session() {
  local usage="usage: session [-h] [-H <host>] <name>
       session [-H <host>] -p <project>"
  local name="" project="" host=""

  while [[ $# -gt 0 ]]; do
    case "$1" in
      -h|--help)
        echo "$usage

Create or attach to a tmux session by name either locally or 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.

With -H create or attach to the session on <host> via ssh, where <host> is
either a hostname or a shortname defined in ~/.config/session. Combined with -p
the project is resolved on <host>, not locally. Not allowed inside a tmux
session.
"
        return 0
        ;;
      -p|--project)
        if [[ "$2" == "" ]]; then
          echo "$fg[red]error:$reset_color missing <project> argument"
          return 1
        fi
        project=${2%/}
        shift 2
        ;;
      -H|--host)
        if [[ "$2" == "" ]]; then
          echo "$fg[red]error:$reset_color missing <host> argument"
          return 1
        fi
        host=$2
        shift 2
        ;;
      -*)
        echo "$fg[red]error:$reset_color unknown option: $1"
        return 1
        ;;
      *)
        if [[ "$name" != "" ]]; then
          echo "$fg[red]error:$reset_color invalid argument: $1"
          return 1
        fi
        name=$1
        shift
        ;;
    esac
  done

  if [[ "$project" != "" ]] && [[ "$name" != "" ]]; then
    echo "$fg[red]error:$reset_color <name> not allowed with -p"
    return 1
  fi
  if [[ "$project" == "" ]] && [[ "$name" == "" ]]; then
    echo "$usage"
    return 0
  fi

  if [[ "$host" != "" ]]; then
    if [[ "$TMUX" != "" ]]; then
      echo "$fg[red]error:$reset_color <host> not allowed inside tmux session"
      return 1
    fi
    declare -A hosts
    if [ -f ~/.config/session ]; then
      source ~/.config/session
    fi
    host=${hosts[$host]:-$host}
  fi

  if [[ "$project" != "" ]]; then
    # tmux rewrites `.` and `:` in session names, so mirror that to keep the
    # name we create and the name we target in sync.
    name=${project//[.:]/_}
    if [[ "$host" != "" ]]; then
      # The project lives in the remote ~/Projects, so resolve it there rather
      # than locally. The remote session-created hook reads TMUX_PROJECT back
      # out of the remote tmux server to build the layout.
      ssh "$host" -t "
        repo=\$HOME/Projects/${(q)project}
        [ -d \"\$repo\" ] ||
          { echo 'error: no such project: ${(q)project}' >&2; exit 1; }
        git -C \"\$repo\" rev-parse --git-dir >/dev/null 2>&1 ||
          { echo \"error: not a git repository: \$repo\" >&2; exit 1; }
        exec tmux new-session -As ${(q)name} -c \"\$HOME\" -e TMUX_PROJECT=${(q)project}
      "
      return
    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: $repo"
      return 1
    fi
    if [[ "$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 "=$name"
    fi
  elif [[ "$host" != "" ]]; then
    ssh "$host" -t tmux new-session -As "$name"
  elif [[ "$TMUX" == "" ]]; then
    tmux new-session -As "$name"
  else
    tmux has-session -t "=$name" &> /dev/null || \
      tmux new-session -Ads "$name" -c $HOME
    tmux switch-client -t "=$name"
  fi
}