2019-03-17 14:40:57 +02:00
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< ul class = "crumbs" > < li > < a href = "../webs.html" > ★ < / a > < / li > < li > < a href = "index.html" > inblorb 4< / a > < / li > < li > < a href = "index.html#3" > Chapter 3: Other Material< / a > < / li > < li > < b > Base64< / b > < / li > < / ul > < p class = "purpose" > To produce base64-encoded story files ready for in-browser play by a Javascript-based interpreter such as Parchment.< / p >
< p class = "inwebparagraph" > < a id = "SP1" > < / a > < b > § 1. < / b > The Base64 encoding scheme is defined by the Internet standard RFC 1113.
Broadly, the idea is to take a binary stream of bytes, break it into
threes, and then convert this into a sequence of four emailable
characters. To encode 24 bits in four characters, we need six bits per
character, so we need 2^6 = 64 characters in all. Since 64 = 26 +
26 + 10 + 2, we can nearly get there with alphanumeric characters alone,
adding just two others — conventionally, plus and forward-slash.
That's more or less the whole thing, except that we use an equals sign
to indicate incompleteness of the final triplet (which might have only
1 or 2 bytes in it).
< / p >
< p class = "inwebparagraph" > RFC 1113 permits white space to be used freely, including in particular
line breaks, but we don't avail ourselves.
< / p >
< pre class = "display" >
< span class = "reserved" > char< / span > < span class = "plain" > *< / span > < span class = "identifier" > RFC1113_table< / span > < span class = "plain" > = < / span > < span class = "string" > "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="< / span > < span class = "plain" > ;< / span >
< / pre >
< p class = "inwebparagraph" > < / p >
< p class = "inwebparagraph" > < a id = "SP2" > < / a > < b > § 2. < / b > The encoding routine is as follows.
< / p >
< pre class = "display" >
< span class = "reserved" > void< / span > < span class = "plain" > < / span > < span class = "functiontext" > Base64::encode< / span > < span class = "plain" > (< / span > < span class = "reserved" > filename< / span > < span class = "plain" > *< / span > < span class = "identifier" > in_filename< / span > < span class = "plain" > , < / span > < span class = "reserved" > filename< / span > < span class = "plain" > *< / span > < span class = "identifier" > out_filename< / span > < span class = "plain" > , < / span > < span class = "reserved" > text_stream< / span > < span class = "plain" > *< / span > < span class = "identifier" > top< / span > < span class = "plain" > , < / span > < span class = "reserved" > text_stream< / span > < span class = "plain" > *< / span > < span class = "identifier" > tail< / span > < span class = "plain" > ) {< / span >
< span class = "reserved" > FILE< / span > < span class = "plain" > *< / span > < span class = "identifier" > IN< / span > < span class = "plain" > = < / span > < span class = "functiontext" > BinaryFiles::open_for_reading< / span > < span class = "plain" > (< / span > < span class = "identifier" > in_filename< / span > < span class = "plain" > );< / span >
< span class = "reserved" > text_stream< / span > < span class = "plain" > < / span > < span class = "identifier" > TO_struct< / span > < span class = "plain" > ;< / span >
< span class = "reserved" > text_stream< / span > < span class = "plain" > *< / span > < span class = "identifier" > TO< / span > < span class = "plain" > = & < / span > < span class = "identifier" > TO_struct< / span > < span class = "plain" > ;< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > STREAM_OPEN_TO_FILE< / span > < span class = "plain" > (< / span > < span class = "identifier" > TO< / span > < span class = "plain" > , < / span > < span class = "identifier" > out_filename< / span > < span class = "plain" > , < / span > < span class = "constant" > UTF8_ENC< / span > < span class = "plain" > ) == < / span > < span class = "constant" > FALSE< / span > < span class = "plain" > )< / span >
< span class = "functiontext" > BlorbErrors::fatal_fs< / span > < span class = "plain" > (< / span > < span class = "string" > "can't open base-64 encoded story file for output"< / span > < span class = "plain" > , < / span > < span class = "identifier" > out_filename< / span > < span class = "plain" > );< / span >
< span class = "functiontext" > Base64::encode_inner< / span > < span class = "plain" > (< / span > < span class = "identifier" > TO< / span > < span class = "plain" > , < / span > < span class = "identifier" > IN< / span > < span class = "plain" > , < / span > < span class = "identifier" > top< / span > < span class = "plain" > , < / span > < span class = "identifier" > tail< / span > < span class = "plain" > );< / span >
< span class = "functiontext" > BinaryFiles::close< / span > < span class = "plain" > (< / span > < span class = "identifier" > IN< / span > < span class = "plain" > );< / span >
< span class = "identifier" > STREAM_CLOSE< / span > < span class = "plain" > (< / span > < span class = "identifier" > TO< / span > < span class = "plain" > );< / span >
< span class = "plain" > }< / span >
< span class = "reserved" > void< / span > < span class = "plain" > < / span > < span class = "functiontext" > Base64::encode_inner< / span > < span class = "plain" > (< / span > < span class = "constant" > OUTPUT_STREAM< / span > < span class = "plain" > , < / span > < span class = "reserved" > FILE< / span > < span class = "plain" > *< / span > < span class = "identifier" > IN< / span > < span class = "plain" > , < / span > < span class = "reserved" > text_stream< / span > < span class = "plain" > *< / span > < span class = "identifier" > top< / span > < span class = "plain" > , < / span > < span class = "reserved" > text_stream< / span > < span class = "plain" > *< / span > < span class = "identifier" > tail< / span > < span class = "plain" > ) {< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > top< / span > < span class = "plain" > ) < / span > < span class = "identifier" > WRITE< / span > < span class = "plain" > (< / span > < span class = "string" > "%S"< / span > < span class = "plain" > , < / span > < span class = "identifier" > top< / span > < span class = "plain" > );< / span >
< span class = "reserved" > while< / span > < span class = "plain" > (< / span > < span class = "constant" > TRUE< / span > < span class = "plain" > ) {< / span >
< span class = "reserved" > int< / span > < span class = "plain" > < / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [3], < / span > < span class = "identifier" > triplet_size< / span > < span class = "plain" > = 0;< / span >
< < span class = "cwebmacro" > Read the triplet of binary bytes, storing 0 to 3 in the size read< / span > < span class = "cwebmacronumber" > 2.1< / span > > < span class = "plain" > ;< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet_size< / span > < span class = "plain" > == 0) < / span > < span class = "reserved" > break< / span > < span class = "plain" > ;< / span >
< span class = "reserved" > int< / span > < span class = "plain" > < / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [4];< / span >
< < span class = "cwebmacro" > Convert triplet to a quartet< / span > < span class = "cwebmacronumber" > 2.2< / span > > < span class = "plain" > ;< / span >
< span class = "reserved" > int< / span > < span class = "plain" > < / span > < span class = "identifier" > i< / span > < span class = "plain" > ; < / span > < span class = "reserved" > for< / span > < span class = "plain" > (< / span > < span class = "identifier" > i< / span > < span class = "plain" > =0; < / span > < span class = "identifier" > i< / span > < span class = "plain" > < 4; < / span > < span class = "identifier" > i< / span > < span class = "plain" > ++) < / span > < span class = "identifier" > PUT< / span > < span class = "plain" > (< / span > < span class = "identifier" > RFC1113_table< / span > < span class = "plain" > [< / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [< / span > < span class = "identifier" > i< / span > < span class = "plain" > ]]);< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet_size< / span > < span class = "plain" > < 3) < / span > < span class = "reserved" > break< / span > < span class = "plain" > ;< / span >
< span class = "plain" > }< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > tail< / span > < span class = "plain" > ) < / span > < span class = "identifier" > WRITE< / span > < span class = "plain" > (< / span > < span class = "string" > "%S"< / span > < span class = "plain" > , < / span > < span class = "identifier" > tail< / span > < span class = "plain" > );< / span >
< span class = "plain" > }< / span >
< / pre >
< p class = "inwebparagraph" > < / p >
< p class = "endnote" > The function Base64::encode is used in 3/rls (< a href = "3-rls.html#SP6_5" > § 6.5< / a > ).< / p >
< p class = "endnote" > The function Base64::encode_inner appears nowhere else.< / p >
< p class = "inwebparagraph" > < a id = "SP2_1" > < / a > < b > § 2.1. < / b > If the file ends in mid-triplet, we pad out with zeros.
< / p >
< p class = "macrodefinition" > < code class = "display" >
< < span class = "cwebmacrodefn" > Read the triplet of binary bytes, storing 0 to 3 in the size read< / span > < span class = "cwebmacronumber" > 2.1< / span > > =
< / code > < / p >
< pre class = "displaydefn" >
< span class = "identifier" > triplet< / span > < span class = "plain" > [0] = < / span > < span class = "identifier" > fgetc< / span > < span class = "plain" > (< / span > < span class = "identifier" > IN< / span > < span class = "plain" > );< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [0] != < / span > < span class = "identifier" > EOF< / span > < span class = "plain" > ) {< / span >
< span class = "identifier" > triplet_size< / span > < span class = "plain" > ++;< / span >
< span class = "identifier" > triplet< / span > < span class = "plain" > [1] = < / span > < span class = "identifier" > fgetc< / span > < span class = "plain" > (< / span > < span class = "identifier" > IN< / span > < span class = "plain" > );< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [1] != < / span > < span class = "identifier" > EOF< / span > < span class = "plain" > ) {< / span >
< span class = "identifier" > triplet_size< / span > < span class = "plain" > ++;< / span >
< span class = "identifier" > triplet< / span > < span class = "plain" > [2] = < / span > < span class = "identifier" > fgetc< / span > < span class = "plain" > (< / span > < span class = "identifier" > IN< / span > < span class = "plain" > );< / span >
< span class = "reserved" > if< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [2] != < / span > < span class = "identifier" > EOF< / span > < span class = "plain" > )< / span >
< span class = "identifier" > triplet_size< / span > < span class = "plain" > ++;< / span >
< span class = "plain" > }< / span >
< span class = "plain" > }< / span >
< span class = "reserved" > int< / span > < span class = "plain" > < / span > < span class = "identifier" > i< / span > < span class = "plain" > ; < / span > < span class = "reserved" > for< / span > < span class = "plain" > (< / span > < span class = "identifier" > i< / span > < span class = "plain" > =< / span > < span class = "identifier" > triplet_size< / span > < span class = "plain" > ; < / span > < span class = "identifier" > i< / span > < span class = "plain" > < 3; < / span > < span class = "identifier" > i< / span > < span class = "plain" > ++) < / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [< / span > < span class = "identifier" > i< / span > < span class = "plain" > ] = 0;< / span >
< / pre >
< p class = "inwebparagraph" > < / p >
< p class = "endnote" > This code is used in < a href = "#SP2" > § 2< / a > .< / p >
< p class = "inwebparagraph" > < a id = "SP2_2" > < / a > < b > § 2.2. < / b > < code class = "display" >
< < span class = "cwebmacrodefn" > Convert triplet to a quartet< / span > < span class = "cwebmacronumber" > 2.2< / span > > =
< / code > < / p >
< pre class = "displaydefn" >
< span class = "reserved" > int< / span > < span class = "plain" > < / span > < span class = "identifier" > i< / span > < span class = "plain" > ; < / span > < span class = "reserved" > for< / span > < span class = "plain" > (< / span > < span class = "identifier" > i< / span > < span class = "plain" > =0; < / span > < span class = "identifier" > i< / span > < span class = "plain" > < 4; < / span > < span class = "identifier" > i< / span > < span class = "plain" > ++) < / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [< / span > < span class = "identifier" > i< / span > < span class = "plain" > ] = 0;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [0] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [0] & 0< / span > < span class = "identifier" > xFC< / span > < span class = "plain" > ) > > 2;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [1] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [0] & 0< / span > < span class = "identifier" > x03< / span > < span class = "plain" > ) < < 4;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [1] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [1] & 0< / span > < span class = "identifier" > xF0< / span > < span class = "plain" > ) > > 4;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [2] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [1] & 0< / span > < span class = "identifier" > x0F< / span > < span class = "plain" > ) < < 2;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [2] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [2] & 0< / span > < span class = "identifier" > xC0< / span > < span class = "plain" > ) > > 6;< / span >
< span class = "identifier" > quartet< / span > < span class = "plain" > [3] += (< / span > < span class = "identifier" > triplet< / span > < span class = "plain" > [2] & 0< / span > < span class = "identifier" > x3F< / span > < span class = "plain" > ) < < 0;< / span >
< span class = "reserved" > switch< / span > < span class = "plain" > (< / span > < span class = "identifier" > triplet_size< / span > < span class = "plain" > ) {< / span >
< span class = "reserved" > case< / span > < span class = "plain" > 1: < / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [2] = 64; < / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [3] = 64; < / span > < span class = "reserved" > break< / span > < span class = "plain" > ;< / span >
< span class = "reserved" > case< / span > < span class = "plain" > 2: < / span > < span class = "identifier" > quartet< / span > < span class = "plain" > [3] = 64; < / span > < span class = "reserved" > break< / span > < span class = "plain" > ;< / span >
< span class = "plain" > }< / span >
< / pre >
< p class = "inwebparagraph" > < / p >
< p class = "endnote" > This code is used in < a href = "#SP2" > § 2< / a > .< / p >
< hr class = "tocbar" >
< ul class = "toc" > < li > < a href = "3-wm.html" > Back to 'Website Maker'< / a > < / li > < li > < i > (This section ends Chapter 3: Other Material.)< / i > < / li > < / ul > < hr class = "tocbar" >
2019-04-22 17:42:10 +03:00
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2019-03-17 14:40:57 +02:00
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