decoration/underline: draws a line at the bottom of the particle

decorations/underline.c

@@ -0,0 +1,41 @@
+#include "underline.h"
+
+#include <stdlib.h>
+
+struct private {
+    int size;
+    struct rgba color;
+};
+
+static void
+destroy(struct deco *deco)
+{
+    struct private *d = deco->private;
+    free(d);
+    free(deco);
+}
+
+static void
+expose(const struct deco *deco, cairo_t *cr, int x, int y, int width, int height)
+{
+    const struct private *d = deco->private;
+    cairo_set_source_rgba(
+        cr, d->color.red, d->color.green, d->color.blue, d->color.alpha);
+    cairo_rectangle(cr, x, y + height - d->size, width, d->size);
+    cairo_fill(cr);
+}
+
+struct deco *
+deco_underline(int size, struct rgba color)
+{
+    struct private *priv = malloc(sizeof(*priv));
+    priv->size = size;
+    priv->color = color;
+
+    struct deco *deco = malloc(sizeof(*deco));
+    deco->private = priv;
+    deco->expose = &expose;
+    deco->destroy = &destroy;
+
+    return deco;
+}

decorations/underline.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#include "../color.h"
+#include "../decoration.h"
+
+struct deco *deco_underline(int size, struct rgba color);