feat: add agenda plugin

Import the package from my dotfiles.

plugin/agenda.lua

@@ -0,0 +1,121 @@
+local agenda = require 'agenda'
+local getopts = require 'agenda.util'.getopts
+
+-- If neovim was started by `agenda` the `AGENDA_FILE` env var will be set
+-- with the file name.
+if vim.env.AGENDA_FILE == vim.api.nvim_buf_get_name(0) then
+  agenda.on_attach(0)
+end
+
+-- Callbacks in the middle of a function signature are PAIN.
+local function create_user_command(name, opts)
+  local command = opts.command
+  opts.command = nil
+  vim.api.nvim_create_user_command(name, command, opts)
+end
+
+-- TODO: Handle subcommands (kinda like git-fugitive)? Could be useful if I'll
+--       add a remove subcommand.
+-- TODO: Handle -E option (why not?).
+create_user_command('Agenda', {
+  desc = 'Open a note file/buffer for the given day or the current day',
+  nargs = '*',
+  bang = true,
+  bar = true,
+  command = function (command)
+    local opts = {
+      create = command.bang,
+    }
+
+    local i = 1
+    for optind, opt, optval in getopts(command.fargs, 'bce:t:') do
+      if opt == 'b' then
+        opts.backlog = true
+      elseif opt == 'c' then
+        opts.create = true
+      elseif opt == 'e' then
+        opts.extension = optval
+      elseif opt == 't' then
+        opts.name = optval
+      else
+        local err = string.format("agenda: invalid option: '%s'", opt)
+        vim.notify(err, vim.log.levels.WARN)
+        return
+      end
+
+      i = optind
+    end
+
+    local args = {}
+    while command.fargs[i] do
+      args[#args + 1] = command.fargs[i]
+      i = i + 1
+    end
+
+    local date
+    if #args > 0 then
+      date = table.concat(args, ' ')
+    else
+      date = opts.backlog and 'backlog' or 'today'
+    end
+
+    agenda.open(date, opts)
+  end,
+  -- TODO: Handle supported agenda edit options.
+  complete = function (arg_lead, cmd_line, cursor_pos)
+    local modifiers = {
+      next = true,
+      last = true,
+    }
+
+    local completions = {}
+
+    local final = {}
+
+    if cmd_line:match '^Agenda!? [%w-]*$' then
+      completions = {
+        'next ',
+        'last ',
+        'yesterday',
+        'tomorrow',
+        'today',
+        'day',
+        'week',
+        'month',
+        'year',
+        'monday',
+        'tuesday',
+        'wednesday',
+        'thursday',
+        'friday',
+        'saturday',
+        'sunday',
+      }
+    else
+      local first = cmd_line:match '^Agenda (%w+)%s+%w*$'
+      if modifiers[first] then
+        completions = {
+          'day',
+          'week',
+          'month',
+          'year',
+          'monday',
+          'tuesday',
+          'wednesday',
+          'thursday',
+          'friday',
+          'saturday',
+          'sunday',
+        }
+      end
+    end
+
+    for _, can in pairs(completions) do
+      if vim.startswith(can, arg_lead) then
+        final[#final + 1] = can
+      end
+    end
+
+    return final
+  end,
+})