feat: truncate strings from the middle (#276)

Author: l-kershaw

lua/plenary/strings.lua

@@ -92,10 +92,7 @@ M.strcharpart = (function()
   end
 end)()
 
-M.truncate = function(str, len, dots, direction)
-  str = tostring(str) -- We need to make sure its an actually a string and not a number
-  dots = dots or "…"
-  direction = direction or 1
+local truncate = function(str, len, dots, direction)
   if M.strdisplaywidth(str) <= len then
     return str
   end
@@ -123,6 +120,24 @@ M.truncate = function(str, len, dots, direction)
   return result
 end
 
+M.truncate = function(str, len, dots, direction)
+  str = tostring(str) -- We need to make sure its an actually a string and not a number
+  dots = dots or "…"
+  direction = direction or 1
+  if direction ~= 0 then
+    return truncate(str, len, dots, direction)
+  else
+    if M.strdisplaywidth(str) <= len then
+      return str
+    end
+    local len1 = math.floor((len + M.strdisplaywidth(dots)) / 2)
+    local s1 = truncate(str, len1, dots, 1)
+    local len2 = len - M.strdisplaywidth(s1) + M.strdisplaywidth(dots)
+    local s2 = truncate(str, len2, dots, -1)
+    return s1 .. s2:sub(dots:len() + 1)
+  end
+end
+
 M.align_str = function(string, width, right_justify)
   local str_len = M.strdisplaywidth(string)
   return right_justify and string.rep(" ", width - str_len) .. string or string .. string.rep(" ", width - str_len)

tests/plenary/strings_spec.lua

@@ -71,11 +71,11 @@ describe("strings", function()
       { args = { "abcde", 5, nil, -1 }, expected = { single = "abcde", double = "abcde" } },
       { args = { "abcde", 4, nil, -1 }, expected = { single = "…cde", double = "…de" } },
       {
-        args = { "アイウエオ", 11, nil, 1 },
+        args = { "アイウエオ", 11, nil, -1 },
         expected = { single = "アイウエオ", double = "アイウエオ" },
       },
       {
-        args = { "アイウエオ", 10, nil, 1 },
+        args = { "アイウエオ", 10, nil, -1 },
         expected = { single = "アイウエオ", double = "アイウエオ" },
       },
       {
@@ -89,11 +89,34 @@ describe("strings", function()
       { args = { "├─┤", 4, nil, -1 }, expected = { single = "├─┤", double = "…┤" } },
       { args = { "├─┤", 3, nil, -1 }, expected = { single = "├─┤", double = "…" } },
       { args = { "├─┤", 2, nil, -1 }, expected = { single = "…┤", double = "…" } },
+      -- truncations from the middle
+      { args = { "abcde", 6, nil, 0 }, expected = { single = "abcde", double = "abcde" } },
+      { args = { "abcde", 5, nil, 0 }, expected = { single = "abcde", double = "abcde" } },
+      { args = { "abcde", 4, nil, 0 }, expected = { single = "a…de", double = "a…e" } },
+      {
+        args = { "アイウエオ", 11, nil, 0 },
+        expected = { single = "アイウエオ", double = "アイウエオ" },
+      },
+      {
+        args = { "アイウエオ", 10, nil, 0 },
+        expected = { single = "アイウエオ", double = "アイウエオ" },
+      },
+      {
+        args = { "アイウエオ", 9, nil, 0 },
+        expected = { single = "アイ…エオ", double = "ア…エオ" },
+      },
+      { args = { "アイウエオ", 8, nil, 0 }, expected = { single = "ア…エオ", double = "ア…エオ" } },
+      { args = { "├─┤", 7, nil, 0 }, expected = { single = "├─┤", double = "├─┤" } },
+      { args = { "├─┤", 6, nil, 0 }, expected = { single = "├─┤", double = "├─┤" } },
+      { args = { "├─┤", 5, nil, 0 }, expected = { single = "├─┤", double = "…┤" } },
+      { args = { "├─┤", 4, nil, 0 }, expected = { single = "├─┤", double = "…┤" } },
+      { args = { "├─┤", 3, nil, 0 }, expected = { single = "├─┤", double = "…" } },
+      { args = { "├─┤", 2, nil, 0 }, expected = { single = "…┤", double = "…" } },
     } do
       for _, ambiwidth in ipairs { "single", "double" } do
         local msg = ("ambiwidth = %s, direction = %s, [%s, %d] -> %s"):format(
           ambiwidth,
-          (case.args[4] > 0) and "right" or "left",
+          (case.args[4] > 0) and "right" or (case.args[4] < 0) and "left" or "middle",
           case.args[1],
           case.args[2],
           case.expected[ambiwidth]