wip: use a sliding window instead of memmove() to scroll

Instead of memmoving a large amount of data on every scroll, use a
sliding window. That is, each time we scroll, we offset origin.

grid.c

@@ -6,3 +6,37 @@
 #define LOG_MODULE "grid"
 #define LOG_ENABLE_DBG 1
 #include "log.h"
+
+struct cell *
+grid_get_range(struct grid *grid, size_t start, size_t *length)
+{
+#define min(x, y) ((x) < (y) ? (x) : (y))
+    assert(*length <= grid->size);
+
+    size_t real_start = (grid->offset + start) % grid->size;
+    assert(real_start < grid->size);
+
+    *length = min(*length, grid->size - real_start);
+    assert(real_start + *length <= grid->size);
+
+    return &grid->cells[real_start];
+#undef min
+}
+
+void
+grid_memset(struct grid *grid, size_t start, int c, size_t length)
+{
+    size_t left = length;
+    while (left > 0) {
+        size_t count = left;
+        struct cell *cells = grid_get_range(grid, start, &count);
+
+        assert(count > 0);
+        assert(count <= left);
+
+        memset(cells, c, count * sizeof(cells[0]));
+
+        left -= count;
+        start += count;
+    }
+}