Add update-index-state.sh script

This is kinda heavy, but it should work well on 32bit and 64bit arches.
The alternative was to copy-paste a lot of cabal-install code
that is not exposed. There seems to be no value in that for a
development script.

The alternative to this script is simply running 'cabal v2-update'
twice and then copy-pasting the output manually to cabal.project.

Author: hasufell

update-index-state.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+
+set -eu
+
+status_message() {
+    printf "\\033[0;32m%s\\033[0m\\n" "$1"
+}
+
+error_message() {
+    printf "\\033[0;31m%s\\033[0m\\n" "$1"
+}
+
+SCRIPTPATH="$( cd "$(dirname "$0")" ; pwd -P )"
+CACHE_LOCATION="${HOME}/.cabal/packages/hackage.haskell.org/01-index.cache"
+
+if [ ! -f "${CACHE_LOCATION}" ] ; then
+	error_message "${CACHE_LOCATION} does not exist, did you run 'cabal update'?"
+	exit 1
+fi
+
+if [ ! -f "${SCRIPTPATH}/cabal.project" ] ; then
+	error_message "Could not find ${SCRIPTPATH}/cabal.project, skipping index state update."
+	exit 3
+fi
+
+cabal v2-update
+
+arch=$(getconf LONG_BIT)
+
+case "${arch}" in
+32)
+	byte_size=4
+	magic_word="CABA1002"
+    ;;
+64)
+	byte_size=8
+	magic_word="00000000CABA1002"
+    ;;
+*)
+	error_message "Unknown architecture (long bit): ${arch}"
+	exit 2
+    ;;
+esac
+
+# This is the logic to parse the binary format of 01-index.cache.
+# The first word is a magic 'caba1002', the second one is the timestamp in unix epoch.
+# Better than copying the cabal-install source code.
+if [ "$(xxd -u -p -l${byte_size} -s 0 "${CACHE_LOCATION}")" != "${magic_word}" ] ; then
+	error_message "Magic word does not match!"
+	exit 4
+fi
+cache_timestamp=$(echo "ibase=16;obase=A;$(xxd -u -p -l${byte_size} -s ${byte_size} "${CACHE_LOCATION}")" | bc)
+
+# If we got junk from the binary file, this should fail.
+cache_date=$(date --utc --date "@${cache_timestamp}" "+%FT%TZ")
+
+
+status_message "Updating index state in ${SCRIPTPATH}/cabal.project"
+
+if grep -q "^index-state: .*" "${SCRIPTPATH}/cabal.project" ; then
+	awk '/index-state:/ {gsub(/.*/, "index-state: '${cache_date}'")}; { print }' "${SCRIPTPATH}/cabal.project" > "${SCRIPTPATH}/cabal.project.tmp"
+	mv "${SCRIPTPATH}/cabal.project.tmp" "${SCRIPTPATH}/cabal.project"
+else
+	printf "index-state: %s\n" "${cache_date}" >> "${SCRIPTPATH}/cabal.project"
+fi
+