utf8.offset returns also final position of character

'utf8.offset' returns two values: the initial and the final position
of the given character.

lutf8lib.c

@@ -181,8 +181,8 @@ static int utfchar (lua_State *L) {
 
 
 /*
-** offset(s, n, [i])  -> index where n-th character counting from
-**   position 'i' starts; 0 means character at 'i'.
+** offset(s, n, [i])  -> indices where n-th character counting from
+**   position 'i' starts and ends; 0 means character at 'i'.
 */
 static int byteoffset (lua_State *L) {
   size_t len;
@@ -217,12 +217,20 @@ static int byteoffset (lua_State *L) {
        }
      }
   }
-  if (n == 0)  /* did it find given character? */
-    lua_pushinteger(L, posi + 1);
-  else  /* no such character */
+  if (n != 0) {  /* did not find given character? */
     luaL_pushfail(L);
     return 1;
   }
+  lua_pushinteger(L, posi + 1);  /* initial position */
+  if ((s[posi] & 0x80) != 0) {  /* multi-byte character? */
+    do {
+      posi++;
+    } while (iscontp(s + posi + 1));  /* skip to final byte */
+  }
+  /* else one-byte character: final position is the initial one */
+  lua_pushinteger(L, posi + 1);  /* 'posi' now is the final position */
+  return 2;
+}
 
 
 static int iter_aux (lua_State *L, int strict) {

manual/manual.of

@@ -7958,21 +7958,27 @@ returns @fail plus the position of the first invalid byte.
 
 @LibEntry{utf8.offset (s, n [, i])|
 
-Returns the position (in bytes) where the encoding of the
-@id{n}-th character of @id{s}
-(counting from position @id{i}) starts.
+Returns the the position of the @id{n}-th character of @id{s}
+(counting from byte position @id{i}) as two integers:
+The index (in bytes) where its encoding starts and the
+index (in bytes) where it ends.
+
+If the specified character is right after the end of @id{s},
+the function behaves as if there was a @Char{\0} there.
+If the specified character is neither in the subject
+nor right after its end,
+the function returns @fail.
+
 A negative @id{n} gets characters before position @id{i}.
 The default for @id{i} is 1 when @id{n} is non-negative
 and @T{#s + 1} otherwise,
 so that @T{utf8.offset(s, -n)} gets the offset of the
 @id{n}-th character from the end of the string.
-If the specified character is neither in the subject
-nor right after its end,
-the function returns @fail.
 
 As a special case,
-when @id{n} is 0 the function returns the start of the encoding
-of the character that contains the @id{i}-th byte of @id{s}.
+when @id{n} is 0 the function returns the start and end
+of the encoding of the character that contains the
+@id{i}-th byte of @id{s}.
 
 This function assumes that @id{s} is a valid UTF-8 string.
 

testes/utf8.lua

@@ -52,14 +52,16 @@ local function check (s, t, nonstrict)
   for i = 1, #t do assert(t[i] == t1[i]) end   -- 't' is equal to 't1'
 
   for i = 1, l do   -- for all codepoints
-    local pi = utf8.offset(s, i)        -- position of i-th char
+    local pi, pie = utf8.offset(s, i)        -- position of i-th char
     local pi1 = utf8.offset(s, 2, pi)   -- position of next char
+    assert(pi1 == pie + 1)
     assert(string.find(string.sub(s, pi, pi1 - 1), justone))
     assert(utf8.offset(s, -1, pi1) == pi)
     assert(utf8.offset(s, i - l - 1) == pi)
     assert(pi1 - pi == #utf8.char(utf8.codepoint(s, pi, pi, nonstrict)))
     for j = pi, pi1 - 1 do
-      assert(utf8.offset(s, 0, j) == pi)
+      local off1, off2 = utf8.offset(s, 0, j)
+      assert(off1 == pi and off2 == pi1 - 1)
     end
     for j = pi + 1, pi1 - 1 do
       assert(not utf8.len(s, j))
@@ -71,6 +73,14 @@ local function check (s, t, nonstrict)
     assert(utf8.len(s, 1, pi, nonstrict) == i)
   end
 
+  local expected = 1    -- expected position of "current" character
+  for i = 1, l + 1 do
+    local p, e = utf8.offset(s, i)
+    assert(p == expected)
+    expected = e + 1
+  end
+  assert(expected - 1 == #s + 1)
+
   local i = 0
   for p, c in utf8.codes(s, nonstrict) do
     i = i + 1
@@ -94,20 +104,20 @@ end
 
 
 do    -- error indication in utf8.len
-  local function check (s, p)
+  local function checklen (s, p)
     local a, b = utf8.len(s)
     assert(not a and b == p)
   end
-  check("abc\xE3def", 4)
-  check("\xF4\x9F\xBF", 1)
-  check("\xF4\x9F\xBF\xBF", 1)
+  checklen("abc\xE3def", 4)
+  checklen("\xF4\x9F\xBF", 1)
+  checklen("\xF4\x9F\xBF\xBF", 1)
   -- spurious continuation bytes
-  check("汉字\x80", #("汉字") + 1)
-  check("\x80hello", 1)
-  check("hel\x80lo", 4)
-  check("汉字\xBF", #("汉字") + 1)
-  check("\xBFhello", 1)
-  check("hel\xBFlo", 4)
+  checklen("汉字\x80", #("汉字") + 1)
+  checklen("\x80hello", 1)
+  checklen("hel\x80lo", 4)
+  checklen("汉字\xBF", #("汉字") + 1)
+  checklen("\xBFhello", 1)
+  checklen("hel\xBFlo", 4)
 end
 
 -- errors in utf8.codes