Boolean expression evaluator

A simple evaluator for infix boolean expressions, for handling conditionals in parsers and similar code. This one was written to be used in VICE to handle contionals in hotkeys files.

The evaluator supports seven different tokens: false, true, && (and), ||, (or), ! (not), ( and ). Precedence of operators is the same as used in C, so in order of higher to lower precedence: !, &&, ||.

Building

Simply run make. Some POSIX-specific code is present in main.c, the test driver, which serves my purposes just fine. The "library" (boolexpr.c and boolexpr.h) should be clean C99.

Running the tests

After running make, an executable expr-test will be present that can be used to parse a text file with tests. Usage is as follows: expr-test <filename>.

The input file is expected to contain boolean expressions, prefixed with expected error code and expected result (if error code is 0). For example:

0   false   true && false
0   true    false || true && true
4           true && false )
6           ( true && false

Empty lines are allowed in the file, as are comments starting with #.

API

Use of the API is straightforward: initialize for use, feed expression, attempt to evaluate, clean up when done.

Initializiation

Declare a boolexpr_t object and initialize it using bexpr_init():

#include <boolexpr.h>

bexpr_init();

This allocates memory for the evaluator and initializes some internal state.

Evaluating an expression

An expression can be either parsed from text, using bexpr_tokenize(), or fed to the evaluator token by token with bexpr_add_token().

Once an expression is made available through either method, the expression can be evaluated with bexpr_evaluate().

For example:

bool result;

/* tokenize text */
if (!bexpr_tokenize("false || true")) {
    fprintf(stderr, "oops!\n");
    exit(1);
}

/* try to evaluate expression */
if (bexpr_evaluate(&result)) {
    printf("result = %s\n", result ? "true" : "false");
} else{
    fprintf(stderr, "oops!");
    exit(1);
}

Reusing the evaluator and cleaning up

The memory used by the evaluator must be freed after use with bexpr_free().

When evaluating multiple expressions during the lifetime of a program, the evaluator can be reset for reuse without freeing and reallocating internal resources with bexpr_reset(). Cleaning up (on program exit or so) still requires bexpr_free() to properly release used resources.

Error codes and messages

A simple mechanism is provided to check for types of errors and translating them into user-readable messages:

The variable extern int bexpr_errno provides the error code set by the evaluator in case of errors. Error messages can be obtained with bexpr_strerror(int errnum).

Currently there is no possibility to pinpoint where in an expression the error occurred, so reporting the column number in a source line won't work.

Compyx / boolexpr