1 files changed, 70 insertions, 0 deletions
diff --git a/utf8.lua b/utf8.lua
new file mode 100644
index 0000000..f661b1b
--- /dev/null
+++ b/utf8.lua
@@ -0,0 +1,70 @@
+local function iter(s, i)
+	if i >= #s then return end
+	local b, nbytes = s:byte(i+1,i+1), 1
+
+	-- determine width of the codepoint by counting the number of set bits in the first byte
+	-- warning: there is no validation of the following bytes!
+	if     b >= 0xc0 and b <= 0xdf then nbytes = 2 -- 1100 0000 to 1101 1111
+	elseif b >= 0xe0 and b <= 0xef then nbytes = 3 -- 1110 0000 to 1110 1111
+	elseif b >= 0xf0 and b <= 0xf7 then nbytes = 4 -- 1111 0000 to 1111 0111
+	elseif b >= 0xf8 and b <= 0xfb then nbytes = 5 -- 1111 1000 to 1111 1011
+	elseif b >= 0xfc and b <= 0xfd then nbytes = 6 -- 1111 1100 to 1111 1101
+	elseif b <  0x00 or  b >  0x7f then error(("Invalid codepoint: 0x%02x"):format(b))
+	end
+	return i+nbytes, s:sub(i+1,i+nbytes), nbytes
+end
+
+local function chars(s)
+	return iter, s, 0
+end
+
+local function len(s)
+	-- assumes sane utf8 string: count the number of bytes that is *not* 10xxxxxx
+	local _, c = s:gsub('[^\128-\191]', '')
+	return c
+end
+
+local function sub(s, i, j)
+	local l = len(s)
+	j = j or l
+	if i < 0 then i = l + i + 1 end
+	if j < 0 then j = l + j + 1 end
+	if j < i then return '' end
+
+	local k, t = 1, {}
+	for _, c in chars(s) do
+		if k >= i then t[#t+1] = c end
+		if k >= j then break end
+		k = k + 1
+	end
+	return table.concat(t)
+end
+
+local function split(s, i)
+	local l = len(s)
+	if i < 0 then i = l + i + 1 end
+
+	local k, pos = 1, 0
+	for byte in chars(s) do
+		if k > i then break end
+		pos, k = byte, k + 1
+	end
+	return s:sub(1, pos), s:sub(pos+1, -1)
+end
+
+local function reverse(s)
+	local t = {}
+	for _, c in chars(s) do
+		table.insert(t, 1, c)
+	end
+	return table.concat(t)
+end
+
+return {
+	iter    = iter,
+	chars   = chars,
+	len     = len,
+	sub     = sub,
+	split   = split,
+	reverse = reverse,
+}