UTF-16LE to UTF-8 conversion

gistfile1.c

#include <stdio.h>

#define BOM 0xFEFF
#define SUR1 0xD800
#define SUR2 0xDC00

int read16(void) {
  int a = getc(stdin), b = getc(stdin);
	if (a == EOF || b == EOF)
		return EOF;
	return (b << 8) + a;
}

int readc(void) {
	static int save = 0;
	
	int a = save ? save : read16();
	save = 0;
	if (a < SUR1 || a >= SUR2)
		return a;
	
	int b = read16();
	if (b >= SUR2 && b < SUR2 + SUR2 - SUR1)
		return 0x10000 + (a - SUR1) * 0x400 + (b - SUR2);
	save = b;
	return a;
}

void writec(int c, int bs) {
	int s = 6 * (bs - 1);
	putc(0xff - (0xff >> bs) + (c >> s), stdout);
	for (s -= 6; s >= 0; s -= 6)
		putc(((c >> s) & 0x3f) | 0x80, stdout);
}

int main(void) {
	int c = readc();
	if (c == BOM)
		c = readc();
	
	while (c != EOF) {
		if (c < 0x80) putc(c, stdout);
		else if (c < 0x800) writec(c, 2);
		else if (c < 0x10000) writec(c, 3);
		else writec(c, 4);
		
		c = readc();
	}
	return 0;
}