Gentoo Archives: gentoo-dev

From: Steven J Long <slong@××××××××××××××××××.uk>
To: gentoo-dev@l.g.o
Subject: [gentoo-dev] Re: estack_{push,pop}: cool new helpers or over engineering?
Date: Fri, 16 Dec 2011 07:22:30
Message-Id: jcerhf$t8v$1@dough.gmane.org
In Reply to: Re: [gentoo-dev] estack_{push,pop}: cool new helpers or over engineering? by Mike Frysinger
Just to point out that arithmetic context can be more efficient; no bugs, 
except for a /minor/ possibility (second last comment.)

Mike Frysinger wrote:
> --- eutils.eclass 14 Dec 2011 17:36:18 -0000 1.372 > +++ eutils.eclass 14 Dec 2011 23:46:37 -0000 > @@ -100,6 +100,54 @@ esvn_clean() { > find "$@" -type d -name '.svn' -prune -print0 | xargs -0 rm -rf > } > > +# @FUNCTION: estack_push > +# @USAGE: <stack> [items to push] > +# @DESCRIPTION: > +# Push any number of items onto the specified stack. Pick a name that > +# is a valid variable (i.e. stick to alphanumerics), and push as many > +# items as you like onto the stack at once. > +# > +# The following code snippet will echo 5, then 4, then 3, then ... > +# @CODE > +# estack_push mystack 1 2 3 4 5 > +# while estack_pop mystack i ; do > +# echo ${i}
A minor #bash point in passing: although these values of i are safe, for tutorial code, I really would recommend quoting: echo "$i" (or "${i}"). It's better to get people used to quoting by default, and only not quoting iff they need field-splitting on parameter expansions (eg for a variable used for command options.)
> +# done > +# @CODE > +estack_push() { > + [[ $# -eq 0 ]] && die "estack_push: incorrect # of arguments" > + local stack_name="__ESTACK_$1__" ; shift > + eval ${stack_name}+=\( \"\$@\" \) > +}
((..)) is quicker than [[ .. ]] for arithmetic stuff, and usually easier to grok swiftly. (($#)) || die .. is how this would normally be done.
> + > +# @FUNCTION: estack_pop > +# @USAGE: <stack> [variable] > +# @DESCRIPTION: > +# Pop a single item off the specified stack. If a variable is specified, > +# the popped item is stored there. If no more items are available, > return > +# 1, else return 0. See estack_push for more info. > +estack_pop() { > + ( [[ $# -eq 0 ]] || [[ $# -gt 2 ]] ) && die "estack_pop: incorrect
# of arguments" (($# == 0 || $# > 2)) && die.. # does it in one command, with no subshell. [[ $# -eq 0 || $# -gt 2 ]] && die .. would work too, but more slowly. In general if you want to do complex chains without a subshell, you would use: { } && .. instead of: ( ) && .. TBH I would type (($#==0||$#>2)) in bash, though I space in C, where it doesn't affect execution time. But it's not as clear, especially if you're not in a highlighting editor.
> + # We use the fugly __estack_xxx var names to avoid collision with > + # passing back the return value. If we used "local i" and the > + # caller ran `estack_pop ... i`, we'd end up setting the local > + # copy of "i" rather than the caller's copy. The __estack_xxx > + # garbage is preferable to using $1/$2 everywhere as that is a > + # bit harder to read. > + local __estack_name="__ESTACK_$1__" ; shift > + local __estack_retvar=$1 ; shift > + eval local __estack_i=\${#${__estack_name}[@]} > + # Don't warn -- let the caller interpret this as a failure > + # or as normal behavior (akin to `shift`) > + [[ $(( --__estack_i )) -eq -1 ]] && return 1
((--__estack_i == -1)) && ..
> + > + if [[ -n ${__estack_retvar} ]] ; then > + eval ${__estack_retvar}=\"\${${__estack_name}
[${__estack_i}]}\"
> + fi > + eval unset ${__estack_name}[${__estack_i}] > +} > + > # @FUNCTION: eshopts_push > # @USAGE: [options to `set` or `shopt`] > # @DESCRIPTION: > @@ -126,15 +174,14 @@ esvn_clean() { > eshopts_push() { > # have to assume __ESHOPTS_SAVE__ isn't screwed with > # as a `declare -a` here will reset its value > - local i=${#__ESHOPTS_SAVE__[@]} > if [[ $1 == -[su] ]] ; then > - __ESHOPTS_SAVE__[$i]=$(shopt -p) > + estack_push eshopts "$(shopt -p)" > [[ $# -eq 0 ]] && return 0
I'm not sure how this will ever match, given that $1 has been checked above? (($#==1)) && return 0 # if that applies (might be a 'bug'.)
> shopt "$@" || die "eshopts_push: bad options to shopt: $*" > else > - __ESHOPTS_SAVE__[$i]=$- > + estack_push eshopts $- > [[ $# -eq 0 ]] && return 0
(($#)) || return 0
> set "$@" || die "eshopts_push: bad options to set: $*" > fi > } >
HTH, Steve. -- #friendly-coders -- We're friendly, but we're not /that/ friendly ;-)

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