Add dependency injection container for vm

include/vm_di.h

@@ -0,0 +1,21 @@
+#ifndef _VM_DI_H_
+#define _VM_DI_H_
+
+enum vm_di_entry {
+    VM_CPU,
+    VM_MACHINE,
+    VM_REFLECT,
+
+    // This value is the size of the DI container we will construct. As
+    // you can see, it's quite a bit higher than what would be implied
+    // by the number of enum values currently defined--and it is so we
+    // don't have to update this value every time we add a new value.
+    // But be wary that you don't add so many entries as to go beyond
+    // VM_DI_SIZE's defined value!
+    VM_DI_SIZE = 100,
+};
+
+extern void *vm_di_get(int);
+extern int vm_di_set(int, void *);
+
+#endif

sources.cmake

@@ -25,6 +25,7 @@ set(erc_sources
 	option.c
 	vm_area.c
 	vm_bitfont.c
+	vm_di.c
 	vm_event.c
 	vm_reflect.c
 	vm_screen.c

src/vm_di.c

@@ -0,0 +1,49 @@
+/*
+ * vm_di.c
+ *
+ * This code defines a very (very!) simplistic form of dependency
+ * injection container. It's up to you to set the entries of the
+ * container appropriately.
+ *
+ * This container is obviously _not type-safe_. It's just a bunch of
+ * void pointers. This container, however, _is_ immutable; once you set
+ * a value for a DI entry, it cannot be changed.
+ */
+
+#include <stdlib.h>
+
+#include "log.h"
+#include "vm_di.h"
+
+/*
+ * I learned something new today: this array will be constructed with
+ * zero-values for each entry because it is statically declared. See:
+ * http://en.cppreference.com/w/c/language/initialization
+ */
+static void *di_table[VM_DI_SIZE];
+
+/*
+ * Set the di table entry `ent` to `val`. If there is a _previous_
+ * value assigned to ent, then this returns ERR_INVALID, and no
+ * further assignment is allowed.
+ */
+int
+vm_di_set(int ent, void *val)
+{
+    if (di_table[ent] != NULL) {
+        return ERR_INVALID;
+    }
+
+    di_table[ent] = val;
+    return OK;
+}
+
+/*
+ * Return the value assigned to the given ent, or NULL if no entry has
+ * been defined before.
+ */
+void *
+vm_di_get(int ent)
+{
+    return di_table[ent];
+}