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Difference between revisions of "Howto:Serializing Nasal data structures"

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(Created page with "{{Stub}} == Objective == == Background == Nasal's hash syntax is already close enough to JSON that Nasal could even serialie a property tree without requiring much work, ana...")
 
m (References)
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debug.dump(result);
 
debug.dump(result);
  
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</syntaxhighlight>
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 +
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== Finally ==
 +
the original Nasal example, already contains a helper function to serialie a Nasal type:<ref>{{cite web
 +
  |url    =  https://forum.flightgear.org/viewtopic.php?p=257123#p257123
 +
  |title  =  <nowiki> Re: String manipulation and file I/O (pm2thread) </nowiki>
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  |author =  <nowiki> Hooray </nowiki>
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  |date  =  Sep 12th, 2015
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  |added  =  Sep 12th, 2015
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  |script_version = 0.40
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  }}</ref>
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http://plausible.org/nasal/sample.nas
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<syntaxhighlight lang="nasal">##
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## A rockin' metaprogramming hack.  Generates and returns a deep copy
 +
## of the object in valid Nasal syntax.  A warning to those who might
 +
## want to use this: it ignores function objects (which cannot be
 +
## inspected from Nasal) and replaces them with nil references.  It
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## also makes no attempt to escape special characters in strings, which
 +
## can break re-parsing in strange (and possibly insecure!) ways.
 +
##
 +
dump = func(o) {
 +
    result = "";
 +
    if(typeof(o) == "scalar") {
 +
        n = num(o);
 +
        if(n == nil) { result = result ~ '"' ~ o ~ '"'; }
 +
        else { result = result ~ o; }
 +
    } elsif(typeof(o) == "vector") {
 +
        result = result ~ "[ ";
 +
        if(size(o) > 0) { result = result ~ dump(o[0]); }
 +
        for(i=1; i<size(o); i=i+1) {
 +
            result = result ~ ", " ~ dump(o[i]);
 +
        }
 +
        result = result ~ " ]";
 +
    } elsif(typeof(o) == "hash") {
 +
        ks = keys(o);
 +
        result = result ~ "{ ";
 +
        if(size(o) > 0) {
 +
            k = ks[0];
 +
            result = result ~ k ~ ":" ~ dump(o[k]);
 +
        }
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        for(i=1; i<size(o); i=i+1) {
 +
            k = ks[i];
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            result = result ~ ", " ~ k ~ " : " ~ dump(o[k]);
 +
        }
 +
        result = result ~ " }";
 +
    } else {
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        result = result ~ "nil";
 +
    }
 +
    return result;
 +
}
 
</syntaxhighlight>
 
</syntaxhighlight>
  

Revision as of 16:16, 12 August 2017

This article is a stub. You can help the wiki by expanding it.

Objective

Background

Nasal's hash syntax is already close enough to JSON that Nasal could even serialie a property tree without requiring much work, analogous to props.getValues() which is already traversing a property tree node and which turns it into a Nasal data structure (hash).

[1]


Another possibility would be using a subset of JSON, i.e. to serialie/unserialize Nasal data structures as scalars, vectors and hashes - which should be pretty straightforward. The format would then be plain text, and it would just be evaluated via Nasal's compile()/call() APIs to turn data structures back into Nasal types.[2]

you could then also directly use io.load_nasal() to load a Nasal file from disk (think mydata.nas) and for serialiation purposes, write to the same file - just make sure that it is valid Nasal[3]


depending on how much can be saved using a space-time or time-memory tradeoff, you could compute lookup tables using Nasal vectors/hashes (or both) and serialie those to disk - e.g. using JSON, which is basically equivalent to the syntax used by Nasal. You would basically compute your lookup tables once and then use a nested for-loop to read the lookup tables and serialize/write those to a custom file in $FG_HOME/Export, at that point you can then copy the whole thing into a folder where it can be loaded at runtime using the helpers available in io.nas (using JSON would be much less verbose than using PropertyList/XML, but obviously not as efficient as using a binary serialization format). Basically, this would be equivalent to programmatically creating a complex texture and storing that on disk to save reduce the runtime overhead (treating the texture as a multi-dimensional lookup table). https://en.wikipedia.org/wiki/Space%E2%80%93time_tradeoff

https://en.wikipedia.org/wiki/JSON[4]


Proof of concept

Nasal Console

var serializeVector = func(vector) {
retVal = "[";
foreach(var item; vector) {
retVal ~= serialize(item) ~ ',';
} 

return retVal~"]";
}

var serializeHash = func(hash) {
var retVal = "{";
foreach(var member; keys(hash)) {
retVal ~= serialize(member) ~ ',';

}

return retVal ~"}";
}


var serialize = func (data) {
var type = typeof(data);
if (type == "scalar") return data;
if (type == "vector") return serializeVector(data);
if (type == "hash") return serializeHash(data);
if (type == "ghost" or type == "func") return "";
die("cannot serialize unsupported data type:", type);
} 

var myVector = [];

for (var i=0;i<=40;i+=1) {
append(myVector,[i, i*i]);
}



# debug.dump(myVector);

var serialized = serialize(myVector);


# debug.dump(serialized);


var compiled = compile(serialized);
var result = compiled();
debug.dump(result);


Finally

the original Nasal example, already contains a helper function to serialie a Nasal type:[5]

http://plausible.org/nasal/sample.nas

##
## A rockin' metaprogramming hack.  Generates and returns a deep copy
## of the object in valid Nasal syntax.  A warning to those who might
## want to use this: it ignores function objects (which cannot be
## inspected from Nasal) and replaces them with nil references.  It
## also makes no attempt to escape special characters in strings, which
## can break re-parsing in strange (and possibly insecure!) ways.
##
dump = func(o) {
    result = "";
    if(typeof(o) == "scalar") {
        n = num(o);
        if(n == nil) { result = result ~ '"' ~ o ~ '"'; }
        else { result = result ~ o; }
    } elsif(typeof(o) == "vector") {
        result = result ~ "[ ";
        if(size(o) > 0) { result = result ~ dump(o[0]); }
        for(i=1; i<size(o); i=i+1) {
            result = result ~ ", " ~ dump(o[i]);
        }
        result = result ~ " ]";
    } elsif(typeof(o) == "hash") {
        ks = keys(o);
        result = result ~ "{ ";
        if(size(o) > 0) {
            k = ks[0];
            result = result ~ k ~ ":" ~ dump(o[k]);
        }
        for(i=1; i<size(o); i=i+1) {
            k = ks[i];
            result = result ~ ", " ~ k ~ " : " ~ dump(o[k]);
        }
        result = result ~ " }";
    } else {
        result = result ~ "nil";
    }
    return result;
}


References

References
  1. Hooray  (Oct 9th, 2016).  Re: Why not use Erlang instead of .
  2. Hooray  (Sep 12th, 2015).  Re: String manipulation and file I/O (pm2thread) .
  3. Hooray  (Sep 12th, 2015).  Re: String manipulation and file I/O (pm2thread) .
  4. Hooray  (Aug 12th, 2017).  Re: Space Shuttle .
  5. Hooray  (Sep 12th, 2015).  Re: String manipulation and file I/O (pm2thread) .