        curr = next_non_system_buffer(s, curr);
+    }
+    curr_next = next_non_system_buffer(s, curr);
+    while (curr_next != orig) {
+        curr = curr_next;
+        curr_next = next_non_system_buffer(s, curr_next);
+    }
+    switch_to_buffer(s, curr);
+}
+
 void do_toggle_read_only(EditState *s)
 {
     s->b->flags ^= BF_READONLY;

qe.h

@@ -1403,6 +1403,8 @@ void display_mode_line(EditState *s);
 int edit_set_mode(EditState *s, ModeDef *m);
 void qe_set_next_mode(EditState *s, int n, int status);
 void do_set_next_mode(EditState *s, int n);
+void do_next_buffer(EditState *s);
+void do_previous_buffer(EditState *s);
 
 /* loading files */
 void do_exit_qemacs(EditState *s, int argval);

qeconfig.h

@@ -224,6 +224,12 @@ static const CmdDef basic_commands[] = {
           do_kill_buffer, ESsi,
           "s{Kill buffer: }[buffer]|buffer|"
           "v", 0)
+    CMD0( "next-buffer", "C-x C-right",
+          "Switch to the next buffer",
+          do_next_buffer)
+    CMD0( "previous-buffer", "C-x C-left",
+          "Switch to the previous buffer",
+          do_previous_buffer)
     CMD0( "toggle-read-only", "C-x C-q, C-c %",
           "Toggle the read-only flag of the current buffer",
           do_toggle_read_only)