particle/map: new particle; maps a tag value to a particle

This particle is basically a dictionary mapping tag values to
particles.

particles/map.c

@@ -0,0 +1,85 @@
+#include "map.h"
+
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+
+struct map {
+    char *tag;
+    struct particle *default_particle;
+    struct particle_map *map;
+    size_t count;
+};
+
+static struct exposable *
+instantiate(const struct particle *particle, const struct tag_set *tags)
+{
+    const struct map *map = particle->private;
+    const struct tag *tag = tag_for_name(tags, map->tag);
+    assert(tag != NULL || map->default_particle != NULL);
+
+    if (tag == NULL)
+        return map->default_particle->instantiate(map->default_particle, tags);
+
+
+    const char *tag_value = tag->as_string(tag);
+    for (size_t i = 0; i < map->count; i++) {
+        const struct particle_map *e = &map->map[i];
+
+        if (strcmp(e->tag_value, tag_value) != 0)
+            continue;
+
+        return e->particle->instantiate(e->particle, tags);
+    }
+
+    assert(map->default_particle != NULL);
+    return map->default_particle->instantiate(map->default_particle, tags);
+}
+
+static void
+particle_destroy(struct particle *particle)
+{
+    struct map *map = particle->private;
+
+    if (map->default_particle != NULL)
+        map->default_particle->destroy(map->default_particle);
+
+    for (size_t i = 0; i < map->count; i++) {
+        struct particle *p = map->map[i].particle;
+        p->destroy(p);
+        free((char *)map->map[i].tag_value);
+    }
+
+    free(map->map);
+    free(map->tag);
+    free(map);
+    free(particle);
+}
+
+struct particle *
+particle_map_new(const char *tag, const struct particle_map *particle_map,
+                 size_t count, struct particle *default_particle,
+                 int left_margin, int right_margin)
+{
+    struct particle *particle = particle_common_new(left_margin, right_margin);
+    particle->destroy = &particle_destroy;
+    particle->instantiate = &instantiate;
+
+    struct map *map = malloc(sizeof(*map));
+    map->tag = strdup(tag);
+    map->default_particle = default_particle;
+    map->count = count;
+    map->map = malloc(count * sizeof(map->map[0]));
+
+    for (size_t i = 0; i < count; i++) {
+        map->map[i].tag_value = strdup(particle_map[i].tag_value);
+        map->map[i].particle = particle_map[i].particle;
+        map->map[i].particle->parent = particle;
+    }
+
+    if (map->default_particle != NULL)
+        map->default_particle->parent = particle;
+
+    particle->private = map;
+    return particle;
+}