Eval

Copyright 2003 Michael B. Allen <mba2000 ioplex.com> mba/eval.h Eval The eval(3m) module will evaluate simple arithmentic expressions consisting of integers, symbols for which the provided symlook_fn returns an integer, and any of the operators |&^+-*/().

Operator Precedence

Operator precedence is roughly the same as the C language.

 ( )  higher
 * /
 + -
^ & | lower

Prefixing integer tokens with minus '-' to indicate a negative value is currently not supported. A Simple Calculator The following example illustrates how to use the eval module with a hashmap(3m) as a symbol table to make a simple commandline calculator.

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#include <mba/hashmap.h>
#include <mba/eval.h>

int
symtab_lookup(const unsigned char *key, unsigned long *val, struct hashmap *symtab)
{
    const unsigned char *str;

    if ((str = hashmap_get(symtab, key))) {
        *val = strtoul(str, NULL, 0);
        return 0;
    }

    return -1;
}
int
main(void)
{
    struct hashmap symtab; 
    unsigned char buf[1024], *bp = buf;
    unsigned long result; 
    struct eval *eval = eval_new((symlook_fn)symtab_lookup, &symtab);

    hashmap_init(&symtab, 0, hash_str, cmp_str, NULL, NULL);

    while ((*bp = fgetc(stdin)) > 0) {
        if (*bp == '\n') { 
            *bp = '\0'; 
            bp = strchr(buf, '=');
            *bp++ = '\0'; 
            eval_expression(eval, bp, bp + strlen(bp), &result);
            sprintf(bp, "%u", result);
            printf(" %s=%ld\n", buf, result);
            hashmap_put(&symtab, strdup(buf), strdup(bp));
            bp = buf;
        } else {
            bp++;
        }
    }

    return EXIT_SUCCESS;
}
output:
$ ./calc 
foo=0xffff
 foo=65535
bar=10 * foo & 0x3333
 bar=13106

The Eval Functions

struct eval *eval_new(symlook_fn symlook, void *context);

The eval_new function creates and returns a new context object for evaluating expressions. The symlook parameter is defined as follows:

typedef int (*symlook_fn)(const tchar *name, unsigned long *val, void *context);

The symlook_fn function will be called to resolve any non-numeric symbols and should place the value identified by name into val and return 0. If the symbol is not found -1 should be returned.

The eval_new function can be used repeatedly to evaluate any number of expressions before being destroyed with the eval_del function. The context parameter is passed uninterpreted to the symlook_fn (e.g. a map perhaps).

void eval_del(void *eval);

The eval_del function deletes the context object eval freeing any memory allocated with eval_new or during the evaluation of expressions.

int eval_expression(struct eval *eval, const tchar *expr, const tchar *elim, unsigned long *result);

The eval_expression function evaluates an infix expression like '(5 + 3) * N', converts it into a stack of tokens in postfix orientation, and reduces it with a simple translation matrix to produce a single integer value. The eval parameter is a context object created with eval_new. The expression at expr is evaluated up to, but not including, the memory at elim, and writes the resulting value in the memory at result. The eval_expression function returns 0 if the expression was successfully reduced or -1 if an error occured in which case errno will be set appropriately (e.g. ENOENT if the symlook_fn could not resolve a symbol).