Fix 1993/vanb/README.md
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@@ -20,8 +20,7 @@ right parameters. Thank you Cody for your assistance!
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./vanb 'exp'
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where:
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'exp' an octal expression
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where 'exp' is an octal expression
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## Try:
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@@ -50,49 +49,47 @@ expression, otherwise it might become hard to understand. :-)
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## Author's comments
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The program does no error checking - erroneous expressions will
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produce spurious results. Note that unary '-' is an operator. Thus,
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decimal -46 would be entered as -d46 and not d-46.
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The program does no error checking - erroneous expressions will
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produce spurious results. Note that unary '-' is an operator. Thus,
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decimal -46 would be entered as -d46 and not d-46.
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What Makes This Program Special:
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What Makes This Program Special:
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0) Of course, the fact that the program takes input and produces output
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in octal, rather than a more useful system like decimal, makes
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it "special".
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0. Of course, the fact that the program takes input and produces output in
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octal, rather than a more useful system like decimal, makes it "special".
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1) The entire program consists of a single expression, returned from main.
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1. The entire program consists of a single expression, returned from main.
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2) There are no global or local variables other than the parameters
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to main
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2. There are no global or local variables other than the parameters to main.
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3) All of the constants in the program are expressed in octal - including
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the printf string.
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3. All of the constants in the program are expressed in octal - including the
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printf string.
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4) The variables are given names which cause visual confusion with the
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octal constants.
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4. The variables are given names which cause visual confusion with the octal
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constants.
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5) The program is formatted like a roman numeral 8. The significance of
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8 is obvious. I chose roman numerals because it is a numbering
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system even more arcane than octal. It's not very 'deceptive',
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but it's difficult to deceptively format a program which has no
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explicit control structures (other than ?:'s).
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5. The program is formatted like a roman numeral 8. The significance of 8 is
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obvious. I chose roman numerals because it is a numbering system even more
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arcane than octal. It's not very 'deceptive', but it's difficult to deceptively
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format a program which has no explicit control structures (other than ?:'s).
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6) Many simple tasks are done many times. I tried to do these differently
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each time, to make the program harder to understand.
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6. Many simple tasks are done many times. I tried to do these differently each
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time, to make the program harder to understand.
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How It Works:
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How It Works:
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It's a recursive descent parser, calling main
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for all of its recursion. O2 is a state variable, telling main
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what grammar non-terminal to parse. **O7 is the next character.
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O3 is an intermediate result. Whenever you see a construct like:
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It's a recursive descent parser, calling main
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for all of its recursion. O2 is a state variable, telling main
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what grammar non-terminal to parse. `**O7` is the next character.
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`O3` is an intermediate result. Whenever you see a construct like:
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!(expression with O2)?
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it's decrementing & testing O2 to see what state it's in. Comparisons
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involving **O7 and octal constants are looking for certain characters.
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it's decrementing & testing O2 to see what state it's in. Comparisons
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involving `**O7` and octal constants are looking for certain characters.
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Here's the grammar ('e' denotes the empty string) :
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Here's the grammar ('e' denotes the empty string) :
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<E> ::= <T><E'>
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<E'> ::= +<T><E'> | -<T><E'> | e
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@@ -104,7 +101,8 @@ expression, otherwise it might become hard to understand. :-)
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<X> ::= [0-9]<X> | [A-F]<X> | [a-f]<X> | e
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<O> ::= [0-7]<O> | e
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Here's how the grammar nonterminals map to octal state numbers:
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Here's how the grammar nonterminals map to octal state numbers:
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<E> is 012
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<E'> is 011
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@@ -116,84 +114,87 @@ expression, otherwise it might become hard to understand. :-)
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<X> is 05
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<O> is 03
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Here's a version of the program before it got formatted into the VIII,
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augmented with comments showing where each state begins. N1 and N2 are
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notes:
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Here's a version of the program before it got formatted into the VIII,
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augmented with comments showing where each state begins. N1 and N2 are
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notes:
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#define O5 main
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O5(O2,O7,O3)char**O7;
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{
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return
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/* N1 */ !(O2+=~01+01) ?
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00:
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/* N2 */ !(O2-=02>01) ?
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printf("\045\157\012",O5(012,O7+01,00)):
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/* O */ !(O2-=02>>01) ?
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(**O7<=067 && **O7>057 ? O5(03,O7,*(*O7)++-060+010*O3):O3):
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/* D */ !(O2-=-O3-~O3) ?
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(072>**O7 && 060<=**O7 ? O5(04,O7,012*O3-060+*(*O7)++):O3):
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/* X */ !(O2-=!O3+!!O3) ?
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(**O7>057 && **O7<=071 ? O5(05,O7,*(*O7)+++O3*020-060):
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**O7<=0106 && 0101<=**O7 ? O5(05,O7,020*O3+*(*O7)++-067):
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0140<**O7 && **O7<0147 ? O5(05,O7,-0127+*(*O7)+++020*O3):O3):
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/* F */ !(O2-=02-01) ?
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(**O7==050 ? 050**++*O7,O5(013,O7,O5(012,O7,00)):
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**O7<056 && 054<**O7 ? 055**++*O7,-O5(06,O7,00):
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054>**O7 && 052<**O7 ? 050**(*O7)++,O5(06,O7,00):
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!(**O7^0170)||!(0130^**O7) ? *++*O7,O5(05,O7,00):
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**O7==0144||**O7==0104 ? ++*O7,O5(04,O7,00):O5(03,O7,00)):
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/* T' */ !--O2 ?
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(**O7==052 ? O5(07,O7,O3*(*++*O7,O5(06,O7,00))):
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!(045-**O7) ? O5(07,O7,O3%(03+(*O7)++,O5(06,O7,00))):
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!(**O7^057) ? O5(07,O7,O3/(03-*++*O7,O5(06,O7,00))):O3):
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/* T */ !(O2+=01-02) ?
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O5(07,O7,O5(06,O7,00)):
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/* E' */ !(O2+=-02/02) ?
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(!(**O7-053)?O5(011,O7,O3+(++*O7,O5(010,O7,00))):
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!(055^**O7) ? O5(011,O7,O3-(03+*(*O7)++,O5(010,O7,00))):O3):
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/* E */ !(O2-=0563&0215) ?
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O5(011,O7,O5(010,O7,00)):
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/* C */ (++*O7,O3);
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}
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Note N1: It should never enter this state, unless the user invokes the
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program with no parameters, in which case it just returns 0.
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Note N2: Since the program is properly invoked with 1 parameter, this is
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the first state it will go into. This state just invokes
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printf, and sends the parser to state 012 (which is <E>).
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The E and T states work like this:
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int e(){ return eprime( t() ); }
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The E' and T' states work like this:
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int eprime( int intermediate )
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{
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if( ch == '+' )
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#define O5 main
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O5(O2,O7,O3)char**O7;
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{
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ch = nextchar();
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return eprime( intermediate + t() );
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return
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/* N1 */ !(O2+=~01+01) ?
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00:
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/* N2 */ !(O2-=02>01) ?
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printf("\045\157\012",O5(012,O7+01,00)):
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/* O */ !(O2-=02>>01) ?
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(**O7<=067 && **O7>057 ? O5(03,O7,*(*O7)++-060+010*O3):O3):
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/* D */ !(O2-=-O3-~O3) ?
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(072>**O7 && 060<=**O7 ? O5(04,O7,012*O3-060+*(*O7)++):O3):
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/* X */ !(O2-=!O3+!!O3) ?
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(**O7>057 && **O7<=071 ? O5(05,O7,*(*O7)+++O3*020-060):
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**O7<=0106 && 0101<=**O7 ? O5(05,O7,020*O3+*(*O7)++-067):
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0140<**O7 && **O7<0147 ? O5(05,O7,-0127+*(*O7)+++020*O3):O3):
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/* F */ !(O2-=02-01) ?
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(**O7==050 ? 050**++*O7,O5(013,O7,O5(012,O7,00)):
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**O7<056 && 054<**O7 ? 055**++*O7,-O5(06,O7,00):
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054>**O7 && 052<**O7 ? 050**(*O7)++,O5(06,O7,00):
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!(**O7^0170)||!(0130^**O7) ? *++*O7,O5(05,O7,00):
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**O7==0144||**O7==0104 ? ++*O7,O5(04,O7,00):O5(03,O7,00)):
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/* T' */ !--O2 ?
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(**O7==052 ? O5(07,O7,O3*(*++*O7,O5(06,O7,00))):
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!(045-**O7) ? O5(07,O7,O3%(03+(*O7)++,O5(06,O7,00))):
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!(**O7^057) ? O5(07,O7,O3/(03-*++*O7,O5(06,O7,00))):O3):
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/* T */ !(O2+=01-02) ?
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O5(07,O7,O5(06,O7,00)):
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/* E' */ !(O2+=-02/02) ?
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(!(**O7-053)?O5(011,O7,O3+(++*O7,O5(010,O7,00))):
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!(055^**O7) ? O5(011,O7,O3-(03+*(*O7)++,O5(010,O7,00))):O3):
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/* E */ !(O2-=0563&0215) ?
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O5(011,O7,O5(010,O7,00)):
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/* C */ (++*O7,O3);
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}
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else return intermediate;
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}
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The D, X and O states work like this (they assume that they're initially
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called with 0):
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Note N1: It should never enter this state, unless the user invokes the
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program with no parameters, in which case it just returns 0.
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int octal( int intermediate )
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{
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if( ch>='0' && ch<='7' )
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Note N2: Since the program is properly invoked with 1 parameter, this is
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the first state it will go into. This state just invokes
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printf, and sends the parser to state 012 (which is `<E>`).
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The E and T states work like this:
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int e(){ return eprime( t() ); }
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The E' and T' states work like this:
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int eprime( int intermediate )
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{
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intermediate = intermediate * 8 + ch - '0';
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ch = nextchar();
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return octal( intermediate );
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if( ch == '+' )
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{
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ch = nextchar();
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return eprime( intermediate + t() );
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}
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else return intermediate;
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}
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else return intermediate;
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}
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F and C work similarly.
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The D, X and O states work like this (they assume that they're initially
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called with 0):
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int octal( int intermediate )
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{
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if( ch>='0' && ch<='7' )
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{
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intermediate = intermediate * 8 + ch - '0';
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ch = nextchar();
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return octal( intermediate );
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}
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else return intermediate;
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}
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F and C work similarly.
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## Copyright and CC BY-SA 4.0 License:
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