session() {
  if [[ "$1" == "" ]]; then
    echo "usage: session [-h] <name> [<host>]
       session -p <project>"
  elif [[ "$1" == "-h" ]] || [[ "$1" == "--help" ]]; then
    echo "usage: session [-h] <name> [<host>]
       session -p <project>

Create or attach to a tmux session by name either locally on on a remote host
via ssh.

With -p create or attach to a session for <project>, a path in ~/Projects with
the prefix removed, which must be a git repository. The session is named after
the project and holds a home window plus a window at the project, named after
its default branch.
"
  elif [[ "$1" == "-p" ]] || [[ "$1" == "--project" ]]; then
    local project=${2%/}
    if [[ "$project" == "" ]]; then
      echo "$fg[red]error:$reset_color missing <project> argument"
      return 1
    fi
    if [[ "$3" != "" ]]; then
      echo "$fg[red]error:$reset_color invalid argument: $3"
      return 1
    fi
    local repo=$HOME/Projects/$project
    if [ ! -d "$repo" ]; then
      echo "$fg[red]error:$reset_color no such project: $project"
      return 1
    fi
    if ! git -C "$repo" rev-parse --git-dir &>/dev/null; then
      echo "$fg[red]error:$reset_color not a git repository: $repo"
      return 1
    fi
    # tmux rewrites `.` and `:` in session names, so mirror that to keep the
    # name we create and the name we target in sync.
    local name=${project//[.:]/_}
    if [[ "$TMUX" == "" ]]; then
      tmux new-session -As "$name" -c $HOME -e "TMUX_PROJECT=$project"
    else
      tmux has-session -t "=$name" &> /dev/null || \
        tmux new-session -Ads "$name" -c $HOME -e "TMUX_PROJECT=$project"
      tmux switch-client -t "=$name"
    fi
  else
    local name=$1
    local host=$2
    if [[ "$3" != "" ]]; then
      echo "$fg[red]error:$reset_color invalid argument: $3"
      return 1
    fi
    declare -A hosts
    if [ -f ~/.config/session ]; then
      source ~/.config/session
    fi
    local url=$hosts[$host]
    host=${url:-$host}
    if [[ "$TMUX" == "" ]]; then
      local cmd="tmux new-session -As $name"
      if [[ "$host" != "" ]]; then
        cmd="ssh $host -t $cmd"
      fi
      eval $cmd
    else
      if [[ "$host" != "" ]]; then
        echo "$fg[red]error:$reset_color <host> not allowed inside tmux session"
        return 1
      fi
      tmux list-sessions | grep "$name:" &> /dev/null || \
        tmux new-session -Ads $name -c $HOME
      tmux switch-client -t $name
    fi
  fi
}