const (
	RUNE_ERROR = '\ufffd';
	RUNE_SELF  = 0x80;
	RUNE_BOM   = 0xfeff;
	RUNE_EOF   = ~(0 as rune);
	MAX_RUNE   = '\U0010ffff';
	UTF_MAX    = 4;

	SURROGATE_MIN = 0xd800;
	SURROGATE_MAX = 0xdfff;
);

type Accept_Range struct {
	lo, hi u8;
};

var accept_ranges = [5]Accept_Range{
	{0x80, 0xbf},
	{0xa0, 0xbf},
	{0x80, 0x9f},
	{0x90, 0xbf},
	{0x80, 0x8f},
};

var accept_sizes = [256]byte{
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x00-0x0f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x10-0x1f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x20-0x2f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x30-0x3f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x40-0x4f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x50-0x5f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x60-0x6f
	0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, 0xf0, // 0x70-0x7f

	0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, // 0x80-0x8f
	0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, // 0x90-0x9f
	0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, // 0xa0-0xaf
	0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, // 0xb0-0xbf
	0xf1, 0xf1, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, // 0xc0-0xcf
	0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, 0x02, // 0xd0-0xdf
	0x13, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x23, 0x03, 0x03, // 0xe0-0xef
	0x34, 0x04, 0x04, 0x04, 0x44, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, 0xf1, // 0xf0-0xff
};

proc encode_rune(r rune) -> ([4]byte, int) {
	var buf [4]byte;
	var i = r as u32;
	const mask = 0x3f as byte;
	if i <= 1<<7-1 {
		buf[0] = r as byte;
		return buf, 1;
	}
	if i <= 1<<11-1 {
		buf[0] = 0xc0 | (r>>6) as byte;
		buf[1] = 0x80 | (r)    as byte & mask;
		return buf, 2;
	}

	// Invalid or Surrogate range
	if i > 0x0010ffff ||
	   (0xd800 <= i && i <= 0xdfff) {
		r = 0xfffd;
	}

	if i <= 1<<16-1 {
		buf[0] = 0xe0 | (r>>12) as byte;
		buf[1] = 0x80 | (r>>6)  as byte & mask;
		buf[2] = 0x80 | (r)     as byte & mask;
		return buf, 3;
	}

	buf[0] = 0xf0 | (r>>18) as byte;
	buf[1] = 0x80 | (r>>12) as byte & mask;
	buf[2] = 0x80 | (r>>6)  as byte & mask;
	buf[3] = 0x80 | (r)     as byte & mask;
	return buf, 4;
}

proc decode_rune(s string) -> (rune, int) {
	var n = s.count;
	if n < 1 {
		return RUNE_ERROR, 0;
	}
	var b0 = s[0];
	var x = accept_sizes[b0];
	if x >= 0xf0 {
		var mask = (x as rune << 31) >> 31; // all zeros or all ones
		return (b0 as rune) &~ mask | RUNE_ERROR&mask, 1;
	}
	var size = x & 7;
	var ar = accept_ranges[x>>4];
	if n < size as int {
		return RUNE_ERROR, 1;
	}
	var b1 = s[1];
	if b1 < ar.lo || ar.hi < b1 {
		return RUNE_ERROR, 1;
	}

	const MASK_X = 0b00111111;
	const MASK_2 = 0b00011111;
	const MASK_3 = 0b00001111;
	const MASK_4 = 0b00000111;

	if size == 2 {
		return (b0&MASK_2) as rune <<6 | (b1&MASK_X) as rune, 2;
	}
	var b2 = s[2];
	if b2 < 0x80 || 0xbf < b2 {
		return RUNE_ERROR, 1;
	}
	if size == 3 {
		return (b0&MASK_3) as rune <<12 | (b1&MASK_X) as rune <<6 | (b2&MASK_X) as rune, 3;
	}
	var b3 = s[3];
	if b3 < 0x80 || 0xbf < b3 {
		return RUNE_ERROR, 1;
	}
	return (b0&MASK_4) as rune <<18 | (b1&MASK_X) as rune <<12 | (b3&MASK_X) as rune <<6 | (b3&MASK_X) as rune, 4;

}


proc valid_rune(r rune) -> bool {
	if r < 0 {
		return false;
	} else if SURROGATE_MIN <= r && r <= SURROGATE_MAX {
		return false;
	} else if r > MAX_RUNE {
		return false;
	}
	return true;
}

proc valid_string(s string) -> bool {
	var n = s.count;
	for var i = 0; i < n; {
		var si = s[i];
		if si < RUNE_SELF { // ascii
			i++;
			continue;
		}
		var x = accept_sizes[si];
		if x == 0xf1 {
			return false;
		}
		var size = (x & 7) as int;
		if i+size > n {
			return false;
		}
		var ar = accept_ranges[x>>4];
		if var b = s[i+1]; b < ar.lo || ar.hi < b {
			return false;
		} else if size == 2 {
			// Okay
		} else if var b = s[i+2]; b < 0x80 || 0xbf < b {
			return false;
		} else if size == 3 {
			// Okay
		} else if var b = s[i+3]; b < 0x80 || 0xbf < b {
			return false;
		}
		i += size;
	}
	return true;
}

proc rune_count(s string) -> int {
	var count = 0;
	var n = s.count;
	for var i = 0; i < n; count++ {
		var si = s[i];
		if si < RUNE_SELF { // ascii
			i++;
			continue;
		}
		var x = accept_sizes[si];
		if x == 0xf1 {
			i++;
			continue;
		}
		var size = (x & 7) as int;
		if i+size > n {
			i++;
			continue;
		}
		var ar = accept_ranges[x>>4];
		if var b = s[i+1]; b < ar.lo || ar.hi < b {
			size = 1;
		} else if size == 2 {
			// Okay
		} else if var b = s[i+2]; b < 0x80 || 0xbf < b {
			size = 1;
		} else if size == 3 {
			// Okay
		} else if var b = s[i+3]; b < 0x80 || 0xbf < b {
			size = 1;
		}
		i += size;
	}
	return count;
}


proc rune_size(r rune) -> int {
	match {
	case r < 0:          return -1;
	case r <= 1<<7  - 1: return 1;
	case r <= 1<<11 - 1: return 2;
	case SURROGATE_MIN <= r && r <= SURROGATE_MAX: return -1;
	case r <= 1<<16 - 1: return 3;
	case r <= MAX_RUNE:  return 4;
	}
	return -1;
}