Two-line ZSH prompt

two-line-prompt.zsh

# Example of two-line ZSH prompt with four components.
#
#   top-left                         top-right
#   bottom-left                   bottom-right
#
# Components can be customized by editing set-prompt function.
#
# Installation:
#
#   (cd && curl -fsSLO https://gist.githubusercontent.com/romkatv/2a107ef9314f0d5f76563725b42f7cab/raw/two-line-prompt.zsh)
#   echo 'source ~/two-line-prompt.zsh' >>~/.zshrc
#
# Accompanying article:
# https://www.reddit.com/r/zsh/comments/cgbm24/multiline_prompt_the_missing_ingredient/
#
# This is only an example. If you are looking for a good ZSH prompt,
# try https://github.com/romkatv/powerlevel10k/.

# Usage: prompt-length TEXT [COLUMNS]
#
# If you run `print -P TEXT`, how many characters will be printed
# on the last line?
#
# Or, equivalently, if you set PROMPT=TEXT with prompt_subst
# option unset, on which column will the cursor be?
#
# The second argument specifies terminal width. Defaults to the
# real terminal width.
#
# The result is stored in REPLY.
#
# Assumes that `%{%}` and `%G` don't lie.
#
# Examples:
#
#   prompt-length ''            => 0
#   prompt-length 'abc'         => 3
#   prompt-length $'abc\nxy'    => 2
#   prompt-length '❎'          => 2
#   prompt-length $'\t'         => 8
#   prompt-length $'\u274E'     => 2
#   prompt-length '%F{red}abc'  => 3
#   prompt-length $'%{a\b%Gb%}' => 1
#   prompt-length '%D'          => 8
#   prompt-length '%1(l..ab)'   => 2
#   prompt-length '%(!.a.)'     => 1 if root, 0 if not
function prompt-length() {
  emulate -L zsh
  local -i COLUMNS=${2:-COLUMNS}
  local -i x y=${#1} m
  if (( y )); then
    while (( ${${(%):-$1%$y(l.1.0)}[-1]} )); do
      x=y
      (( y *= 2 ))
    done
    while (( y > x + 1 )); do
      (( m = x + (y - x) / 2 ))
      (( ${${(%):-$1%$m(l.x.y)}[-1]} = m ))
    done
  fi
  typeset -g REPLY=$x
}

# Usage: fill-line LEFT RIGHT
#
# Sets REPLY to LEFT<spaces>RIGHT with enough spaces in
# the middle to fill a terminal line.
function fill-line() {
  emulate -L zsh
  prompt-length $1
  local -i left_len=REPLY
  prompt-length $2 9999
  local -i right_len=REPLY
  local -i pad_len=$((COLUMNS - left_len - right_len - ${ZLE_RPROMPT_INDENT:-1}))
  if (( pad_len < 1 )); then
    # Not enough space for the right part. Drop it.
    typeset -g REPLY=$1
  else
    local pad=${(pl.$pad_len.. .)}  # pad_len spaces
    typeset -g REPLY=${1}${pad}${2}
  fi
}

# Sets PROMPT and RPROMPT.
#
# Requires: prompt_percent and no_prompt_subst.
function set-prompt() {
  emulate -L zsh
  local git_branch="$(git rev-parse --abbrev-ref HEAD 2>/dev/null)"
  git_branch=${git_branch//\%/%%}  # escape '%'

  # ~/foo/bar                     master
  # % █                            10:51
  #
  # Top left:      Blue current directory.
  # Top right:     Green Git branch.
  # Bottom left:   '#' if root, '%' if not; green on success, red on error.
  # Bottom right:  Yellow current time.

  local top_left='%F{blue}%~%f'
  local top_right="%F{green}${git_branch}%f"
  local bottom_left='%B%F{%(?.green.red)}%#%f%b '
  local bottom_right='%F{yellow}%T%f'

  local REPLY
  fill-line "$top_left" "$top_right"
  PROMPT=$REPLY$'\n'$bottom_left
  RPROMPT=$bottom_right
}

setopt no_prompt_{bang,subst} prompt_{cr,percent,sp}
autoload -Uz add-zsh-hook
add-zsh-hook precmd set-prompt

First of all, thank you for sharing the prompt-length function. It really is the "missing ingredient" as you say, and it inspired me to make a prompt of my own.

However, I've found that when the measured string happens to be the exact width as the terminal, it computes a length of 0. In fact, it seems that the calculated length is modulo the number of columns. Is it possible to find the length of the printed string independently of the $COLUMNS variable?

@Moarram Pass 1000000 as the second (optional) argument.