sixel: wip: split up image being overwritten, rather than erasing it

Instead of completely erasing a sixel image when it is being
"overwritten" (text is printed somewhere within the image, or another
sixel image is emitted within the first image), split it up into up to
four pieces: 'above', 'below', 'to-the-left' and 'to-the-right'.

This is currently very un-optimized, but seems to produce correct
results.

sixel.c

@@ -154,10 +154,143 @@ sixel_delete_at_cursor(struct terminal *term)
         term, term->grid->cursor.point.row, term->grid->cursor.point.col);
 }
 
+static void
+sixel_split(struct terminal *term, struct sixel *six, int row, int col)
+{
+    assert(row >= six->pos.row);
+    assert(row < six->pos.row + six->rows);
+    assert(col >= six->pos.col);
+    assert(col < six->pos.col + six->cols);
+
+    int rel_row = row - six->pos.row;
+    int rel_col = col - six->pos.col;
+
+    if (rel_row > 0) {
+        struct sixel above = {
+            .width = six->width,
+            .height = rel_row * term->cell_height,
+            .rows = rel_row,
+            .cols = six->cols,
+            .pos = six->pos,
+        };
+        above.data = malloc(above.width * above.height * sizeof(uint32_t));
+        memcpy(above.data, six->data, above.width * above.height * sizeof(uint32_t));
+
+        above.pix = pixman_image_create_bits_no_clear(
+            PIXMAN_a8r8g8b8,
+            above.width, above.height,
+            above.data, above.width * sizeof(uint32_t));
+
+        tll_push_back(term->grid->sixel_images, above);
+    }
+
+    if (rel_row + 1 < six->rows) {
+        struct sixel below = {
+            .width = six->width,
+            .height = six->height - (rel_row + 1) * term->cell_height,
+            .rows = six->rows - (rel_row + 1),
+            .cols = six->cols,
+            .pos = (struct coord){
+                six->pos.col,
+                (six->pos.row + rel_row + 1) & (term->grid->num_rows - 1)},
+        };
+        below.data = malloc(below.width * below.height * sizeof(uint32_t));
+        memcpy(
+            below.data,
+            &((const uint32_t *)six->data)[(rel_row + 1) * term->cell_height * six->width],
+            below.width * below.height * sizeof(uint32_t));
+        below.pix = pixman_image_create_bits_no_clear(
+            PIXMAN_a8r8g8b8,
+            below.width, below.height,
+            below.data, below.width * sizeof(uint32_t));
+
+        tll_push_back(term->grid->sixel_images, below);
+    }
+
+    if (rel_col > 0) {
+        struct sixel left = {
+            .width = rel_col * term->cell_width,
+            .height = min(term->cell_height, six->height - rel_row * term->cell_height),
+            .rows = 1,
+            .cols = rel_col,
+            .pos = (struct coord){six->pos.col,
+                                  (six->pos.row + rel_row) & (term->grid->num_rows - 1)},
+        };
+        left.data = malloc(left.width * left.height * sizeof(uint32_t));
+        for (size_t i = 0; i < term->cell_height; i++)
+            memcpy(
+                &((uint32_t *)left.data)[i * left.width],
+                &((const uint32_t *)six->data)[(rel_row * term->cell_height + i) * six->width],
+                left.width * sizeof(uint32_t));
+        left.pix = pixman_image_create_bits_no_clear(
+            PIXMAN_a8r8g8b8,
+            left.width, left.height,
+            left.data, left.width * sizeof(uint32_t));
+        tll_push_back(term->grid->sixel_images, left);
+
+    }
+
+    if (rel_col + 1 < six->cols) {
+        struct sixel right = {
+            .width = six->width - (rel_col + 1) * term->cell_width,
+            .height = min(term->cell_height, six->height - rel_row * term->cell_height),
+            .rows = 1,
+            .cols = six->cols - (rel_col + 1),
+            .pos = (struct coord){six->pos.col + rel_col + 1,
+                                  (six->pos.row + rel_row) & (term->grid->num_rows - 1)},
+        };
+        right.data = malloc(right.width * right.height * sizeof(uint32_t));
+        for (size_t i = 0; i < term->cell_height; i++)
+            memcpy(
+                &((uint32_t *)right.data)[i * right.width],
+                &((const uint32_t *)six->data)[(rel_row * term->cell_height + i) * six->width + (rel_col + 1) * term->cell_width],
+                right.width * sizeof(uint32_t));
+        right.pix = pixman_image_create_bits_no_clear(
+            PIXMAN_a8r8g8b8,
+            right.width, right.height,
+            right.data, right.width * sizeof(uint32_t));
+        tll_push_back(term->grid->sixel_images, right);
+    }
+}
+
+void
+sixel_split_at_point(struct terminal *term, int _row, int col)
+{
+    if (likely(tll_length(term->grid->sixel_images) == 0))
+        return;
+
+    const int row = (term->grid->offset + _row) & (term->grid->num_rows - 1);
+
+    tll_foreach(term->grid->sixel_images, it) {
+        struct sixel *six = &it->item;
+
+        const int six_start = six->pos.row;
+        const int six_end = six_start + six->rows - 1;
+
+        if (row >= six_start && row <= six_end) {
+            const int col_start = six->pos.col;
+            const int col_end = six->pos.col + six->cols;
+
+            if (col >= col_start && col < col_end) {
+                //sixel_erase(term, six);
+                sixel_split(term, six, row, col);
+                sixel_erase(term, six);
+                tll_remove(term->grid->sixel_images, it);
+            }
+        }
+    }
+}
+
+void
+sixel_split_at_cursor(struct terminal *term)
+{
+    sixel_split_at_point(term, term->grid->cursor.point.row, term->grid->cursor.point.col);
+}
+
 void
 sixel_unhook(struct terminal *term)
 {
-    sixel_delete_at_cursor(term);
+    //sixel_delete_at_cursor(term);
 
     struct sixel image = {
         .data = term->sixel.image.data,
@@ -170,6 +303,11 @@ sixel_unhook(struct terminal *term)
             (term->grid->offset + term->grid->cursor.point.row) & (term->grid->num_rows - 1)},
     };
 
+    for (int row = 0; row < image.rows; row++) {
+        for (int col = 0; col < image.cols; col++)
+            sixel_split_at_point(term, term->grid->cursor.point.row + row, term->grid->cursor.point.col + col);
+    }
+
     LOG_DBG("generating %dx%d pixman image", image.width, image.height);
 
     image.pix = pixman_image_create_bits_no_clear(