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

char *reverse_string(const char *word) {
    if (!word)
        return NULL;
    size_t length = strlen(word);
    char *result = malloc(length + 1);
    if (!result)
        return NULL;
    for (size_t i = 0; i < length; i++)
        result[i] = word[length - i - 1];
    result[length] = '\0';
    return result;
}

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

void append_number_from_0_to_99_as_text(char *string, unsigned value) {
	static char *from_0_to_19_as_text[] = { "ZERO", "ONE", "TWO", "THREE", "FOUR", "FIVE", "SIX", "SEVEN", "EIGHT", "NINE", "TEN", "ELEVEN", "TWELVE", "THIRTEEN", "FOURTEEN", "FIFTEEN", "SIXTEEN", "SEVENTEEN", "EIGHTEEN", "NINETEEN", };
	static char *tens_digit_to_ty_form[] = { "", "", "TWENTY", "THIRTY", "FORTY", "FIFTY", "SIXTY", "SEVENTY", "EIGHTY", "NINETY"};
	if (value > 19) {
		strcat(string, tens_digit_to_ty_form[value / 10]);
		if (value % 10) {
			strcat(string, " ");
			strcat(string, from_0_to_19_as_text[value % 10]);
		}
	}
	else
		strcat(string, from_0_to_19_as_text[value]);
}

char *checkAmount(double amount) {
	if (amount >= 10000)
		return NULL;
	char *buffer = (char[64]) { 0 };
	unsigned dollars = (unsigned)amount;
	unsigned cents = (unsigned)(amount * 100) % 100;
	append_number_from_0_to_99_as_text(buffer, dollars);
	strcat(buffer, " and ");
	char *tiny_buffer = &(char[8]) { 0 };
	sprintf(tiny_buffer, "%d", cents);
	strcat(buffer, tiny_buffer);
	strcat(buffer, "/100");
	return strdup(buffer);
}

unsigned char invertbits(unsigned char value) { return ~value; }

int just_why(int a, int b) { return b ? a + just_why(a, b - 1) : 0; }
int multiply(int a, int b) { return just_why(a, b); }

const char *strpbrknul(const char *s, const char *accept) 
{
	char *character[256] = { 0 };
	while (*accept)
	{
		character[*accept++]++;
	}
	while (*s)
	{
		if (character[*s])
		{
			return s;
		}
		s++;
	}
	return s;
}

#include<stdbool.h>

const char *bool_to_word (bool value)
{
    return value ? "Yes" : "No";
}

#include <stdint.h>

// naive implementation
uint8_t number_of_digits(uint64_t n)
{
	uint8_t digits = 0;
	do
	{
		n /= 10;
		digits++;
	} while (n > 0);
	return digits;
}

int reverse_int(int num)
{
    int rev = 0;
    while (num > 9 || -9 >= num)
    {
        rev += num % 10;
        rev *= 10;
        num /= 10;
    }
    return rev + num;
}