tui: Move screen state tracking to new "ugrid" module

The ugrid module implements a unicode "drawing" grid and is used to store TUI
screen state. Later this module will be reused in other layers.

src/nvim/ugrid.c

@@ -0,0 +1,137 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <limits.h>
+
+#include "nvim/vim.h"
+#include "nvim/ui.h"
+#include "nvim/ugrid.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "ugrid.c.generated.h"
+#endif
+
+void ugrid_init(UGrid *grid)
+{
+  grid->attrs = EMPTY_ATTRS;
+  grid->fg = grid->bg = -1;
+  grid->cells = NULL;
+}
+
+void ugrid_free(UGrid *grid)
+{
+  destroy_cells(grid);
+}
+
+void ugrid_resize(UGrid *grid, int width, int height)
+{
+  destroy_cells(grid);
+  grid->cells = xmalloc((size_t)height * sizeof(UCell *));
+  for (int i = 0; i < height; i++) {
+    grid->cells[i] = xcalloc((size_t)width, sizeof(UCell));
+  }
+
+  grid->top = 0;
+  grid->bot = height - 1;
+  grid->left = 0;
+  grid->right = width - 1;
+  grid->row = grid->col = 0;
+  grid->width = width;
+  grid->height = height;
+}
+
+void ugrid_clear(UGrid *grid)
+{
+  clear_region(grid, grid->top, grid->bot, grid->left, grid->right);
+}
+
+void ugrid_eol_clear(UGrid *grid)
+{
+  clear_region(grid, grid->row, grid->row, grid->col, grid->right);
+}
+
+void ugrid_goto(UGrid *grid, int row, int col)
+{
+  grid->row = row;
+  grid->col = col;
+}
+
+void ugrid_set_scroll_region(UGrid *grid, int top, int bot, int left, int right)
+{
+  grid->top = top;
+  grid->bot = bot;
+  grid->left = left;
+  grid->right = right;
+}
+
+void ugrid_scroll(UGrid *grid, int count, int *clear_top, int *clear_bot)
+{
+  // Compute start/stop/step for the loop below
+  int start, stop, step;
+  if (count > 0) {
+    start = grid->top;
+    stop = grid->bot - count + 1;
+    step = 1;
+  } else {
+    start = grid->bot;
+    stop = grid->top - count - 1;
+    step = -1;
+  }
+
+  int i;
+
+  // Copy cell data
+  for (i = start; i != stop; i += step) {
+    UCell *target_row = grid->cells[i] + grid->left;
+    UCell *source_row = grid->cells[i + count] + grid->left;
+    memcpy(target_row, source_row,
+        sizeof(UCell) * (size_t)(grid->right - grid->left + 1));
+  }
+
+  // clear cells in the emptied region,
+  if (count > 0) {
+    *clear_top = stop;
+    *clear_bot = stop + count - 1;
+  } else {
+    *clear_bot = stop;
+    *clear_top = stop + count + 1;
+  }
+  clear_region(grid, *clear_top, *clear_bot, grid->left, grid->right);
+}
+
+UCell *ugrid_put(UGrid *grid, uint8_t *text, size_t size)
+{
+  UCell *cell = grid->cells[grid->row] + grid->col;
+  cell->data[size] = 0;
+  cell->attrs = grid->attrs;
+
+  if (text) {
+    memcpy(cell->data, text, size);
+  }
+
+  grid->col += 1;
+  return cell;
+}
+
+static void clear_region(UGrid *grid, int top, int bot, int left, int right)
+{
+  HlAttrs clear_attrs = EMPTY_ATTRS;
+  clear_attrs.foreground = grid->fg;
+  clear_attrs.background = grid->bg;
+  UGRID_FOREACH_CELL(grid, top, bot, left, right, {
+    cell->data[0] = ' ';
+    cell->data[1] = 0;
+    cell->attrs = clear_attrs;
+  });
+}
+
+static void destroy_cells(UGrid *grid)
+{
+  if (grid->cells) {
+    for (int i = 0; i < grid->height; i++) {
+      xfree(grid->cells[i]);
+    }
+    xfree(grid->cells);
+  }
+}
+