perf(vim.text): use lookup table implementation for hex encoding (#30080)

Co-authored-by: glepnir <glephunter@gmail.com>

runtime/lua/vim/text.lua

@@ -2,6 +2,18 @@
 
 local M = {}
 
+local alphabet = '0123456789ABCDEF'
+local atoi = {} ---@type table<string, integer>
+local itoa = {} ---@type table<integer, string>
+do
+  for i = 1, #alphabet do
+    local char = alphabet:sub(i, i)
+    itoa[i - 1] = char
+    atoi[char] = i - 1
+    atoi[char:lower()] = i - 1
+  end
+end
+
 --- Hex encode a string.
 ---
 --- @param str string String to encode
@@ -9,7 +21,9 @@ local M = {}
 function M.hexencode(str)
   local enc = {} ---@type string[]
   for i = 1, #str do
-    enc[i] = string.format('%02X', str:byte(i, i + 1))
+    local byte = str:byte(i)
+    enc[2 * i - 1] = itoa[math.floor(byte / 16)]
+    enc[2 * i] = itoa[byte % 16]
   end
   return table.concat(enc)
 end
@@ -26,8 +40,12 @@ function M.hexdecode(enc)
 
   local str = {} ---@type string[]
   for i = 1, #enc, 2 do
-    local n = assert(tonumber(enc:sub(i, i + 1), 16))
-    str[#str + 1] = string.char(n)
+    local u = atoi[enc:sub(i, i)]
+    local l = atoi[enc:sub(i + 1, i + 1)]
+    if not u or not l then
+      return nil, 'string must contain only hex characters'
+    end
+    str[(i + 1) / 2] = string.char(u * 16 + l)
   end
   return table.concat(str), nil
 end