Group by Key

#include <stdio.h>
#include <string.h>

#define TABLE_SIZE 2
#define MAX_KEYS 5
#define MAX_VALUES 5

typedef struct Entry {
    const char *key;
    int values[MAX_VALUES];
    int value_count;
    struct Entry *next;
} Entry;

static size_t hash_key(const char *key) {
    size_t hash = 0;
    while (*key != '\0') {
        hash = hash * 31u + (unsigned char)*key;
        key++;
    }
    return hash % TABLE_SIZE;
}

static Entry *find_entry(Entry *bucket, const char *key) {
    for (Entry *entry = bucket; entry != NULL; entry = entry->next) {
        if (strcmp(entry->key, key) == 0) {
            return entry;
        }
    }
    return NULL;
}

static Entry *insert_entry(Entry *table[], Entry entries[], int *entry_count, const char *key) {
    size_t bucket = hash_key(key);
    Entry *entry = find_entry(table[bucket], key);
    if (entry != NULL) {
        return entry;
    }

    entry = &entries[*entry_count];
    entry->key = key;
    entry->value_count = 0;
    entry->next = table[bucket];
    table[bucket] = entry;
    (*entry_count)++;
    return entry;
}

static void print_groups(const Entry entries[], int entry_count) {
    printf("{");
    for (int i = 0; i < entry_count; ++i) {
        if (i > 0) {
            printf(", ");
        }
        printf("%s: [", entries[i].key);
        for (int j = 0; j < entries[i].value_count; ++j) {
            if (j > 0) {
                printf(", ");
            }
            printf("%d", entries[i].values[j]);
        }
        printf("]");
    }
    printf("}\n");
}

int main(void) {
    const char *keys[] = {"a", "b", "a", "c", "b"};
    int values[] = {1, 2, 3, 4, 5};
    size_t n = sizeof(values) / sizeof(values[0]);
    Entry entries[MAX_KEYS] = {0};
    Entry *table[TABLE_SIZE] = {0};
    int entry_count = 0;

    for (size_t i = 0; i < n; ++i) {
        Entry *entry = insert_entry(table, entries, &entry_count, keys[i]);
        entry->values[entry->value_count] = values[i];
        entry->value_count++;
    }

    print_groups(entries, entry_count);
    return 0;
}