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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,
}
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