#!/usr/bin/env bash
# vim:noexpandtab:ts=2:sw=2:
#
#+  Usage: $(basename $0) [flags] [go-version] [version-prefix]
#+  -
#+  Version: ${GIMME_VERSION}
#+  Copyright: ${GIMME_COPYRIGHT}
#+  License URL: ${GIMME_LICENSE_URL}
#+  -
#+  Install go!  There are multiple types of installations available, with 'auto' being the default.
#+  If either 'auto' or 'binary' is specified as GIMME_TYPE, gimme will first check for an existing
#+  go installation.  This behavior may be disabled by providing '-f/--force/force' as first positional
#+  argument.
#+  -
#+  Option flags:
#+          -h --help help - show this help text and exit
#+    -V --version version - show the version only and exit
#+        -f --force force - remove the existing go installation if present prior to install
#+          -l --list list - list installed go versions and exit
#+        -k --known known - list known go versions and exit
#+    --force-known-update - when used with --known, ignores the cache and updates
#+    -r --resolve resolve - resolve a version specifier to a version, show that and exit
#+  -
#+  Influential env vars:
#+  -
#+        GIMME_GO_VERSION - version to install (*REQUIRED*, may be given as first positional arg)
#+    GIMME_VERSION_PREFIX - prefix for installed versions (default '${GIMME_VERSION_PREFIX}',
#+                           may be given as second positional arg)
#+              GIMME_ARCH - arch to install (default '${GIMME_ARCH}')
#+        GIMME_BINARY_OSX - darwin-specific binary suffix (default '${GIMME_BINARY_OSX}')
#+        GIMME_ENV_PREFIX - prefix for env files (default '${GIMME_ENV_PREFIX}')
#+     GIMME_GO_GIT_REMOTE - git remote for git-based install (default '${GIMME_GO_GIT_REMOTE}')
#+                GIMME_OS - os to install (default '${GIMME_OS}')
#+               GIMME_TMP - temp directory (default '${GIMME_TMP}')
#+              GIMME_TYPE - install type to perform ('auto', 'binary', 'source', or 'git')
#+                           (default '${GIMME_TYPE}')
#+      GIMME_INSTALL_RACE - install race directory after compile if non-empty.
#+                           If the install type is 'binary', this option is ignored.
#+             GIMME_DEBUG - enable tracing if non-empty
#+      GIMME_NO_ENV_ALIAS - disable creation of env 'alias' file when os and arch match host
#+        GIMME_SILENT_ENV - omit the 'go version' line from env file
#+       GIMME_CGO_ENABLED - enable build of cgo support
#+     GIMME_CC_FOR_TARGET - cross compiler for cgo support
#+     GIMME_DOWNLOAD_BASE - override base URL dir for download (default '${GIMME_DOWNLOAD_BASE}')
#+        GIMME_LIST_KNOWN - override base URL for known go versions (default '${GIMME_LIST_KNOWN}')
#+   GIMME_KNOWN_CACHE_MAX - seconds the cache for --known is valid for (default '${GIMME_KNOWN_CACHE_MAX}')
#+  -
#
set -e
shopt -s nullglob
shopt -s dotglob
shopt -s extglob
set -o pipefail

[[ ${GIMME_DEBUG} ]] && set -x

readonly GIMME_VERSION="v1.5.4"
readonly GIMME_COPYRIGHT="Copyright (c) 2015-2020 gimme contributors"
readonly GIMME_LICENSE_URL="https://raw.githubusercontent.com/travis-ci/gimme/${GIMME_VERSION}/LICENSE"
export GIMME_VERSION
export GIMME_COPYRIGHT
export GIMME_LICENSE_URL

program_name="$(basename "$0")"
# shellcheck disable=SC1117
warn() { printf >&2 "%s: %s\n" "${program_name}" "${*}"; }
die() {
	warn "$@"
	exit 1
}

# We don't want to go around hitting Google's servers with requests for
# files named HEAD@{date}.tar so we only try binary/source downloads if
# it looks like a plausible name to us.
# We don't need to support 0. releases of Go.
# We don't support 5 digit major-versions of Go (limit back-tracking in RE).
# We don't support very long versions
#   (both to avoid annoying download server operators with attacks and
#    because regexp backtracking can be pathological).
# Per _assert_version_given we do assume 2.0 not 2
ALLOWED_UPSTREAM_VERSION_RE='^[1-9][0-9]{0,3}(\.[0-9][0-9a-zA-Z_-]{0,9})+$'
#
# The main path which allowed these to leak upstream before has been closed
# but a valid git repo tag or branch-name will still reach the point of
# being _tried_ upstream.

# _do_curl "url" "file"
_do_curl() {
	mkdir -p "$(dirname "${2}")"

	if command -v curl >/dev/null; then
		curl -sSLf "${1}" -o "${2}" 2>/dev/null
		return
	fi

	if command -v wget >/dev/null; then
		wget -q "${1}" -O "${2}" 2>/dev/null
		return
	fi

	if command -v fetch >/dev/null; then
		fetch -q "${1}" -o "${2}" 2>/dev/null
		return
	fi

	echo >&2 'error: no curl, wget, or fetch found'
	exit 1
}

# _sha256sum "file"
_sha256sum() {
	if command -v sha256sum &>/dev/null; then
		sha256sum "$@"
	elif command -v gsha256sum &>/dev/null; then
		gsha256sum "$@"
	else
		shasum -a 256 "$@"
	fi
}

# sort versions, handling 1.10 after 1.9, not before 1.2
# FreeBSD sort has --version-sort, none of the others do
# Looks like --general-numeric-sort is the safest; checked macOS 10.12.6, FreeBSD 10.3, Ubuntu Trusty
if sort --version-sort </dev/null &>/dev/null; then
	_version_sort() { sort --version-sort; }
else
	_version_sort() {
		# If we go to four-digit minor or patch versions, then extend the padding here
		# (but in such a world, perhaps --version-sort will have become standard by then?)
		sed -E 's/\.([0-9](\.|$))/.00\1/g; s/\.([0-9][0-9](\.|$))/.0\1/g' |
			sort --general-numeric-sort |
			sed 's/\.00*/./g'
	}
fi

# _do_curls "file" "url" ["url"...]
_do_curls() {
	f="${1}"
	shift
	if _sha256sum -c "${f}.sha256" &>/dev/null; then
		return 0
	fi
	for url in "${@}"; do
		if _do_curl "${url}" "${f}"; then
			if _do_curl "${url}.sha256" "${f}.sha256"; then
				echo "$(cat "${f}.sha256")  ${f}" >"${f}.sha256.tmp"
				mv "${f}.sha256.tmp" "${f}.sha256"
				if ! _sha256sum -c "${f}.sha256" &>/dev/null; then
					warn "sha256sum failed for '${f}'"
					warn 'continuing to next candidate URL'
					continue
				fi
			fi
			return
		fi
	done
	rm -f "${f}"
	return 1
}

# _binary "version" "file.tar.gz" "arch"
_binary() {
	local version=${1}
	local file=${2}
	local arch=${3}
	urls=(
		"${GIMME_DOWNLOAD_BASE}/go${version}.${GIMME_OS}-${arch}.tar.gz"
	)
	if [[ "${GIMME_OS}" == 'darwin' && "${GIMME_BINARY_OSX}" ]]; then
		urls=(
			"${GIMME_DOWNLOAD_BASE}/go${version}.${GIMME_OS}-${arch}-${GIMME_BINARY_OSX}.tar.gz"
			"${urls[@]}"
		)
	fi
	if [ "${arch}" = 'arm' ]; then
		# attempt "armv6l" vs just "arm" first (since that's what's officially published)
		urls=(
			"${GIMME_DOWNLOAD_BASE}/go${version}.${GIMME_OS}-${arch}v6l.tar.gz" # go1.6beta2 & go1.6rc1
			"${GIMME_DOWNLOAD_BASE}/go${version}.${GIMME_OS}-${arch}6.tar.gz" # go1.6beta1
			"${urls[@]}"
		)
	fi
	if [ "${GIMME_OS}" = 'windows' ]; then
		urls=(
			"${GIMME_DOWNLOAD_BASE}/go${version}.${GIMME_OS}-${arch}.zip"
		)
	fi
	_do_curls "${file}" "${urls[@]}"
}

# _source "version" "file.src.tar.gz"
_source() {
	urls=(
		"${GIMME_DOWNLOAD_BASE}/go${1}.src.tar.gz"
		"https://github.com/golang/go/archive/go${1}.tar.gz"
	)
	_do_curls "${2}" "${urls[@]}"
}

# _fetch "dir"
_fetch() {
	mkdir -p "$(dirname "${1}")"

	if [[ -d "${1}/.git" ]]; then
		(
			cd "${1}"
			git remote set-url origin "${GIMME_GO_GIT_REMOTE}"
			git fetch -q --all && git fetch -q --tags
		)
		return
	fi

	git clone -q "${GIMME_GO_GIT_REMOTE}" "${1}"
}

# _checkout "version" "dir"
# NB: might emit a "renamed version" on stdout
_checkout() {
	local spec="${1:?}" godir="${2:?}"
	# We are called twice, once during validation that a version was given and
	# later during build.  We don't want to fetch twice, so we are fetching
	# during the validation only, in the caller.

	if [[ "${spec}" =~ ^[0-9a-f]{6,}$ ]]; then
		# We always treat this as a commit sha, whether instead of doing
		# branch tests etc.  It looks like a commit sha and the Go maintainers
		# aren't daft enough to use pure hex for a tag or branch.
		git -C "$godir" reset -q --hard "${spec}" || return 1
		return 0
	fi

	# If spec looks like HEAD^{something} or HEAD^^^ then trying
	# origin/$spec would succeed but we'd write junk to the filesystem,
	# propagating annoying characters out.
	local retval probe_named disallow rev

	probe_named=1
	disallow='[@^~:{}]'
	if [[ "${spec}" =~ $disallow ]]; then
		probe_named=0
		[[ "${spec}" != "@" ]] || spec="HEAD"
	fi

	try_spec() { git -C "${godir}" reset -q --hard "$@" -- 2>/dev/null; }

	retval=1
	if ((probe_named)); then
		retval=0
		try_spec "origin/${spec}" ||
			try_spec "origin/go${spec}" ||
			{ [[ "${spec}" == "tip" ]] && try_spec origin/master; } ||
			try_spec "refs/tags/${spec}" ||
			try_spec "refs/tags/go${spec}" ||
			retval=1
	fi

	if ((retval)); then
		retval=0
		# We're about to reset anyway, if we succeed, so we should reset to a
		# known state before parsing what might be relative specs
		try_spec origin/master &&
			rev="$(git -C "${godir}" rev-parse --verify -q "${spec}^{object}")" &&
			try_spec "${rev}" &&
			git -C "${godir}" rev-parse --verify -q --short=12 "${rev}" ||
			retval=1
		# that rev-parse prints to stdout, so we can affect the version seen
	fi

	unset -f try_spec
	return $retval
}

# _extract "file.tar.gz" "dir"
_extract() {
	mkdir -p "${2}"

	if [[ "${1}" == *.tar.gz ]]; then
		tar -xf "${1}" -C "${2}" --strip-components 1
	else
		unzip -q "${1}" -d "${2}"
		mv "${2}"/go/* "${2}"
		rmdir "${2}"/go
	fi
}

# _setup_bootstrap
_setup_bootstrap() {
	local versions=("1.18" "1.17" "1.16" "1.15" "1.14" "1.13" "1.12" "1.11" "1.10" "1.9" "1.8" "1.7" "1.6" "1.5" "1.4")

	# try existing
	for v in "${versions[@]}"; do
		for candidate in "${GIMME_ENV_PREFIX}/go${v}"*".env"; do
			if [ -s "${candidate}" ]; then
				# shellcheck source=/dev/null
				GOROOT_BOOTSTRAP="$(source "${candidate}" 2>/dev/null && go env GOROOT)"
				export GOROOT_BOOTSTRAP
				return 0
			fi
		done
	done

	# try binary
	for v in "${versions[@]}"; do
		if [ -n "$(_try_binary "${v}" "${GIMME_HOSTARCH}")" ]; then
			export GOROOT_BOOTSTRAP="${GIMME_VERSION_PREFIX}/go${v}.${GIMME_OS}.${GIMME_HOSTARCH}"
			return 0
		fi
	done

	echo >&2 "Unable to setup go bootstrap from existing or binary"
	return 1
}

# _compile "dir"
_compile() {
	(
		if grep -q GOROOT_BOOTSTRAP "${1}/src/make.bash" &>/dev/null; then
			_setup_bootstrap || return 1
		fi
		cd "${1}"
		if [[ -d .git ]]; then
			git clean -dfx -q
		fi
		cd src
		export GOOS="${GIMME_OS}" GOARCH="${GIMME_ARCH}"
		export CGO_ENABLED="${GIMME_CGO_ENABLED}"
		export CC_FOR_TARGET="${GIMME_CC_FOR_TARGET}"

		local make_log="${1}/make.${GOOS}.${GOARCH}.log"
		if [[ "${GIMME_DEBUG}" -ge "2" ]]; then
			./make.bash -v 2>&1 | tee "${make_log}" 1>&2 || return 1
		else
			./make.bash &>"${make_log}" || return 1
		fi
	)
}

_try_install_race() {
	if [[ ! "${GIMME_INSTALL_RACE}" ]]; then
		return 0
	fi
	"${1}/bin/go" install -race std
}

_can_compile() {
	cat >"${GIMME_TMP}/test.go" <<'EOF'
package main
import "os"
func main() {
	os.Exit(0)
}
EOF
	"${1}/bin/go" run "${GIMME_TMP}/test.go"
}

# _env "dir"
_env() {
	[[ -d "${1}/bin" && -x "${1}/bin/go" ]] || return 1

	# if we try to run a Darwin binary on Linux, we need to fail so 'auto' can fallback to cross-compiling from source
	# automatically
	GOROOT="${1}" GOFLAGS="" "${1}/bin/go" version &>/dev/null || return 1

	# https://twitter.com/davecheney/status/431581286918934528
	# we have to GOROOT sometimes because we use official release binaries in unofficial locations :(
	#
	# Issue 87 leads to:
	#   No, we should _always_ set GOROOT when using official release binaries, and sanest to just always set it.
	#   The "avoid setting it" is _only_ for people using official releases in official locations.
	#   Tools like `gimme` are the reason that GOROOT-in-env exists.

	echo
	if [[ "$(GOROOT="${1}" "${1}/bin/go" env GOHOSTOS)" == "${GIMME_OS}" ]]; then
		echo 'unset GOOS;'
	else
		echo 'export GOOS="'"${GIMME_OS}"'";'
	fi
	if [[ "$(GOROOT="${1}" "${1}/bin/go" env GOHOSTARCH)" == "${GIMME_ARCH}" ]]; then
		echo 'unset GOARCH;'
	else
		echo 'export GOARCH="'"${GIMME_ARCH}"'";'
	fi

	echo "export GOROOT='${1}';"

	# shellcheck disable=SC2016
	echo 'export PATH="'"${1}/bin"':${PATH}";'
	if [[ -z "${GIMME_SILENT_ENV}" ]]; then
		echo 'go version >&2;'
	fi
	echo
}

# _env_alias "dir" "env-file"
_env_alias() {
	if [[ "${GIMME_NO_ENV_ALIAS}" ]]; then
		echo "${2}"
		return
	fi

	if [[ "$(GOROOT="${1}" "${1}/bin/go" env GOHOSTOS)" == "${GIMME_OS}" && "$(GOROOT="${1}" "${1}/bin/go" env GOHOSTARCH)" == "${GIMME_ARCH}" ]]; then
		# GIMME_GO_VERSION might be a branch, which can contain '/'
		local dest="${GIMME_ENV_PREFIX}/go${GIMME_GO_VERSION//\//__}.env"
		cp "${2}" "${dest}"
		ln -sf "${dest}" "${GIMME_ENV_PREFIX}/latest.env"
		echo "${dest}"
	else
		echo "${2}"
	fi
}

_try_existing() {
	case "${1}" in
	binary)
		local existing_ver="${GIMME_VERSION_PREFIX}/go${GIMME_GO_VERSION}.${GIMME_OS}.${GIMME_ARCH}"
		local existing_env="${GIMME_ENV_PREFIX}/go${GIMME_GO_VERSION}.${GIMME_OS}.${GIMME_ARCH}.env"
		;;
	source)
		local existing_ver="${GIMME_VERSION_PREFIX}/go${GIMME_GO_VERSION}.src"
		local existing_env="${GIMME_ENV_PREFIX}/go${GIMME_GO_VERSION}.src.env"
		;;
	*)
		_try_existing binary || _try_existing source
		return $?
		;;
	esac

	if [[ -x "${existing_ver}/bin/go" && -s "${existing_env}" ]]; then
		# newer envs have existing semi-colon at end of line, because newer gimme
		# puts them there; envs created before that change lack those semi-colons
		# and should gain them, to make it easier for people using eval without
		# double-quoting the command substition.
		sed -e 's/\([^;]\)$/\1;/' <"${existing_env}"
		# gimme is the corner-case where GOROOT _should_ be overriden, since if the
		# ancilliary tooling's system-internal DefaultGoroot exists, and GOROOT is
		# unset, then it will be used and the wrong golang will be picked up.
		# Lots of old installs won't have GOROOT; munge it from $PATH
		if grep -qs '^unset GOROOT' -- "${existing_env}"; then
			sed -n -e 's/^export PATH="\(.*\)\/bin:.*$/export GOROOT='"'"'\1'"'"';/p' <"${existing_env}"
			echo
		fi
		# Export the same variables whether building new or using existing
		echo "export GIMME_ENV='${existing_env}';"
		return
	fi

	return 1
}

# _try_binary "version" "arch"
_try_binary() {
	local version=${1}
	local arch=${2}
	local bin_tgz="${GIMME_TMP}/go${version}.${GIMME_OS}.${arch}.tar.gz"
	local bin_dir="${GIMME_VERSION_PREFIX}/go${version}.${GIMME_OS}.${arch}"
	local bin_env="${GIMME_ENV_PREFIX}/go${version}.${GIMME_OS}.${arch}.env"

	[[ "${version}" =~ ${ALLOWED_UPSTREAM_VERSION_RE} ]] || return 1

	if [ "${GIMME_OS}" = 'windows' ]; then
		bin_tgz=${bin_tgz%.tar.gz}.zip
	fi

	_binary "${version}" "${bin_tgz}" "${arch}" || return 1
	_extract "${bin_tgz}" "${bin_dir}" || return 1
	_env "${bin_dir}" | tee "${bin_env}" || return 1
	echo "export GIMME_ENV=\"$(_env_alias "${bin_dir}" "${bin_env}")\""
}

_try_source() {
	local src_tgz="${GIMME_TMP}/go${GIMME_GO_VERSION}.src.tar.gz"
	local src_dir="${GIMME_VERSION_PREFIX}/go${GIMME_GO_VERSION}.src"
	local src_env="${GIMME_ENV_PREFIX}/go${GIMME_GO_VERSION}.src.env"

	[[ "${GIMME_GO_VERSION}" =~ ${ALLOWED_UPSTREAM_VERSION_RE} ]] || return 1

	_source "${GIMME_GO_VERSION}" "${src_tgz}" || return 1
	_extract "${src_tgz}" "${src_dir}" || return 1
	_compile "${src_dir}" || return 1
	_try_install_race "${src_dir}" || return 1
	_env "${src_dir}" | tee "${src_env}" || return 1
	echo "export GIMME_ENV=\"$(_env_alias "${src_dir}" "${src_env}")\""
}

# We do _not_ try to use any version caching with _try_existing(), but instead
# build afresh each time.  We don't want to deal with someone moving the repo
# to other-version, doing an install, then resetting it back to
# last-version-we-saw and thus introducing conflicts.
#
# If you want to re-use a built-at-spec version, then avoid moving the repo
# and source the generated .env manually.
# Note that the env will just refer to the 'go' directory, so it's not safe
# to reuse anyway.
_try_git() {
	local git_dir="${GIMME_VERSION_PREFIX}/go"
	local git_env="${GIMME_ENV_PREFIX}/go.git.${GIMME_OS}.${GIMME_ARCH}.env"
	local resolved_sha

	# Any tags should have been resolved when we asserted that we were
	# given a version, so no need to handle that here.
	_checkout "${GIMME_GO_VERSION}" "${git_dir}" >/dev/null || return 1
	_compile "${git_dir}" || return 1
	_try_install_race "${git_dir}" || return 1
	_env "${git_dir}" | tee "${git_env}" || return 1
	echo "export GIMME_ENV=\"$(_env_alias "${git_dir}" "${git_env}")\""
}

_wipe_version() {
	local env_file="${GIMME_ENV_PREFIX}/go${1}.${GIMME_OS}.${GIMME_ARCH}.env"

	if [[ -s "${env_file}" ]]; then
		rm -rf "$(awk -F\" '/GOROOT/ { print $2 }' "${env_file}")"
		rm -f "${env_file}"
	fi
}

_list_versions() {
	if [ ! -d "${GIMME_VERSION_PREFIX}" ]; then
		return 0
	fi

	local current_version
	current_version="$(go env GOROOT 2>/dev/null)"
	current_version="${current_version##*/go}"
	current_version="${current_version%%.${GIMME_OS}.*}"

	# 1.1 1.10 1.2 is bad; zsh has `setopt numeric_glob_sort` but bash
	# doesn't appear to have anything like that.
	for d in "${GIMME_VERSION_PREFIX}/go"*".${GIMME_OS}."*; do
		local cleaned="${d##*/go}"
		cleaned="${cleaned%%.${GIMME_OS}.*}"
		echo "${cleaned}"
	done | _version_sort | while read -r cleaned; do
		echo -en "${cleaned}"
		if [[ "${cleaned}" == "${current_version}" ]]; then
			echo -en ' <= current' >&2
		fi
		echo
	done
}

_update_remote_known_list_if_needed() {
	# shellcheck disable=SC1117
	local exp="go([[:alnum:]\.]*)\.src.*" # :alnum: catches beta versions too
	local list="${GIMME_VERSION_PREFIX}/known-versions.txt"
	local dlfile="${GIMME_TMP}/known-dl"

	if [[ -e "${list}" ]] &&
		! ((force_known_update)) &&
		! _file_older_than_secs "${list}" "${GIMME_KNOWN_CACHE_MAX}"; then
		echo "${list}"
		return 0
	fi

	[[ -d "${GIMME_VERSION_PREFIX:?}" ]] || mkdir -p -- "${GIMME_VERSION_PREFIX}"

	_do_curl "${GIMME_LIST_KNOWN}" "${dlfile}"

	while read -r line; do
		if [[ "${line}" =~ ${exp} ]]; then
			echo "${BASH_REMATCH[1]}"
		fi
	done <"${dlfile}" | _version_sort | uniq >"${list}.new"
	rm -f "${list}" &>/dev/null
	mv "${list}.new" "${list}"

	rm -f "${dlfile}"
	echo "${list}"
	return 0
}

_list_known() {
	local knownfile
	knownfile="$(_update_remote_known_list_if_needed)"

	(
		_list_versions 2>/dev/null
		cat -- "${knownfile}"
	) | grep . | _version_sort | uniq
}

# For the "invoked on commandline" case, we want to always pass unknown
# strings through, so that we can be a uniqueness filter, but for unknown
# names we want to exit with a value other than 1, so we document that
# we'll exit 2.  For use by other functions, 2 is as good as 1.
_resolve_version() {
	case "${1}" in
	stable)
		_get_curr_stable
		return 0
		;;
	oldstable)
		_get_old_stable
		return 0
		;;
	tip)
		echo "tip"
		return 0
		;;
	*.x)
		true
		;;
	*)
		echo "${1}"
		local GIMME_GO_VERSION="$1"
		local ASSERT_ABORT='return'
		if _assert_version_given 2>/dev/null; then
			return 0
		fi
		warn "version specifier '${1}' unknown"
		return 2
		;;
	esac
	# We have a .x suffix
	local base="${1%.x}"
	local ver last='' known
	known="$(_update_remote_known_list_if_needed)" # will be version-sorted
	if [[ ! "${base}" =~ ^[0-9.]+$ ]]; then
		warn "resolve pattern '${base}.x' invalid for .x finding"
		return 2
	fi
	# The `.x` is optional; "1.10" matches "1.10.x"
	local search="^${base//./\\.}(\\.[0-9.]+)?\$"
	# avoid regexp attacks
	while read -r ver; do
		[[ "${ver}" =~ $search ]] || continue
		last="${ver}"
	done <"$known"
	if [[ -n "${last}" ]]; then
		echo "${last}"
		return 0
	fi
	echo "${1}"
	warn "given '${1}' but no release for '${base}' found"
	return 2
}

_realpath() {
	# shellcheck disable=SC2005
	[ -d "$1" ] && echo "$(cd "$1" && pwd)" || echo "$(cd "$(dirname "$1")" && pwd)/$(basename "$1")"
}

_get_curr_stable() {
	local stable="${GIMME_VERSION_PREFIX}/stable"

	if _file_older_than_secs "${stable}" 86400; then
		_update_stable "${stable}"
	fi

	cat "${stable}"
}

_get_old_stable() {
	local oldstable="${GIMME_VERSION_PREFIX}/oldstable"

	if _file_older_than_secs "${oldstable}" 86400; then
		_update_oldstable "${oldstable}"
	fi

	cat "${oldstable}"
}

_update_stable() {
	local stable="${1}"
	local url="https://golang.org/VERSION?m=text"

	_do_curl "${url}" "${stable}"
	sed -i.old -e 's/^go\(.*\)/\1/' "${stable}"
	rm -f "${stable}.old"
}

_update_oldstable() {
	local oldstable="${1}"
	local oldstable_x
	oldstable_x=$(_get_curr_stable | awk -F. '{
		$2--;
		print $1 "." $2 "." "x"
	}')
	_resolve_version "${oldstable_x}" >"${oldstable}"
}

_last_mod_timestamp() {
	local filename="${1}"
	case "${GIMME_HOSTOS}" in
	darwin | *bsd)
		stat -f %m "${filename}"
		;;
	linux)
		stat -c %Y "${filename}"
		;;
	esac
}

_file_older_than_secs() {
	local file="${1}"
	local age_secs="${2}"
	local ts
	# if the file does not exist, we return true, as the cache needs updating
	ts="$(_last_mod_timestamp "${file}" 2>/dev/null)" || return 0
	((($(date +%s) - ts) > age_secs))
}

_assert_version_given() {
	# By the time we're called, aliases such as "stable" must have been resolved
	# but we could be a reference in git.
	#
	# Versions can include suffices such as in "1.8beta2", so our assumption is that
	# there will always be a minor present; the first public release was "1.0" so
	# we assume "2.0" not "2".

	if [[ -z "${GIMME_GO_VERSION}" ]]; then
		echo >&2 'error: no GIMME_GO_VERSION supplied'
		echo >&2 "  ex: GIMME_GO_VERSION=1.4.1 ${0} ${*}"
		echo >&2 "  ex: ${0} 1.4.1 ${*}"
		${ASSERT_ABORT:-exit} 1
	fi

	# Note: _resolve_version calls back to us (_assert_version_given), but
	# only for cases where the version does not end with .x, so this should
	# be safe.
	# This should be untangled.  PRs accepted, good starter project.
	if [[ "${GIMME_GO_VERSION}" == *.x ]]; then
		GIMME_GO_VERSION="$(_resolve_version "${GIMME_GO_VERSION}")" || ${ASSERT_ABORT:-exit} 1
	fi

	if [[ "${GIMME_GO_VERSION}" == +([[:digit:]]).+([[:digit:]])* ]]; then
		return 0
	fi

	# Here we resolve symbolic references.  If we don't, then we get some
	# random git tag name being accepted as valid and then we try to
	# curl garbage from upstream.
	if [[ "${GIMME_TYPE}" == "auto" || "${GIMME_TYPE}" == "git" ]]; then
		local git_dir="${GIMME_VERSION_PREFIX}/go"
		local resolved_sha
		_fetch "${git_dir}"
		if resolved_sha="$(_checkout "${GIMME_GO_VERSION}" "${git_dir}")"; then
			if [[ -n "${resolved_sha}" ]]; then
				# Break our normal silence, this one really needs to be seen on stderr
				# always; auditability and knowing what version of Go you got wins.
				warn "resolved '${GIMME_GO_VERSION}' to '${resolved_sha}'"
				GIMME_GO_VERSION="${resolved_sha}"
			fi
			return 0
		fi
	fi

	echo >&2 'error: GIMME_GO_VERSION not recognized as valid'
	echo >&2 "  got: ${GIMME_GO_VERSION}"
	${ASSERT_ABORT:-exit} 1
}

_exclude_from_backups() {
	# Please avoid anything which requires elevated privileges or is obnoxious
	# enough to offend the invoker
	case "${GIMME_HOSTOS}" in
	darwin)
		# Darwin: Time Machine is "standard", we can add others.  The default
		# mechanism is sticky, as an attribute on the dir, requires no
		# privileges, is idempotent (and doesn't support -- to end flags).
		tmutil addexclusion "$@"
		;;
	esac
}

_versint() {
	IFS=" " read -r -a args <<<"${1//[^0-9]/ }"
	printf '1%03d%03d%03d%03d' "${args[@]}"
}

_to_goarch() {
	case "${1}" in
	aarch64) echo "arm64" ;;
	*) echo "${1}" ;;
	esac
}

: "${GIMME_OS:=$(uname -s | tr '[:upper:]' '[:lower:]')}"
: "${GIMME_HOSTOS:=$(uname -s | tr '[:upper:]' '[:lower:]')}"
: "${GIMME_ARCH:=$(_to_goarch "$(uname -m)")}"
: "${GIMME_HOSTARCH:=$(_to_goarch "$(uname -m)")}"
: "${GIMME_ENV_PREFIX:=${HOME}/.gimme/envs}"
: "${GIMME_VERSION_PREFIX:=${HOME}/.gimme/versions}"
: "${GIMME_TMP:=${TMPDIR:-/tmp}/gimme}"
: "${GIMME_GO_GIT_REMOTE:=https://github.com/golang/go.git}"
: "${GIMME_TYPE:=auto}" # 'auto', 'binary', 'source', or 'git'
: "${GIMME_BINARY_OSX:=osx10.8}"
: "${GIMME_DOWNLOAD_BASE:=https://dl.google.com/go}"
: "${GIMME_LIST_KNOWN:=https://golang.org/dl}"
: "${GIMME_KNOWN_CACHE_MAX:=10800}"

# The version prefix must be an absolute path
case "${GIMME_VERSION_PREFIX}" in
/*) true ;;
*)
	echo >&2 " Fixing GIMME_VERSION_PREFIX from relative: $GIMME_VERSION_PREFIX"
	GIMME_VERSION_PREFIX="$(pwd)/${GIMME_VERSION_PREFIX}"
	echo >&2 " to: $GIMME_VERSION_PREFIX"
	;;
esac

case "${GIMME_OS}" in mingw* | msys_nt*)
	# Minimalist GNU for Windows
	GIMME_OS='windows'

	if [ "${GIMME_ARCH}" = 'i686' ]; then
		GIMME_ARCH="386"
	else
		GIMME_ARCH="amd64"
	fi
	;;
esac

force_install=0
force_known_update=0

while [[ $# -gt 0 ]]; do
	case "${1}" in
	-h | --help | help | wat)
		_old_ifs="$IFS"
		IFS=';'
		awk '/^#\+  / {
				sub(/^#\+  /, "", $0) ;
				sub(/-$/, "", $0) ;
				print $0
			}' "$0" | while read -r line; do
			eval "echo \"$line\""
		done
		IFS="$_old_ifs"
		exit 0
		;;
	-V | --version | version)
		echo "${GIMME_VERSION}"
		exit 0
		;;
	-r | --resolve | resolve)
		# The normal mkdir of versions is below; we don't want to move it up
		# to where we create files just if asked our version; thus
		# _resolve_version has to mkdir the versions dir itself.
		if [[ $# -ge 2 ]]; then
			_resolve_version "${2}"
		elif [[ -n "${GIMME_GO_VERSION:-}" ]]; then
			_resolve_version "${GIMME_GO_VERSION}"
		else
			die "resolve must be given a version to resolve"
		fi
		exit $?
		;;
	-l | --list | list)
		_list_versions
		exit 0
		;;
	-k | --known | known)
		_list_known
		exit 0
		;;
	-f | --force | force)
		force_install=1
		;;
	--force-known-update | force-known-update)
		force_known_update=1
		;;
	-i | install)
		true # ignore a dummy argument
		;;
	*)
		break
		;;
	esac
	shift
done

if [[ -n "${1}" ]]; then
	GIMME_GO_VERSION="${1}"
fi
if [[ -n "${2}" ]]; then
	GIMME_VERSION_PREFIX="${2}"
fi

case "${GIMME_ARCH}" in
x86_64) GIMME_ARCH=amd64 ;;
x86) GIMME_ARCH=386 ;;
arm64)
	if [[ "${GIMME_GO_VERSION}" != master && "$(_versint "${GIMME_GO_VERSION}")" < "$(_versint 1.5)" ]]; then
		echo >&2 "error: ${GIMME_ARCH} is not supported by this go version"
		echo >&2 "try go1.5 or newer"
		exit 1
	fi
	if [[ "${GIMME_HOSTOS}" == "linux" && "${GIMME_HOSTARCH}" != "${GIMME_ARCH}" ]]; then
		: "${GIMME_CC_FOR_TARGET:="aarch64-linux-gnu-gcc"}"
	fi
	;;
arm*) GIMME_ARCH=arm ;;
esac

case "${GIMME_HOSTARCH}" in
x86_64) GIMME_HOSTARCH=amd64 ;;
x86) GIMME_HOSTARCH=386 ;;
arm64) ;;
arm*) GIMME_HOSTARCH=arm ;;
esac

case "${GIMME_GO_VERSION}" in
stable) GIMME_GO_VERSION=$(_get_curr_stable) ;;
oldstable) GIMME_GO_VERSION=$(_get_old_stable) ;;
esac

_assert_version_given "$@"

((force_install)) && _wipe_version "${GIMME_GO_VERSION}"

unset GOARCH
unset GOBIN
unset GOOS
unset GOPATH
unset GOROOT
unset CGO_ENABLED
unset CC_FOR_TARGET
# GO111MODULE breaks build of Go itself
unset GO111MODULE

mkdir -p "${GIMME_VERSION_PREFIX}" "${GIMME_ENV_PREFIX}"
# The envs dir stays small and provides a record of what had been installed
# whereas the versions dir grows by hundreds of MB per version and is not
# intended to support local modifications (as that subverts the point of gimme)
# _and_ is a cache, so we're unilaterally declaring that the contents of
# the versions dir should be excluded from system backups.
_exclude_from_backups "${GIMME_VERSION_PREFIX}"

GIMME_VERSION_PREFIX="$(_realpath "${GIMME_VERSION_PREFIX}")"
GIMME_ENV_PREFIX="$(_realpath "${GIMME_ENV_PREFIX}")"

if ! case "${GIMME_TYPE}" in
	binary) _try_existing binary || _try_binary "${GIMME_GO_VERSION}" "${GIMME_ARCH}" ;;
	source) _try_existing source || _try_source || _try_git ;;
	git) _try_git ;;
	auto) _try_existing || _try_binary "${GIMME_GO_VERSION}" "${GIMME_ARCH}" || _try_source || _try_git ;;
	*)
		echo >&2 "I don't know how to '${GIMME_TYPE}'."
		echo >&2 "  Try 'auto', 'binary', 'source', or 'git'."
		exit 1
		;;
	esac; then
	echo >&2 "I don't have any idea what to do with '${GIMME_GO_VERSION}'."
	echo >&2 "  (using download type '${GIMME_TYPE}')"
	exit 1
fi