From c5fa68fdf5bc2a175794a2695a0d54ff759f8fcf Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Fri, 30 Mar 2018 22:37:30 -0700
Subject: [PATCH] Implement an initial version of reference counting.

---
 CMakeLists.txt     |  2 +-
 include/refcount.h | 79 ++++++++++++++++++++++++++++++++++++++++++++++
 src/refcount.c     | 57 +++++++++++++++++++++++++++++++++
 3 files changed, 137 insertions(+), 1 deletion(-)
 create mode 100644 include/refcount.h
 create mode 100644 src/refcount.c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b9a588c..d35f0c3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,7 +8,7 @@ project(libabacus)
 
 add_compile_options(-pedantic -Wall)
 
-add_library(abacus STATIC src/lexer.c src/util.c src/table.c src/parser.c src/libabacus.c src/tree.c src/debug.c src/parsetype.c src/reserved.c src/trie.c)
+add_library(abacus STATIC src/lexer.c src/util.c src/table.c src/parser.c src/libabacus.c src/tree.c src/debug.c src/parsetype.c src/reserved.c src/trie.c src/refcount.c)
 add_executable(libabacus src/main.c)
 add_subdirectory(external/liblex)
 
diff --git a/include/refcount.h b/include/refcount.h
new file mode 100644
index 0000000..aa74c23
--- /dev/null
+++ b/include/refcount.h
@@ -0,0 +1,79 @@
+#ifndef LIBABACUS_REFCOUNT_H
+#define LIBABACUS_REFCOUNT_H
+
+#include "result.h"
+
+/**
+ * A struct for holding
+ * the number of references
+ * to a value, as well as the function required
+ * to free the value.
+ */
+struct libab_ref_count_s {
+    /**
+     * The fucntion to free the value.
+     * Can be NULL for no-op.
+     */
+    void (*free_func)(void* data);
+    /**
+     * The number of references that
+     * prevent the deallocation of the value.
+     */
+    int strong;
+    /**
+     * The number of references
+     * that still exist, even to a freed instance.
+     */
+    int weak;
+};
+
+/**
+ * A reference to a value.
+ */
+struct libab_ref_s {
+    /**
+     * Whether this reference is a strong reference.
+     */
+    int strong;
+    /**
+     * The reference count struct keeping track
+     * of how many references are pointing to the value.
+     */
+    struct libab_ref_count_s* count;
+    /**
+     * The value this reference holds.
+     */
+    void* data;
+};
+
+typedef struct libab_ref_s libab_ref;
+typedef struct libab_ref_count_s libab_ref_count;
+
+/**
+ * Creates a new referene, using the given data and free function.
+ * @param ref the reference to initialize with the given data.
+ * @param data the data to reference count.
+ * @param free_func the function to use to realease the data when refcount reaches 0.
+ * @return the result of the construction of the reference.
+ */
+libab_result libab_ref_new(libab_ref* ref, void* data, void (*free_func)(void* data));
+/**
+ * Turns the given reference into a weak reference,
+ * making it not keep the data allocated.
+ */
+void libab_ref_weaken(libab_ref* ref);
+/**
+ * Releases this particular reference to the data.
+ * This doesn't necessarily free the underlying data.
+ */
+void libab_ref_free(libab_ref* ref);
+/**
+ * Copies this reference, thereby increasing the reference count.
+ */
+void libab_ref_copy(libab_ref* ref, libab_ref* into);
+/**
+ * Gets the value of the reference.
+ */
+void* libab_ref_get(libab_ref* ref);
+
+#endif
diff --git a/src/refcount.c b/src/refcount.c
new file mode 100644
index 0000000..aa426e1
--- /dev/null
+++ b/src/refcount.c
@@ -0,0 +1,57 @@
+#include "refcount.h"
+#include <stdlib.h>
+#include <string.h>
+
+libab_result libab_ref_new(libab_ref* ref, void* data, void (*free_func)(void* data)) {
+    libab_result result = LIBAB_SUCCESS;
+    ref->strong = 1;
+    ref->data = data;
+    if((ref->count = malloc(sizeof(*(ref->count))))) {
+        ref->count->strong = ref->count->weak = 1;
+        ref->count->free_func = free_func;
+    } else {
+        result = LIBAB_MALLOC;
+    }
+    return result;
+}
+
+void _libab_ref_changed(libab_ref* ref) {
+    if(ref->count->strong == 0) {
+        ref->count->strong--;
+        if(ref->count->free_func) {
+            ref->count->free_func(ref->data);
+        }
+        free(ref->data);
+    }
+    if(ref->count->weak == 0) {
+        free(ref->count);
+    }
+}
+
+void libab_ref_weaken(libab_ref* ref) {
+    if(ref->strong) {
+        ref->count->strong--;
+        ref->strong = 0;
+        _libab_ref_changed(ref);
+    }
+}
+
+void libab_ref_free(libab_ref* ref) {
+    ref->count->strong -= ref->strong;
+    ref->count->weak--;
+    _libab_ref_changed(ref);
+}
+
+void libab_ref_copy(libab_ref* ref, libab_ref* into) {
+    ref->count->strong++;
+    ref->count->weak++;
+    memcpy(into, ref, sizeof(*ref));
+}
+
+void* libab_ref_get(libab_ref* ref) {
+    void* to_return = NULL;
+    if(ref->count->strong > 0) {
+        to_return = ref->data;
+    }
+    return to_return;
+}