#define _GNU_SOURCE //asprintf
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <ctype.h>
#include <inttypes.h>

__attribute__((noreturn)) void usage(int argc,char **argv){
	(void)argc;
	fprintf(stderr,
		"Usage: %s [-co] <number>\n"
		"Converts a number to (British) English word form.\n"
		"Currently supports only integers.\n"
		"    -c  Capitalise the first output letter\n"
		"    -o  Write as ordinal (first) instead of cardinal (one)\n",
		argv[0]);
	exit(1);
}

char *thousands[6]={"thousand","million","billion","trillion","quadrillion","quintillion"};
char *twenty[20]={
	"zero","one","two","three","four","five","six","seven","eight","nine","ten",
	"eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"
};
char *tens[10]={
	"zero","ten","twenty","thirty","forty","fifty","sixty","seventy","eighty","ninety"
};

char *ordtwenty[20]={
	"zeroth","first","second","third","fourth","fifth","sixth","seventh","eighth","ninth","tenth",
	"eleventh","twelfth","thirteenth","fourteenth","fifteenth","sixteenth","seventeenth","eighteenth","nineteenth"
};
char *ordtens[10]={
	"zeroth","tenth","twentieth","thirtieth","fortieth","fiftieth","sixtieth","seventieth","eightieth","ninetieth"
};

void addtobuf(char *text,char **buf){
	int len=strlen(text);
	strcpy(*buf,text);
	*buf=*buf+len;
}

char* printnumber(intmax_t num,char *buf){
	if(num<0){
		addtobuf("minus ",&buf);
		num=-num;
	}

	bool hadth=false;
	if(num>=1000){
		hadth=true;
		intmax_t th[6],nth=0,rest;
		rest=num%1000;
		num/=1000;
		while(num){
			th[nth++]=num%1000;
			num/=1000;
		}
		bool first=true;
		while(nth-->0){
			if(th[nth]==0)continue;
			if(!first)addtobuf(" ",&buf);
			buf=printnumber(th[nth],buf);
			addtobuf(" ",&buf);
			addtobuf(thousands[nth],&buf);
			first=false;
		}
		num=rest;
		if(num==0)return buf;
		addtobuf(" ",&buf);
	}

	bool and=(num>=100||hadth)&&num%100!=0;
	if(num>=100){
		buf=printnumber(num/100,buf);
		addtobuf(" ",&buf);
		addtobuf("hundred",&buf);
		num%=100;
		if(num)addtobuf(" ",&buf);
		else return buf;
	}

	if(and)addtobuf("and ",&buf);

	if(num%10==0)addtobuf(tens[num/10],&buf);
	else if(num<20)addtobuf(twenty[num],&buf);
	else {
		addtobuf(tens[num/10],&buf);
		addtobuf("-",&buf);
		addtobuf(twenty[num%10],&buf);
	}
	return buf;
}

char* printordinal(intmax_t num,char *buf){
	if(num<0){
		addtobuf("minus ",&buf);
		num=-num;
	}

	bool hadth=false;
	if(num>=1000){
		hadth=true;
		buf=printnumber(num/1000*1000,buf);
		num%=1000;
		if(num==0){
			addtobuf("th",&buf);
			return buf;
		}
		addtobuf(" ",&buf);
	}

	if(hadth&&num/100==0&&num%100!=0)addtobuf("and ",&buf);

	if(num<20){
		addtobuf(ordtwenty[num],&buf);
		return buf;
	}

	if(num<100){
		if(num%10==0)addtobuf(ordtens[num/10],&buf);
		else {
			addtobuf(tens[num/10],&buf);
			addtobuf("-",&buf);
			addtobuf(ordtwenty[num%10],&buf);
		}
		return buf;
	}

	buf=printnumber(num/100*100,buf);
	if(num%100==0){
		addtobuf("th",&buf);
		return buf;
	}
	addtobuf(" and ",&buf);
	return printordinal(num%100,buf);
}

int main(int argc,char **argv){
	char *numstr=NULL;
	bool ordinal=false,capfirst=false;
	for(int i=1;i<argc;i++){
		if(argv[i][0]=='\0'){
			fprintf(stderr,"Unexpected empty argument\n");
			usage(argc,argv);
		}
		if(argv[i][0]=='-'){
			if(isdigit(argv[i][1])){
				if(numstr==NULL)numstr=argv[i];
				else {
					fprintf(stderr,"Unexpected second number '%s'\n",argv[i]);
					usage(argc,argv);
				}
				continue;
			}
			for(int j=1;argv[i][j];j++){
				switch(argv[i][j]){
					case 'c': capfirst=true; break;
					case 'o': ordinal=true; break;
					default:
						fprintf(stderr,"Unrecognised flag -%c\n",argv[i][j]);
						usage(argc,argv);
				}
			}
		} else {
			if(numstr==NULL)numstr=argv[i];
			else {
				fprintf(stderr,"Unexpected second number '%s'\n",argv[i]);
				usage(argc,argv);
			}
		}
	}
	if(numstr==NULL){
		fprintf(stderr,"No number given\n");
		usage(argc,argv);
	}

	char *endp;
	intmax_t num=strtoimax(numstr,&endp,10);
	if(*endp!='\0'){
		fprintf(stderr,"Invalid number: '%s'\n",numstr);
		return 1;
	}
	char *repr;
	asprintf(&repr,"%jd",num);
	if(strcmp(numstr,repr)!=0){
		fprintf(stderr,"Outside range of %lu-bit number: '%s'\n",8*sizeof(intmax_t),numstr);
		return 1;
	}

	char buf[300]={'\0'};
	
	if(ordinal)printordinal(num,buf);
	else printnumber(num,buf);
	if(capfirst)buf[0]-='a'-'A';
	printf("%s\n",buf);
	return 0;
}