From 8d45509d687f45249b46d7eceafef3dbca332876 Mon Sep 17 00:00:00 2001
From: Bailey Thompson <bkthomps@edu.uwaterloo.ca>
Date: Sun, 9 Jun 2019 00:30:30 -0400
Subject: [PATCH] Forward list optimization (#53)

Add a tail pointer which is either NULL or points to the back of the list. This improves the performance when adding to the back of the list many times in a row.
---
 src/forward_list.c | 19 ++++++++++++++-----
 src/list.c         |  5 ++---
 tst/forward_list.c | 20 ++++++++++++++++++++
 3 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/src/forward_list.c b/src/forward_list.c
index cbed266..fcc904b 100644
--- a/src/forward_list.c
+++ b/src/forward_list.c
@@ -28,6 +28,7 @@ struct internal_forward_list {
     size_t bytes_per_item;
     int item_count;
     struct node *head;
+    struct node *tail;
 };
 
 struct node {
@@ -57,6 +58,7 @@ forward_list forward_list_init(const size_t data_size)
     init->bytes_per_item = data_size;
     init->item_count = 0;
     init->head = NULL;
+    init->tail = NULL;
     return init;
 }
 
@@ -113,9 +115,15 @@ static struct node *forward_list_get_node_at(forward_list me, const int index)
 {
     struct node *traverse = me->head;
     int i;
+    if (me->tail && index == me->item_count - 1) {
+        return me->tail;
+    }
     for (i = 0; i < index; i++) {
         traverse = traverse->next;
     }
+    if (!traverse->next) {
+        me->tail = traverse;
+    }
     return traverse;
 }
 
@@ -175,6 +183,9 @@ int forward_list_add_at(forward_list me, const int index, void *const data)
         struct node *const traverse = forward_list_get_node_at(me, index - 1);
         add->next = traverse->next;
         traverse->next = add;
+        if (!add->next) {
+            me->tail = add;
+        }
     }
     me->item_count++;
     return 0;
@@ -238,17 +249,15 @@ int forward_list_remove_at(forward_list me, const int index)
         me->head = temp->next;
         free(temp->data);
         free(temp);
-    } else if (index == me->item_count - 1) {
-        struct node *const traverse = forward_list_get_node_at(me, index - 1);
-        free(traverse->next->data);
-        free(traverse->next);
-        traverse->next = NULL;
     } else {
         struct node *const traverse = forward_list_get_node_at(me, index - 1);
         struct node *const backup = traverse->next;
         traverse->next = traverse->next->next;
         free(backup->data);
         free(backup);
+        if (!backup->next) {
+            me->tail = NULL;
+        }
     }
     me->item_count--;
     return 0;
diff --git a/src/list.c b/src/list.c
index abbd4b4..617774a 100644
--- a/src/list.c
+++ b/src/list.c
@@ -140,11 +140,10 @@ static struct node *list_get_node_from_tail(list me, const int index)
  */
 static struct node *list_get_node_at(list me, const int index)
 {
-    if (index <= me->item_count / 2) {
+    if (index < me->item_count / 2) {
         return list_get_node_from_head(me, index);
-    } else {
-        return list_get_node_from_tail(me, index);
     }
+    return list_get_node_from_tail(me, index);
 }
 
 /**
diff --git a/tst/forward_list.c b/tst/forward_list.c
index bada179..a2937ce 100644
--- a/tst/forward_list.c
+++ b/tst/forward_list.c
@@ -150,6 +150,25 @@ static void test_basic(void)
     assert(!forward_list_destroy(me));
 }
 
+static void test_add_back(void)
+{
+    int i;
+    forward_list me = forward_list_init(sizeof(int));
+    assert(me);
+    for (i = 1; i < 10000; i++) {
+        int get = 0xdeadbeef;
+        forward_list_add_last(me, &i);
+        forward_list_get_last(&get, me);
+        assert(get == i);
+        if (i % 5 == 0) {
+            forward_list_remove_last(me);
+            forward_list_get_last(&get, me);
+            assert(get == i - 1);
+        }
+    }
+    assert(!forward_list_destroy(me));
+}
+
 static void test_init_out_of_memory(void)
 {
     fail_malloc = 1;
@@ -264,6 +283,7 @@ void test_forward_list(void)
 {
     test_invalid_init();
     test_basic();
+    test_add_back();
     test_init_out_of_memory();
     test_add_first_out_of_memory();
     test_add_at_out_of_memory();