# Gets the UTF-8 char at index `pos`
	#
	# Index is expressed in Unicode chars
	#
	# ~~~raw
	#     assert "かきく".as(FlatString).items.char_at(0) == 'か'
	# ~~~
	#
	# If the char at position pos is an invalid Unicode char,
	# the Unicode replacement character � (0xFFFD) will be used.
	#
	# ~~~raw
	#     assert "かきく".as(FlatString).items.char_at(1) == '�'
	# ~~~
	fun char_at(pos: Int): Char do
		var c = self[pos]
		if c & 0x80 == 0 then return c.code_point
		var b = fetch_4_hchars(pos)
		var ret = 0u32
		if b & 0xC00000u32 != 0x800000u32 then return 0xFFFD.code_point
		if b & 0xE0000000u32 == 0xC0000000u32 then
			ret |= (b & 0x1F000000u32) >> 18
			ret |= (b & 0x3F0000u32) >> 16
			return ret.code_point
		end
		if not b & 0xC000u32 == 0x8000u32 then return 0xFFFD.code_point
		if b & 0xF0000000u32 == 0xE0000000u32 then
			ret |= (b & 0xF000000u32) >> 12
			ret |= (b & 0x3F0000u32) >> 10
			ret |= (b & 0x3F00u32) >> 8
			return ret.code_point
		end
		if not b & 0xC0u32 == 0x80u32 then return 0xFFFD.code_point
		if b & 0xF8000000u32 == 0xF0000000u32 then
			ret |= (b & 0x7000000u32) >> 6
			ret |= (b & 0x3F0000u32) >> 4
			ret |= (b & 0x3F00u32) >> 2
			ret |= b & 0x3Fu32
			return ret.code_point
		end
		return 0xFFFD.code_point
	end