bucket_sort.c

/*
 * Algorithm : Bucket Sort
 * Time-Complexity : O(n)
 */
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#define NARRAY 8    /* array size */
#define NBUCKET 5   /* bucket size */
#define INTERVAL 10 /* bucket range */

struct Node
{
    int data;
    struct Node *next;
};

void BucketSort(int arr[]);
struct Node *InsertionSort(struct Node *list);
void print(int arr[]);
void printBuckets(struct Node *list);
int getBucketIndex(int value);

void BucketSort(int arr[])
{
    int i, j;
    struct Node **buckets;

    /* allocate memory for array of pointers to the buckets */
    buckets = (struct Node **)malloc(sizeof(struct Node *) * NBUCKET);

    /* initialize pointers to the buckets */
    for (i = 0; i < NBUCKET; ++i)
    {
        buckets[i] = NULL;
    }

    /* put items into the buckets */
    /* creates a link list in each bucket slot */
    for (i = 0; i < NARRAY; ++i)
    {
        struct Node *current;
        int pos = getBucketIndex(arr[i]);
        current = (struct Node *)malloc(sizeof(struct Node));
        current->data = arr[i];
        current->next = buckets[pos];
        buckets[pos] = current;
    }

    /* check what's in each bucket */
    for (i = 0; i < NBUCKET; i++)
    {
        printf("Bucket[\"%d\"] : ", i);
        printBuckets(buckets[i]);
        printf("\n");
    }

    /* sorting bucket using Insertion Sort */
    for (i = 0; i < NBUCKET; ++i)
    {
        buckets[i] = InsertionSort(buckets[i]);
    }

    /* check what's in each bucket */
    printf("--------------\n");
    printf("Buckets after sorted\n");
    for (i = 0; i < NBUCKET; i++)
    {
        printf("Bucket[\"%d\"] : ", i);
        printBuckets(buckets[i]);
        printf("\n");
    }

    /* put items back to original array */
    for (j = 0, i = 0; i < NBUCKET; ++i)
    {
        struct Node *node;
        node = buckets[i];
        while (node)
        {
            // precondition for avoiding out of bounds by the array
            assert(j < NARRAY);
            arr[j++] = node->data;
            node = node->next;
        }
    }

    /* free memory */
    for (i = 0; i < NBUCKET; ++i)
    {
        struct Node *node;
        node = buckets[i];
        while (node)
        {
            struct Node *tmp;
            tmp = node;
            node = node->next;
            free(tmp);
        }
    }
    free(buckets);
    return;
}

/* Insertion Sort */
struct Node *InsertionSort(struct Node *list)
{
    struct Node *k, *nodeList;
    /* need at least two items to sort */
    if (list == NULL || list->next == NULL)
    {
        return list;
    }

    nodeList = list;
    k = list->next;
    nodeList->next = NULL; /* 1st node is new list */
    while (k != NULL)
    {
        struct Node *ptr;
        /* check if insert before first */
        if (nodeList->data > k->data)
        {
            struct Node *tmp;
            tmp = k;
            k = k->next;  // important for the while
            tmp->next = nodeList;
            nodeList = tmp;
            continue;
        }

        // from begin up to end
        // finds [i] > [i+1]
        for (ptr = nodeList; ptr->next != NULL; ptr = ptr->next)
        {
            if (ptr->next->data > k->data)
                break;
        }

        // if found (above)
        if (ptr->next != NULL)
        {
            struct Node *tmp;
            tmp = k;
            k = k->next;  // important for the while
            tmp->next = ptr->next;
            ptr->next = tmp;
            continue;
        }
        else
        {
            ptr->next = k;
            k = k->next;  // important for the while
            ptr->next->next = NULL;
            continue;
        }
    }
    return nodeList;
}

int getBucketIndex(int value) { return value / INTERVAL; }

void print(int ar[])
{
    int i;
    for (i = 0; i < NARRAY; ++i)
    {
        printf("%d ", ar[i]);
    }
    printf("\n");
}

void printBuckets(struct Node *list)
{
    struct Node *cur = list;
    while (cur)
    {
        printf("%d ", cur->data);
        cur = cur->next;
    }
}

int main(void)
{
    int array[NARRAY] = {29, 25, -1, 49, 9, 37, 21, 43};

    printf("Initial array\n");
    print(array);
    printf("------------\n");

    BucketSort(array);
    printf("------------\n");
    printf("Sorted array\n");
    print(array);
    return 0;
}