Switch to processing in Lua

runtime/lua/man.lua

@@ -0,0 +1,75 @@
+local function highlight_backspaced(line, linenr)
+  local chars = {}
+  local prev_char = ''
+  local overstrike = false
+  local hls = {} -- Store highlight groups as { attr, start, end }
+  local NONE, BOLD, UNDERLINE = 0, 1, 2
+  local attr = NONE
+  local byte = 0 -- byte offset
+
+  -- Break input into UTF8 characters
+  for char in line:gmatch("[^\128-\191][\128-\191]*") do
+    if overstrike then
+      local last_hl = hls[#hls]
+      if char == prev_char then
+        if char == '_' and attr == UNDERLINE and last_hl and last_hl[3] == byte then
+          -- This underscore is in the middle of an underlined word
+          attr = UNDERLINE
+        else
+          attr = BOLD
+        end
+      elseif prev_char == '_' then
+        -- char is underlined
+        attr = UNDERLINE
+      elseif prev_char == '+' and char == 'o' then
+        -- bullet (overstrike text '+^Ho')
+        attr = BOLD
+        char = [[·]]
+      elseif prev_char == [[·]] and char == 'o' then
+        -- bullet (additional handling for '+^H+^Ho^Ho')
+        attr = BOLD
+        char = [[·]]
+      else
+        -- use plain char
+        attr = NONE
+      end
+
+      -- Grow the previous highlight group if possible
+      if last_hl and last_hl[1] == attr and last_hl[3] == byte then
+        last_hl[3] = byte + #char
+      else
+        hls[#hls + 1] = {attr, byte, byte + #char}
+      end
+
+      overstrike = false
+      prev_char = ''
+      byte = byte + #char
+      chars[#chars + 1] = char
+    elseif char == "\b" then
+      overstrike = true
+      prev_char = chars[#chars]
+      byte = byte - #prev_char
+      chars[#chars] = nil
+    else
+      byte = byte + #char
+      chars[#chars + 1] = char
+    end
+  end
+
+  for i, hl in ipairs(hls) do
+    if hl[1] ~= NONE then
+      vim.api.nvim_buf_add_highlight(
+        0,
+        -1,
+        hl[1] == BOLD and "manBold" or "manUnderline",
+        linenr - 1,
+        hl[2],
+        hl[3]
+      )
+    end
+  end
+
+  return table.concat(chars, '')
+end
+
+return { highlight_backspaced = highlight_backspaced }