written_amount.c

// Convert number 'amount' to written English words
// In short scale
// Requires C99 for uint64_t
// Supports large 64-bit integer number up to 18,446,744,073,709,551,615
 
#include <stdio.h>	
#include <stdlib.h>
#include <stdint.h>
 
const char* decades[] = {"", "", "TWENTY", "THIRTY", "FORTY", "FIFTY", "SIXTY", "SEVENTY", "EIGHTY", "NINETY"};
const char* digits[] = {"ZERO", "ONE", "TWO", "THREE", "FOUR", "FIVE", "SIX", "SEVEN", "EIGHT", "NINE", "TEN", 
	"ELEVEN", "TWELVE", "THIRTEEN", "FOURTEEN", "FIFTEEN", "SIXTEEN", "SEVENTEEN", "EIGHTTEEN", "NINETEEN"};
 
typedef enum {QUINTILLION, QUADRILLION, TRILLION, BILLION, MILLION, THOUSAND, HUNDRED} Numeral;
const char* Numeral_Names[] = {"QUINTILLION", "QUADRILLION", "TRILLION","BILLION", "MILLION", "THOUSAND", "HUNDRED"};
const uint64_t Numeral_Values[] = {1000000000000000000, 1000000000000000, 1000000000000, 1000000000, 1000000, 1000, 100};
 
void appendstr(char** pstr, const char* append)
{
	register char c;
	while (1) 
	{ 
		c = *append++;
		if (c == '') break;
		*(*pstr)++ = c;
	}
}
 
void write(uint64_t amount, char** pstr)
{
	for (uint64_t n = QUINTILLION; n <= HUNDRED; n++) 
	{
		if (amount < Numeral_Values[n]) continue;
 
		write(amount / Numeral_Values[n], pstr);
		appendstr(pstr, " ");
		appendstr(pstr, Numeral_Names[n]);
		appendstr(pstr, " ");
		amount %= Numeral_Values[n];
	}
 
	if (amount > 20)
	{
		appendstr(pstr, decades[amount / 10]);
		amount %= 10;
		if (amount != 0) appendstr(pstr, "-"); else return;
	}
 
	appendstr(pstr, digits[amount]);
}
 
void written_amount(uint64_t amount, char* buffer)
{
	write(amount, &buffer);
	*buffer = '';
}
 
uint64_t rand64()
{
	// Combin 4 parts of low 16-bit of each random()
	uint64_t R0 = (uint64_t)random() << 48;
	uint64_t R1 = (uint64_t)random() << 48 >> 16;
	uint64_t R2 = (uint64_t)random() << 48 >> 32;
	uint64_t R3 = (uint64_t)random() << 48 >> 48;
	return R0 | R1 | R2 | R3;
}
 
int main (int argc, char const *argv[])
{
	for (int i=0; i < 20; i++) 
	{
		uint64_t amount = rand64();
		amount >>= (20 - i) * 3; // scale numbers 
		char buffer[1024] = { 0 };
		written_amount(amount, buffer);
		printf("%20llu = %sn", amount, buffer);
	}
	return 0;
}