Re: XS Prepending space to an SVs PV

I had to do something similar on my Tie::Array::Packed module:

static char *
my_sv_unchop(pTHX_ SV *sv, STRLEN size) {
    STRLEN len;
    char *pv = SvPV(sv, len);
    IV off = SvOOK(sv) ? SvIVX(sv) : 0;
    if (!size)
        return pv;

    if (off >= size) {
        SvLEN_set(sv, SvLEN(sv) + size);
        SvCUR_set(sv, len + size);
        SvPV_set(sv, pv - size);
        if (off == size)
            SvFLAGS(sv) &= ~SVf_OOK;
        else
            SvIV_set(sv, off - size);
    }
    else if (len + size <= off + SvLEN(sv)) {
        if (off) {
            SvLEN_set(sv, SvLEN(sv) + off);
            SvFLAGS(sv) &= ~SVf_OOK;
        }
        SvCUR_set(sv, len + size);
        SvPV_set(sv, pv - off);
        Move(pv, pv + size - off, len, char);
    }
    else {
        SV *tmp = sv_2mortal(newSV(len + size));
        STRLEN tmp_len;
        char *tmp_pv;
        SvPOK_on(tmp);
        tmp_pv = SvPV(tmp, tmp_len);
        Move(pv, tmp_pv + size, len, char);
        SvCUR_set(tmp, size + len);
        sv_setsv(sv, tmp);
    }
    return SvPVX(sv);
}
[download]

this function inserts size bytes in front of the string efficiently and returns a pointer to the start of the new pv.

You would probably want to modify it so that more bytes than requested are reserved on the string to optimize consecutive insertions.

Note that sv_setsv is used to only copy the string contents once as it steals the SV string memory from the source SV when it is a mortal.

Comment on Re: XS Prepending space to an SVs PV Select or Download Code

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Re^2: XS Prepending space to an SVs PV by salva (Canon) on Apr 26, 2006 at 15:20 UTC
modified to accept an additional argument that indicates how much extra space should be allocated when reallocation of the string memory is required: static char * my_sv_unchop(pTHX_ SV sv, STRLEN size, STRLEN reserve) { STRLEN len; char pv = SvPV(sv, len); IV off = SvOOK(sv) ? SvIVX(sv) : 0; if (!size) return pv; if (off >= size) { SvLEN_set(sv, SvLEN(sv) + size); SvCUR_set(sv, len + size); SvPV_set(sv, pv - size); if (off == size) SvFLAGS(sv) &= ~SVf_OOK; else SvIV_set(sv, off - size); } else { size += reserve; if ((size < reserve) \|\| (len + size < size)) Perl_croak(aTHX_ "panic: memory wrap"); if (len + size <= off + SvLEN(sv)) { SvCUR_set(sv, len + size); SvPV_set(sv, pv - off); Move(pv, pv + size - off, len, char); if (off) { SvLEN_set(sv, SvLEN(sv) + off ); SvFLAGS(sv) &= ~SVf_OOK; } } else { SV tmp = sv_2mortal(newSV(len + size)); char tmp_pv; SvPOK_on(tmp); tmp_pv = SvPV_nolen(tmp); Move(pv, tmp_pv + size, len, char); SvCUR_set(tmp, len + size); sv_setsv(sv, tmp); } if (reserve) sv_chop(sv, SvPVX(sv) + reserve); } return SvPVX(sv); } [download]	[reply] [d/l]
Re^3: XS Prepending space to an SVs PV by BrowserUk (Patriarch) on Apr 27, 2006 at 01:58 UTC
Many thanks (again) for this code, it is very generous of you and so much easier to learn from than the pure reference material of the 'guts and 'api docs. I set about adapting the above sub to my needs which are slightly less demanding that yours. The naming of `sv_chop` is (historical; set in stone; not of your making), slightly confusing with respect to Perl's chop as they operate on different ends of the string. That makes `sv_unchop` even more confusing :) Anyway, whilst reading the docs on `sv_chop`, I noticed `sv_insert` and relating that back to creamygoodness' explanation and demonstration that used substr to prepend to a string, I thought I'd see if I could use that to simplify things a little. What I came up with is this: `if( *a == 9 ) { ## Time to prepend another byte if( SvOOK( n ) ) { ## If we've some reserve left use it SvLEN_set( n, SvLEN( n ) +1 ); SvCUR_set( n, ++l ); SvPV_set( n, --a ); } else { ## else insert 100 more bytes and use sv_chop ## to reserve 99 of them for later char pad[100] = { 0, }; ## Initialise the reserve to all z +eros sv_insert( n, 0, 0, pad, 100 ); sv_chop( n, SvPVX( n ) + 99 ); a = SvPVX( n ); ++l; } } ...` [download] Whether that could be useful to you I don't know, but if you have the time to cast an eye over it and tell me if you see any obvious fopars... thanks again. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^4: XS Prepending space to an SVs PV by salva (Canon) on Apr 27, 2006 at 08:48 UTC
You have to check that there is still space free when the OOK flag is true and update the IV also: `... if (SvOOK( n ) && (SvIV(sv) > 0)) { /* If we've some reserve left use +it */ SvLEN_set( n, SvLEN( n ) +1 ); SvCUR_set( n, ++l ); SvPV_set( n, --a ); SvIVX(sv)--; } ...` [download]	[reply] [d/l]
Re^2: XS Prepending space to an SVs PV by demerphq (Chancellor) on Apr 26, 2006 at 16:18 UTC
I had to do something similar on my Tie::Array::Packed module I dont mind saying I wish you had sent me a mail about this, I would have been very happy to make Tie::Array::PackedC a pureperl backend to Tie::Array::Packed. (I guess some of the things I did in Tie::Array::PackedC didnt really translate over to XS.) As it is im overjoyed that you did Tie::Array::Packed. Its pretty much exactly what I would have written if I had taken the time to implement what I wanted in XS, except probably better. :-) Anyway, I wonder how hard it would be to make a wrapper or something so that yours is used if its available, falling back to mine if its not... --- $world=~s/war/peace/g	[reply]
Re^3: XS Prepending space to an SVs PV by salva (Canon) on Apr 26, 2006 at 21:47 UTC
I dont mind saying I wish you had sent me a mail about this yes, I should have done it, my apologies for being so unpolite! I wonder how hard it would be to make a wrapper or something so that yours is used if its available, falling back to mine if its not... Quite easy I think, as your package implements a superset of the API provided by mine... I will try to do it. update: Tie::Array::Packed::Auto	[reply]