particle/empty: placeholder particle, renders nothing

But may have margins.

CMakeLists.txt

@@ -40,6 +40,7 @@ add_executable(f00bar
   decorations/stack.c decorations/stack.h
   decorations/underline.c decorations/underline.h
 
+  particles/empty.c particles/empty.h
   particles/list.c particles/list.h
   particles/map.c particles/map.h
   particles/ramp.c particles/ramp.h

config.c

@@ -13,6 +13,7 @@
 #include "decorations/underline.h"
 
 #include "particle.h"
+#include "particles/empty.h"
 #include "particles/list.h"
 #include "particles/map.h"
 #include "particles/ramp.h"
@@ -164,6 +165,21 @@ deco_from_config(const struct yml_node *node)
         assert(false);
 }
 
+static struct particle *
+particle_empty_from_config(const struct yml_node *node,
+                           const struct font *parent_font)
+{
+    const struct yml_node *left_margin = yml_get_value(node, "left_margin");
+    const struct yml_node *right_margin = yml_get_value(node, "right_margin");
+
+    assert(left_margin == NULL || yml_is_scalar(left_margin));
+    assert(right_margin == NULL || yml_is_scalar(right_margin));
+
+    return particle_empty_new(
+        left_margin != NULL ? yml_value_as_int(left_margin) : 0,
+        right_margin != NULL ? yml_value_as_int(right_margin) : 0);
+}
+
 static struct particle *
 particle_string_from_config(const struct yml_node *node,
                             const struct font *parent_font)
@@ -318,7 +334,9 @@ particle_from_config(const struct yml_node *node, const struct font *parent_font
     const char *type = yml_value_as_string(pair.key);
 
     struct particle *ret = NULL;
-    if (strcmp(type, "string") == 0)
+    if (strcmp(type, "empty") == 0)
+        ret = particle_empty_from_config(pair.value, parent_font);
+    else if (strcmp(type, "string") == 0)
         ret = particle_string_from_config(pair.value, parent_font);
     else if (strcmp(type, "list") == 0)
         ret = particle_list_from_config(pair.value, parent_font);

particles/empty.c

@@ -0,0 +1,45 @@
+#include "empty.h"
+
+#include <stdlib.h>
+
+static void
+exposable_destroy(struct exposable *exposable)
+{
+    free(exposable);
+}
+
+static int
+begin_expose(struct exposable *exposable, cairo_t *cr)
+{
+    exposable->width = exposable->particle->left_margin +
+        exposable->particle->right_margin;
+    return exposable->width;
+}
+
+static void
+expose(const struct exposable *exposable, cairo_t *cr, int x, int y, int height)
+{
+    const struct deco *deco = exposable->particle->deco;
+    if (deco != NULL)
+        deco->expose(deco, cr, x, y, exposable->width, height);
+}
+
+static struct exposable *
+instantiate(const struct particle *particle, const struct tag_set *tags)
+{
+    struct exposable *exposable = malloc(sizeof(*exposable));
+    exposable->particle = particle;
+    exposable->destroy = &exposable_destroy;
+    exposable->begin_expose = &begin_expose;
+    exposable->expose = &expose;
+    return exposable;
+}
+
+struct particle *
+particle_empty_new(int left_margin, int right_margin)
+{
+    struct particle *particle = particle_common_new(left_margin, right_margin);
+    particle->destroy = &particle_default_destroy;
+    particle->instantiate = &instantiate;
+    return particle;
+}

particles/empty.h

@@ -0,0 +1,4 @@
+#pragma once
+#include "../particle.h"
+
+struct particle *particle_empty_new(int left_margin, int right_margin);