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#include <stdio.h>
/* The C Programming Language: 2nd Edition
*
* Exercise 1-24: Write a program to check a C program for rudimentary syntax
* errors like unbalanced parentheses, brackets, and braces. Don't forget about
* quotes, both single and double, escape sequences, and comments. (This
* program is hard if you do it in full generality.)
*
* Answer: Proksima from Freenode's ##c helped me understand full generality
* where Ixquick, Wikipedia, and StackOverflow all failed: a program that has
* full generality handles all use cases. In this case, my program should
* report no errors from a well formed C source file, and return correct errors
* for every non-valid C source file.
*
* I can tackle this one the same way I tackled the previous exercise: with a
* FSM. The trick is in catching the mismatched levels. This happens when,
* after the source has been combed, there are values that aren't equal to
* zero. Anything that's non-zero means there are too many or too few of a
* specific syntax construct.
*
* Post-solution note: switch() would've made this MUCH shorter... but it's not
* covered until chapter 3. I think part of the purpose of this exercise is to
* teach the value of the switch structure later on. :)
*/
char c;
/* Set the state/count variables */
int linenr, parens, brackets, braces, singqs, dubqs, escapes, mcomms, scomms = 0;
int main() {
linenr = 1;
// Begin streaming!
while ((c = getchar()) != EOF) {
if (scomms == 1) {
if (c == '\n') {
scomms--;
} else {
continue;
}
} else if (mcomms == 1) {
if (c == '*') {
if (getchar() == '/') {
mcomms--;
}
}
} else {
// Check for escape sequences
if (c == '\\') {
escapes++;
c = getchar();
// This does not detect all sequences; just the ones covered in Chapter 1.
if (c != '\\' && c != 't' && c != '\'' && c != '"' && c != 'n' && c != 'b' && c != '0') {
break;
} else {
escapes--;
}
}
// Newline behavior
if (c == '\n') {
if (singqs > 0 || dubqs > 0) {
break;
}
linenr++;
}
// Parentheses
if (c == '(') {
parens++;
}
if (c == ')') {
parens -= 1;
if (parens < 0) {
break;
}
}
// Brackets
if (c == '[') {
brackets++;
}
if (c == ']') {
brackets--;
if (brackets < 0) {
break;
}
}
// Braces
if (c == '{') {
braces++;
}
if (c == '}') {
braces--;
if (braces < 0) {
break;
}
}
// Double quotes
if (c == '"') {
if (dubqs == 0) {
dubqs++;
} else {
dubqs--;
}
}
// Single quotes, which are only checked outside of doubles.
if (c == '\'' && dubqs == 0) {
if (singqs == 0) {
singqs++;
} else {
singqs--;
}
}
// Comment detection
if (c == '/') {
if (getchar() == '*') {
mcomms++;
}
if (getchar() == '/') {
scomms++;
}
}
}
}
if (escapes > 0) {
printf("Invalid escape sequence on line %d!\n", linenr);
return 1;
}
if (singqs > 0) {
printf("Unclosed single quote on line %d!\n", linenr);
return 1;
}
if (dubqs > 0) {
printf("Unclosed double quote on line %d!\n", linenr);
return 1;
}
if (brackets > 0) {
printf("Unclosed brackets on line %d!\n", linenr);
return 1;
}
if (brackets < 0) {
printf("Too many close brackets on line %d!\n", linenr);
return 1;
}
if (parens > 0) {
printf("Unclosed parenthesis on line %d!\n", linenr);
return 1;
}
if (parens < 0) {
printf("Too many close parentheses on line %d!\n", linenr);
return 1;
}
if (braces > 0) {
printf("Unclosed braces on line %d!\n", linenr);
return 1;
}
if (braces < 0) {
printf("Too many close braces on line %d!\n", linenr);
return 1;
}
if (mcomms == 1) {
printf("Unclosed comment at end of file!\n");
return 1;
}
printf("All clean.\n");
return 0;
}
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