#!/usr/bin/env bash

set -euo pipefail

usage() {
  cat <<'EOF'
Usage:
  preserve-env-values.sh <source-env-file> <target-env-file> <KEY> [KEY...]

For each KEY, copy the value from <source-env-file> into <target-env-file> only
when the target value is missing or still looks like a placeholder.
EOF
}

if [[ $# -lt 3 ]]; then
  usage >&2
  exit 1
fi

source_env_file="$1"
target_env_file="$2"
shift 2

for env_file in "${source_env_file}" "${target_env_file}"; do
  if [[ ! -f "${env_file}" ]]; then
    echo "Missing env file: ${env_file}" >&2
    exit 1
  fi
done

read_env_value() {
  local env_file="$1"
  local key="$2"
  local value

  value="$(
    awk -F= -v key="${key}" '
      /^[[:space:]]*#/ || index($0, "=") == 0 {
        next
      }
      {
        name = $1
        sub(/^[[:space:]]*export[[:space:]]+/, "", name)
        gsub(/^[[:space:]]+|[[:space:]]+$/, "", name)
        if (name == key) {
          value = substr($0, index($0, "=") + 1)
        }
      }
      END {
        print value
      }
    ' "${env_file}"
  )"

  value="${value%$'\r'}"
  value="${value#"${value%%[![:space:]]*}"}"
  value="${value%"${value##*[![:space:]]}"}"
  if [[ "${value}" == \"*\" && "${value}" == *\" ]]; then
    value="${value#\"}"
    value="${value%\"}"
  elif [[ "${value}" == \'*\' && "${value}" == *\' ]]; then
    value="${value#\'}"
    value="${value%\'}"
  fi
  printf '%s' "${value}"
}

should_preserve_value() {
  local value="$1"

  [[ -z "${value}" || "${value}" == placeholder || "${value}" == replace-with-* || "${value}" == *changeme* || "${value}" == *change-me* ]]
}

upsert_env_value() {
  local key="$1"
  local value="$2"
  local tmp_file

  tmp_file="$(mktemp "${target_env_file}.tmp.XXXXXX")"
  awk -v key="${key}" -v value="${value}" '
    BEGIN {
      updated = 0
    }
    /^[[:space:]]*#/ || index($0, "=") == 0 {
      print
      next
    }
    {
      name = $1
      sub(/=.*/, "", name)
      sub(/^[[:space:]]*export[[:space:]]+/, "", name)
      gsub(/^[[:space:]]+|[[:space:]]+$/, "", name)
      if (name == key) {
        print key "=" value
        updated = 1
        next
      }
      print
    }
    END {
      if (!updated) {
        print key "=" value
      }
    }
  ' "${target_env_file}" > "${tmp_file}"

  mv "${tmp_file}" "${target_env_file}"
}

for key in "$@"; do
  source_value="$(read_env_value "${source_env_file}" "${key}")"
  target_value="$(read_env_value "${target_env_file}" "${key}")"

  if [[ -n "${source_value}" ]] && should_preserve_value "${target_value}"; then
    upsert_env_value "${key}" "${source_value}"
  fi
done