From 411af131c2972c124fdb4b3f00460dcdce6c3974 Mon Sep 17 00:00:00 2001
From: Bailey Thompson <bkthomps@edu.uwaterloo.ca>
Date: Tue, 21 May 2019 17:46:53 -0400
Subject: [PATCH] Fix negative modulo for C89 (#45)

In C89, negative integer division and modulo is implementation-defined.

Case 1: division rounds to 0 (C89/90, C99, C11, C18)
-9 / 7 -> -1
-9 % 7 -> -2

Case 2: division rounds to negative infinity (C89/90)
-9 / 7 -> -2
-9 % 7 -> 5

This change fixes the deque data structure from having negative division and modulo to prevent this implementation-defined behavior.
---
 CMakeLists.txt |  2 +-
 src/deque.c    | 17 ++++++++++++-----
 tst/deque.c    | 20 ++++++++++++++++++++
 3 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6eeb4167..051713a7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,7 +3,7 @@ project(Containers C)
 
 set(CMAKE_C_STANDARD 90)
 
-set(CMAKE_C_FLAGS "-pedantic -ldl -g -O0 -Wall -fprofile-arcs -ftest-coverage")
+set(CMAKE_C_FLAGS "-Wall -Wextra -Wpedantic -ldl -g -O0 -fprofile-arcs -ftest-coverage")
 
 add_executable(Containers tst/test.c tst/test.h
         src/array.c src/include/array.h tst/array.c
diff --git a/src/deque.c b/src/deque.c
index 79bc6c6d..6e7a73a7 100644
--- a/src/deque.c
+++ b/src/deque.c
@@ -114,8 +114,12 @@ int deque_is_empty(deque me)
 int deque_trim(deque me)
 {
     int i;
-    const int start_block = me->start_index / BLOCK_SIZE;
-    const int end_block = (me->end_index - 1) / BLOCK_SIZE;
+    /* The start and end blocks are written like this because in C89,   */
+    /* negative integer division and modulo are implementation-defined. */
+    const int start_block =
+            me->start_index == -1 ? 0 : me->start_index / BLOCK_SIZE;
+    const int end_block =
+            me->end_index == 0 ? 0 : (me->end_index - 1) / BLOCK_SIZE;
     const int new_block_count = end_block - start_block + 1;
     void *const new_block = malloc(new_block_count * sizeof(struct node));
     if (!new_block) {
@@ -166,11 +170,14 @@ void deque_copy_to_array(void *const arr, deque me)
 int deque_push_front(deque me, void *const data)
 {
     struct node block_item;
-    int block_index = me->start_index / BLOCK_SIZE;
-    int inner_index = me->start_index % BLOCK_SIZE;
-    if (inner_index == -1) {
+    int block_index;
+    int inner_index;
+    if (me->start_index == -1) {
         block_index = -1;
         inner_index = BLOCK_SIZE - 1;
+    } else {
+        block_index = me->start_index / BLOCK_SIZE;
+        inner_index = me->start_index % BLOCK_SIZE;
     }
     if (inner_index == BLOCK_SIZE - 1) {
         struct node *block_item_reference;
diff --git a/tst/deque.c b/tst/deque.c
index 5d75d5d0..4f7f66a8 100644
--- a/tst/deque.c
+++ b/tst/deque.c
@@ -253,6 +253,25 @@ static void test_clear_out_of_memory(void)
     assert(!deque_destroy(me));
 }
 
+void test_single_full_block(void)
+{
+    int i;
+    int num = 5;
+    deque me = deque_init(sizeof(int));
+    for (i = 0; i < 5; i++) {
+        deque_push_front(me, &num);
+    }
+    for (i = 0; i < 3; i++) {
+        deque_push_back(me, &num);
+    }
+    for (i = 0; i < 8; i++) {
+        deque_pop_back(&num, me);
+    }
+    deque_trim(me);
+    assert(deque_size(me) == 0);
+    deque_destroy(me);
+}
+
 void test_deque(void)
 {
     test_invalid_init();
@@ -263,4 +282,5 @@ void test_deque(void)
     test_push_front_out_of_memory();
     test_push_back_out_of_memory();
     test_clear_out_of_memory();
+    test_single_full_block();
 }