Pad long usage messages

Long usage messages are split into lines. A margin is added so that they line up under "Usage: " part of the message.

spec/help_spec.lua

@@ -44,7 +44,8 @@ describe("tests related to help message generation", function()
          :args "*"
 
       assert.equal(table.concat({
-         "Usage: foo [-h] <first> <second-and-third> <second-and-third> [<maybe-fourth>] [<others>] ...",
+         "Usage: foo [-h] <first> <second-and-third> <second-and-third>",
+         "       [<maybe-fourth>] [<others>] ...",
          "",
          "Arguments: ",
          "   first",

spec/usage_spec.lua

@@ -18,9 +18,10 @@ describe("tests related to usage message generation", function()
       parser:argument "others"
          :args "*"
 
-      assert.equal(
+      assert.equal(table.concat({
-         [=[Usage: foo <first> <second-and-third> <second-and-third> [<maybe-fourth>] [<others>] ...]=],
+         "Usage: foo <first> <second-and-third> <second-and-third>",
-         parser:prepare():get_usage()
+         "       [<maybe-fourth>] [<others>] ..."
+         }, "\r\n"), parser:prepare():get_usage()
       )
    end)
 

src/argparse.lua

@@ -309,28 +309,38 @@ function Parser:update_charset(charset)
    return charset
 end
 
+local max_usage_width = 70
+local usage_welcome = "Usage: "
+
 function Parser:get_usage()
    if not self._usage then
-      local buf = {"Usage:", self._name}
+      local lines = {usage_welcome .. self._name}
+
+      local function add(s)
+         if #lines[#lines]+1+#s <= max_usage_width then
+            lines[#lines] = lines[#lines] .. " " .. s
+         else
+            lines[#lines+1] = (" "):rep(#usage_welcome) .. s
+         end
+      end
 
       for _, elements in ipairs{self._options, self._arguments} do
          for _, element in ipairs(elements) do
-            table.insert(buf, element:get_usage())
+            add(element:get_usage())
          end
       end
 
       if #self._commands > 0 then
          if self._require_command then
-            table.insert(buf, "<command>")
+            add("<command>")
          else
-            table.insert(buf, "[<command>]")
+            add("[<command>]")
          end
 
-         table.insert(buf, "...")
+         add("...")
       end
 
-      -- TODO: prettify
+      self._usage = table.concat(lines, "\r\n")
-      self._usage = table.concat(buf, " ")
    end
 
    return self._usage