feat: add agenda plugin

Import the package from my dotfiles.

lua/agenda/util.lua

@@ -0,0 +1,78 @@
+local M = {}
+
+---Parses a list of arguments **kind of** like the getopts shell builtin.
+---@param args string[]
+---@param optstring string
+---@return function
+function M.getopts(args, optstring)
+  local spec = {}
+  local last
+
+  for i = 1, #optstring do
+    local c = optstring:sub(i, i)
+
+    if c == ':' and last then
+      spec[last] = true
+    elseif i == #optstring then
+      spec[c] = false
+    elseif last and last ~= ':' then
+      spec[last] = false
+    end
+
+    last = c
+  end
+
+  -- 1. if the option takes a value either
+  --      a. take the rest of the slice as its values if at least one char
+  --         remains in the slice
+  --      b. take the next whole argument as its value
+  -- 2. find the next option name (char)
+  local i = 1 -- arg index
+  local n = 1 -- slice index
+  local g = 1 -- I am not very smart
+  return function ()
+    while args[i] do
+      local arg = args[i]
+
+      -- TODO: Maybe write some tests instead :^)
+      if g > 2097152 then
+        error 'logic error'
+      end
+      g = g + 1
+
+      if n == 1 and not vim.startswith(arg, '-') then
+        -- "Normal" arguments end the iterator.
+        return
+      end
+
+      n = n + 1
+      local c = arg:sub(n, n)
+      if #c > n then
+        n = n + 1
+      end
+
+      if #c == 1 then
+        if spec[c] then
+          local optval = arg:sub(n + 1, #arg)
+          n = 1
+          i = i + 1
+          if #optval > 0 then
+            return i, c, optval
+          end
+          -- Remaining argument does not contain optval, consume the next argument
+          -- as optval.
+          i = i + 1
+          return i, c, args[i - 1]
+        end
+
+        -- Do not handle nil. If the opt is invalid the caller must handle it.
+        return i + 1, c, nil
+      end
+
+      -- Stray '-' ends the iterator.
+      return
+    end
+  end
+end
+
+return M