#! /bin/sh
#
# NAME
#   get - evaluate ./default.nix
#
# SYNOPSIS
#   get [-jlprs] [--show-trace] [attrPath]
#
# DESCRIPTION
#   The command nix evaluates the Nix expression in ./default.nix.
#
# OPTIONS
#
#   -j    Format output as JSON. Implies -s.
#
#   -l    List attrNames, one per line.  If used in conjunction with -j, then
#         the list will be output as JSON value.
#
#   -p    Pretty-print output. (Only effectiv in conjunction with -j.)
#
#   -r    Disable the default filter and produce raw output.  Normally, when
#         the result looks like a JSON or Nix string, i.e. begins with a double
#         quote, then the string itself gets printed.
#         Refs jq(1)'s --raw-output.
#
#   -s    Recursively evaluate list elements and attributes.
#         Refs nix-instantiate(1)'s --strict.
#
#   --show-trace
#         Refs nix-instantiate(1)'s --show-trace.
#
set -efu

args=$*

has_lopt() {
  echo "$args" | grep -q '\(^\|\s\)--'"$1"'\(\s\|$\)'
}

has_sopt() {
  echo "$args" | grep -q '\(^\|\s\)-[a-z]*'"$1"'[a-z]*\(\s\|$\)'
}

filter() {
  set -- "$(cat)"
  if echo "$1" | grep -q ^\"; then
    echo "$1" | jq -e -r .
  else
    echo "$1"
  fi
}

if has_sopt j; then
  json=1
  strict=1
fi

if has_sopt l; then
  query='-E builtins.attrNames((import(./.)).&)'
  if ! has_sopt j; then
    json=1
    filter() { jq -r .[]; }
  fi
else
  query='-A &'
fi

if has_sopt p && has_sopt j; then
  filter() { jq .; }
fi

if has_sopt r; then
  filter() { cat; }
fi

if has_sopt s; then
  strict=1
fi

if has_lopt show-trace; then
  show_trace=1
fi


if x=$(nix-instantiate --json --eval -A NIX_PATH 2>/dev/null); then
  NIX_PATH=$(echo "$x" | jq -r .)
  export NIX_PATH
  unset x
fi

# Quote slashes so we can use query as replacement string for sed's s///.
query=$(echo "$query" | sed 's:/:\\&:')

result=$(nix-instantiate \
  ${json+--json} \
  ${show_trace+--show-trace} \
  ${strict+--strict} \
  --eval \
  $(
    for i; do
      echo "$i"
    done | sed -n '
      s/^[A-Za-z_][0-9A-Za-z_"./-]*$/'"$query"'/p
      s/^\([0-9A-Za-z-]\+\)=\([.0-9A-Za-z-]*\)$/--argstr \1 \2/p
    '
  ) \
  --argstr current-date "$(date -Is)" \
  --argstr current-host-name "$HOSTNAME" \
  --argstr current-user-name "$LOGNAME" \
)

echo "$result" | filter