feat(auto update): add utility function to check if update is needed

- compare current and latest version retrived from respective api

Signed-off-by: Aditya Alok <dev.aditya.alok@gmail.com>

scripts/updates/utils/termux_pkg_is_update_needed.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+#
+# NOTE: This function returns true even when CURRENT_VERSION = "1.0" and LATEST_VERSION = "1.0-1".
+# This is logically correct, but repology sometimes returns "1.0-1" as the latest version even
+# if "1.0" is latest. This happens when any of the repositories tracked by repology has specified
+# "1.0-1" as the latest.
+#
+# For example:
+# latest lua:lpeg version (as of 2021-11-20T12:21:31) is "1.0.2" but MacPorts specifies as "1.0.2-1".
+# Hence repology returns "1.0.2-1" as the latest.
+#
+# But hopefully, all this can be avoided if TERMUX_PKG_AUTO_UPDATE_TAG_REGEXP is set.
+#
+termux_pkg_is_update_needed() {
+	if [[ "$#" -lt 2 ]] || [[ "$#" -gt 3 ]]; then
+		termux_error_exit <<-EndOfUsage
+			Usage: ${FUNCNAME[0]} <current-version> <latest-version> [version-regexp]
+			Returns: 0 if update is needed, 1 if not.
+		EndOfUsage
+	fi
+
+	local CURRENT_VERSION="$1"
+	local LATEST_VERSION="$2"
+	local VERSION_REGEX="${3:-}"
+
+	# If needed, filter version numbers from tag by using regexp.
+	if [[ -n "${VERSION_REGEX}" ]]; then
+		LATEST_VERSION="$(grep -oP "${VERSION_REGEX}" <<<"${LATEST_VERSION}" || true)"
+	fi
+
+	if [[ -z "${LATEST_VERSION}" ]]; then
+		termux_error_exit <<-EndOfError
+			ERROR: failed to check latest version. Ensure whether the version regex '${VERSION_REGEX}'
+			works correctly with latest release tag.
+		EndOfError
+	fi
+
+	# Translate "_" into ".": some packages use underscores to seperate
+	# version numbers, but we require them to be separated by dots.
+	LATEST_VERSION="${LATEST_VERSION//_/.}"
+
+	# Compare versions.
+	# shellcheck disable=SC2091
+	if $(
+		cat <<-EOF | python3 -
+			import sys
+
+			from pkg_resources import parse_version
+
+			if parse_version("${CURRENT_VERSION}") < parse_version("${LATEST_VERSION}"):
+			    sys.exit(0)
+			else:
+			    sys.exit(1)
+		EOF
+	); then
+		return 0 # true. Update needed.
+	fi
+	return 1 # false. Update not needed.
+}
+
+# Make script sourceable as well as executable.
+if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+	declare -f termux_error_exit >/dev/null || . "$(dirname "${BASH_SOURCE[0]}")/termux_error_exit.sh"
+	termux_pkg_is_update_needed "$@"
+fi