add optimized version of buffer_get_bline.

`buffer_get_bline_w_hint` begins the bline hunt at a
parameterized bline instead of always starting at the first
bline in the buffer. when editing huge files with millions
of lines, there is a noticeable perf improvement if the
hint is nearby the desired bline.

buffer.c

@@ -694,14 +694,44 @@ int buffer_replace_w_bline(buffer_t *self, bline_t *start_line, bint_t start_col
 
 // Return a line given a line_index
 int buffer_get_bline(buffer_t *self, bint_t line_index, bline_t **ret_bline) {
-    bline_t *tmp_line;
+    return buffer_get_bline_w_hint(self, line_index, self->first_line, ret_bline);
+}
+
+// Return a line given a line_index, starting at hint and iterating outward
+int buffer_get_bline_w_hint(buffer_t *self, bint_t line_index, bline_t *opt_hint, bline_t **ret_bline) {
+    bline_t *fwd, *rev, *found;
     MLBUF_MAKE_GT_EQ0(line_index);
-    for (tmp_line = self->first_line; tmp_line; tmp_line = tmp_line->next) {
-        if (tmp_line->line_index == line_index) {
-            *ret_bline = tmp_line;
-            return MLBUF_OK;
+
+    if (!opt_hint) {
+        opt_hint = self->first_line;
+    }
+
+    fwd = opt_hint;
+    rev = opt_hint->prev;
+    found = NULL;
+
+    while (fwd || rev) {
+        if (fwd) {
+            if (fwd->line_index == line_index) {
+                found = fwd;
+                break;
+            }
+            fwd = fwd->next;
+        }
+        if (rev) {
+            if (rev->line_index == line_index) {
+                found = rev;
+                break;
+            }
+            rev = rev->prev;
         }
     }
+
+    if (found) {
+        *ret_bline = found;
+        return MLBUF_OK;
+    }
+
     *ret_bline = self->last_line;
     return MLBUF_ERR;
 }

bview.c

@@ -347,7 +347,7 @@ int bview_set_viewport_y(bview_t *self, bint_t y, int do_rectify) {
     }
     self->viewport_y = y;
     if (do_rectify) bview_rectify_viewport(self);
-    buffer_get_bline(self->buffer, self->viewport_y, &self->viewport_bline);
+    buffer_get_bline_w_hint(self->buffer, self->viewport_y, self->viewport_bline, &self->viewport_bline);
     return MLE_OK;
 }
 
@@ -385,7 +385,7 @@ int bview_rectify_viewport(bview_t *self) {
     if (_bview_rectify_viewport_dim(self, mark->bline, mark->bline->line_index, self->viewport_scope_y, self->rect_buffer.h, &self->viewport_y)) {
         // TODO viewport_y_vrow (soft-wrapped lines, code folding, etc)
         // Refresh viewport_bline
-        buffer_get_bline(self->buffer, self->viewport_y, &self->viewport_bline);
+        buffer_get_bline_w_hint(self->buffer, self->viewport_y, self->viewport_bline, &self->viewport_bline);
     }
 
     return MLE_OK;
@@ -519,7 +519,7 @@ static void _bview_buffer_callback(buffer_t *buffer, baction_t *action, void *ud
                     bview_resize(bview, bview->x, bview->y, bview->w, bview->h);
                 }
                 // Adjust viewport_bline
-                buffer_get_bline(bview->buffer, bview->viewport_y, &bview->viewport_bline);
+                buffer_get_bline_w_hint(bview->buffer, bview->viewport_y, bview->viewport_bline, &bview->viewport_bline);
             }
         }
     }

mark.c

@@ -89,7 +89,7 @@ int mark_move_to_w_bline(mark_t *self, bline_t *bline, bint_t col) {
 // Move mark to line_index:col
 int mark_move_to(mark_t *self, bint_t line_index, bint_t col) {
     bline_t *bline;
-    buffer_get_bline(self->bline->buffer, line_index, &bline);
+    buffer_get_bline_w_hint(self->bline->buffer, line_index, self->bline, &bline);
     _mark_mark_move_inner(self, bline, col, 1, 1);
     return MLBUF_OK;
 }

mlbuf.h

@@ -178,6 +178,7 @@ int buffer_insert_w_bline(buffer_t *self, bline_t *start_line, bint_t start_col,
 int buffer_delete_w_bline(buffer_t *self, bline_t *start_line, bint_t start_col, bint_t num_chars);
 int buffer_replace_w_bline(buffer_t *self, bline_t *start_line, bint_t start_col, bint_t num_chars, char *data, bint_t data_len);
 int buffer_get_bline(buffer_t *self, bint_t line_index, bline_t **ret_bline);
+int buffer_get_bline_w_hint(buffer_t *self, bint_t line_index, bline_t *opt_hint, bline_t **ret_bline);
 int buffer_get_bline_col(buffer_t *self, bint_t offset, bline_t **ret_bline, bint_t *ret_col);
 int buffer_get_offset(buffer_t *self, bline_t *bline, bint_t col, bint_t *ret_offset);
 int buffer_undo(buffer_t *self);

tests/unit/test_buffer_get_bline.c

@@ -10,4 +10,8 @@ void test(buffer_t *buf, mark_t *cur) {
         buffer_get_bline(buf, i, &line2);
         ASSERT("line", line, line2);
     }
+    for (line = buf->first_line, i = 0; line; line = line->next, i += 1) {
+        buffer_get_bline_w_hint(buf, i, buf->last_line->prev, &line2);
+        ASSERT("line", line, line2);
+    }
 }