Ein Parser für einfache Ausdrücke in C

#include <stdio.h>

#include <math.h>

#include <string.h>

double xpow( double const x, double const y ){ return pow( x, y ); }

double xmul( double const x, double const y ){ return x * y; }

double xdiv( double const x, double const y ){ return x / y; }

double xadd( double const x, double const y ){ return x + y; }

double xsub( double const x, double const y ){ return x - y; }

typedef double( *operator )( double, double );

struct ex { char name; operator op; int left; }ex[] =

{ { 0, 0, 0 }, { '^', xpow, 0 },{ '*', xmul, 1 },{ '/', xdiv, 1 },

{ '+', xadd, 1 },{ '-', xsub, 1 }};

struct ex lookup( char const name )

{ for( size_t i = 0; i < sizeof ex/sizeof 0[ ex ]; ++i )

if( ex[ i ].name == name )return ex[ i ]; return ex[ 0 ]; }

char const * g = 0; char get(){ return *g++; }

char peek(){ return *g; }

char check( char const * const op )

{ return strchr( op, peek() ) ?get(): 0; }

double numeral(){ return get() - '0'; }

double prefix(){ int sign = 1;

while( '-' == peek() ){ get(); sign *= -1; }

return sign * numeral(); }

double parse( char const * const op, double( *next ) () )

{ double result =( *next )();

for( char sym; sym = check( op ); )result = lookup( sym ).op

( result, lookup( sym ).left?( next )(): parse( op, next ));

return result; }

double power(){ return parse( "^", prefix ); }

double product(){ return parse( "/*", power ); }

double sum(){ return parse( "+-", product ); }

double start(){ return sum(); }

void test( char const * s ){ g = s; printf( "%g\n", start() ); }

int main( void ){ test( "2^2^3/2/2" ); test( "2+2*2^2^-3" ); }

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