String manipulation in C

string.c

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

#include <assert.h> /* DEBUG */

static string_err string_enlarge(String *this, size_t add)
{
    string_err err = STRING_ERR_OK;

    size_t size = this->size;
    size_t oldsize = this->size;
    size_t newsize = size + add;
    while (size < newsize)
    {
        size *= 2;
    }

    if (this != NULL)
    {
        this->s = realloc(this->s, size);
        if (this->s != NULL)
        {
            memset(this->s + oldsize, 0, size - oldsize); /* setting underminated memory to 0 */
            this->size = size;
        }
        else
        {
            err = STRING_ERR_MEMORY;
        }
    }
    else
    {
        err = STRING_ERR_NULL;
    }

    return err;
}

static void string_purge(void)
{
    int c;
    while ((c = fgetc(stdin)) != '\n' && c != EOF)
    {
    }
}

String *new_string(void)
{
    String *this = malloc(sizeof *this);
    if (this != NULL)
    {
        this->size = 1;
        this->index = 0;
        this->s = malloc(1);
        if (this->s != NULL)
        {
            this->s[0] = 0;
        }
    }

    return this;
}

string_err string_add_char(String *this, int const c)
{
    string_err err = STRING_ERR_OK;
    if (this != NULL)
    {
        if (this->s != NULL)
        {
            size_t const s_len = 1; /* character's len */
            if (this->index + s_len >= this->size)
            {
                err = string_enlarge(this, s_len);
            }

            if (err == STRING_ERR_OK)
            {
                this->s[this->index++] = c;
                this->s[this->index] = 0;
            }
        }
    }
    else
    {
        err = STRING_ERR_NULL;
    }

    return err;
}

string_err string_add_str(String *this, char const *s)
{
    string_err err = STRING_ERR_NULL;
    if (this != NULL)
    {
        if (this->s != NULL && s != NULL)
        {
            size_t const s_len = strlen(s);
            if (this->index + s_len >= this->size)
            {
                err = string_enlarge(this, s_len);
            }

            if (err == STRING_ERR_OK)
            {
                memcpy(this->s + this->index, s, s_len);
                this->index += s_len;
            }
        }
    }
    else
    {
        err = STRING_ERR_NULL;
    }

    return err;
}

void string_puts(String *this)
{
    if (this != NULL && this->s != NULL)
    {
        puts(this->s);
    }
}

string_err string_gets(String *this)
{
    string_err err = STRING_ERR_OK;

    if (this != NULL && this->s != NULL)
    {
        size_t i = 0;
        int c;
        while ((c = fgetc(stdin)) != '\n' && c != EOF && err == STRING_ERR_OK)
        {
            if (i >= this->size)
            {
                err = string_enlarge(this, this->size * 2); /* growing by 2 */
            }

            if (err == STRING_ERR_OK)
            {
                this->s[this->index] = c;
                this->index++;
            }
            i++;
        }
        this->s[this->index] = 0;
    }

    return err;
}

string_err string_ngets(String *this, size_t n)
{
    string_err err = STRING_ERR_OK;

    if (this != NULL && this->s != NULL)
    {
        size_t i = 0;
        int c;
        while ((c = fgetc(stdin)) != '\n' && c != EOF && err == STRING_ERR_OK)
        {
            if (i < n - 1)
            {
                if (i >= this->size)
                {
                    err = string_enlarge(this, this->size * 2);
                }

                if (err == STRING_ERR_OK)
                {
                    this->s[this->index] = c;
                    this->index++;
                }
                i++;
            }
            this->s[i] = 0;
        }
    }

    return err;
}

char *string_str(String *this)
{
    char *ret = NULL;
    if (this != NULL)
    {
        ret = this->s;
    }
    return ret;
}

void string_clear(String *this)
{
    if (this != NULL)
    {
        free(this->s), this->s = NULL;
        this->index = this->size = 0;
    }
}

void string_delete(String *this)
{
    if (this != NULL)
    {
        free(this->s);
        free(this);
    }
}

string.h

#ifndef STRING_H_INCLUDED
#define STRING_H_INCLUDED

#include <stddef.h> /* size_t */
#include <string.h>

typedef struct string
{
    size_t size;
    size_t index;
    char *s;
} String;

typedef enum
{
    STRING_ERR_OK, STRING_ERR_MEMORY, STRING_ERR_NULL
} string_err;

String *new_string(void);
string_err string_add_char(String *this, int const c);
string_err string_add_str(String *this, char const *s);
void string_puts(String *this);
string_err string_gets(String *this);
string_err string_ngets(String *this, size_t n);
char *string_str(String *this);
void string_clear(String *this);
void string_delete(String *this);

#endif