Fixes #904 : Fixes sort_list function to maintain previous node data after running mergesort

[nathanjjohnson7]

Fixes #904

Previously, the sort_list() function, which is called by cJSONUtils_SortObject, wasn't managing the prev data point of each node, while going through the different steps of mergesort. This caused the ->prev value of the first item in the list, to point to NULL instead of pointing to the last item in the list, leading to errors when you try to add new elements to the list.

This commit fixes that by tracking and correctly assigning the prev value at each step.

Below is a script that tests the sort_list functionality. It creates an object, sorts the object, shows the next and prev values of each element and attempts to add new elements and sort again:

#include <stdio.h>
#include "cJSON.h"
#include "cJSON_Utils.h"

int main() {
    printf("Creating Object ... \n\n");
    cJSON *object = cJSON_Parse("{\"v1\": 1, \"a2\": 2}");

    char *object_str = cJSON_Print(object);
    printf("object=\n%s\n", object_str);
    free(object_str);

    printf("Connections: \n");
    for(cJSON *node = object->child;node!=NULL;node = node->next){
        printf(
            "node: %s, node->prev: %s, node->next: %s\n",
            node->string,
            node->prev ? node->prev->string : "null",
            node->next ? node->next->string : "null"
        );    
    }
    printf("\n");

    cJSONUtils_SortObject(object);

    printf("\n");
    printf("sorted Object: \n");
    object_str = cJSON_Print(object);
    printf("object=\n%s\n", object_str);
    free(object_str);
    printf("\n");

    printf("Connections: \n");
    for(cJSON *node = object->child;node!=NULL;node = node->next){
        printf(
            "node: %s, node->prev: %s, node->next: %s\n",
            node->string,
            node->prev ? node->prev->string : "null",
            node->next ? node->next->string : "null"
        );    
    }
    printf("\n");

    printf("Adding V3: \n");
    cJSON *v3_value = cJSON_CreateNumber(3);
    cJSON_AddItemToObject(object, "v3", v3_value);

    object_str = cJSON_Print(object);
    printf("object=\n%s\n", object_str);
    free(object_str);

    printf("Connections: \n");
    for(cJSON *node = object->child;node!=NULL;node = node->next){
        printf(
            "node: %s, node->prev: %s, node->next: %s\n",
            node->string,
            node->prev ? node->prev->string : "null",
            node->next ? node->next->string : "null"
        );    
    }
    printf("\n");

    printf("Adding d5, a0, z7, k2: \n");
    cJSON *d5_value = cJSON_CreateNumber(5);
    cJSON_AddItemToObject(object, "d5", d5_value);
    cJSON *a0_value = cJSON_CreateNumber(0);
    cJSON_AddItemToObject(object, "a0", a0_value);
    cJSON *z7_value = cJSON_CreateNumber(7);
    cJSON_AddItemToObject(object, "z7", z7_value);
    cJSON *k2_value = cJSON_CreateNumber(2);
    cJSON_AddItemToObject(object, "k2", k2_value);


    object_str = cJSON_Print(object);
    printf("object=\n%s\n", object_str);
    free(object_str);

    printf("Connections: \n");
    for(cJSON *node = object->child;node!=NULL;node = node->next){
        printf(
            "node: %s, node->prev: %s, node->next: %s\n",
            node->string,
            node->prev ? node->prev->string : "null",
            node->next ? node->next->string : "null"
        );    
    }
    printf("\n");

    cJSONUtils_SortObject(object);

    printf("\n");
    printf("sorted Object: \n");
    object_str = cJSON_Print(object);
    printf("object=\n%s\n", object_str);
    free(object_str);
    printf("\n");

    printf("Connections: \n");
    for(cJSON *node = object->child;node!=NULL;node = node->next){
        printf(
            "node: %s, node->prev: %s, node->next: %s\n",
            node->string,
            node->prev ? node->prev->string : "null",
            node->next ? node->next->string : "null"
        );    
    }
    printf("\n");

    return 0;
}

The result of running the above script using the previous implementation of sort_list() is the following:

Creating Object ...

object=
{
        "v1":   1,
        "a2":   2
}
Connections:
node: v1, node->prev: a2, node->next: a2
node: a2, node->prev: v1, node->next: null


sorted Object:
object=
{
        "a2":   2,
        "v1":   1
}

Connections:
node: a2, node->prev: null, node->next: v1
node: v1, node->prev: a2, node->next: null

Adding V3:
object=
{
        "a2":   2,
        "v1":   1
}
Connections:
node: a2, node->prev: null, node->next: v1
node: v1, node->prev: a2, node->next: null

Adding d5, a0, z7, k2:
object=
{
        "a2":   2,
        "v1":   1
}
Connections:
node: a2, node->prev: null, node->next: v1
node: v1, node->prev: a2, node->next: null


sorted Object:
object=
{
        "a2":   2,
        "v1":   1
}

Connections:
node: a2, node->prev: null, node->next: v1
node: v1, node->prev: a2, node->next: null

As shown above, once the object is sorted, the ->prev value of the first element in the list points to NULL instead of the last element of the list. node->prev of a2 points to NULL instead of v1. Once this error is made, users can no longer add to the list, as shown above. Every successive call to cJSON_AddItemToObject results in no change at all.

Below is the result of running the script with the modifications of sort_list() proposed in this commit:

Creating Object ...

object=
{
        "v1":   1,
        "a2":   2
}
Connections:
node: v1, node->prev: a2, node->next: a2
node: a2, node->prev: v1, node->next: null


sorted Object:
object=
{
        "a2":   2,
        "v1":   1
}

Connections:
node: a2, node->prev: v1, node->next: v1
node: v1, node->prev: a2, node->next: null

Adding V3:
object=
{
        "a2":   2,
        "v1":   1,
        "v3":   3
}
Connections:
node: a2, node->prev: v3, node->next: v1
node: v1, node->prev: a2, node->next: v3
node: v3, node->prev: v1, node->next: null

Adding d5, a0, z7, k2:
object=
{
        "a2":   2,
        "v1":   1,
        "v3":   3,
        "d5":   5,
        "a0":   0,
        "z7":   7,
        "k2":   2
}
Connections:
node: a2, node->prev: k2, node->next: v1
node: v1, node->prev: a2, node->next: v3
node: v3, node->prev: v1, node->next: d5
node: d5, node->prev: v3, node->next: a0
node: a0, node->prev: d5, node->next: z7
node: z7, node->prev: a0, node->next: k2
node: k2, node->prev: z7, node->next: null


sorted Object:
object=
{
        "a0":   0,
        "a2":   2,
        "d5":   5,
        "k2":   2,
        "v1":   1,
        "v3":   3,
        "z7":   7
}

Connections:
node: a0, node->prev: z7, node->next: a2
node: a2, node->prev: a0, node->next: d5
node: d5, node->prev: a2, node->next: k2
node: k2, node->prev: d5, node->next: v1
node: v1, node->prev: k2, node->next: v3
node: v3, node->prev: v1, node->next: z7
node: z7, node->prev: v3, node->next: null

As seen above, the new sort_list() ensures that the node->prev value is maintained after sorting. Furthermore, now cJSON_AddItemToObject works as expected.