vex.c

// The stack is 8-byte aligned.
#define ALIGN(p) ((uint64_t *)(((uintptr_t)(p) + 7) & ~7))
#define STACK(stack, size) uint64_t *_stack = ALIGN(stack), *_stack_end = (uint64_t *)((char *)(stack) + size)
#define PUSH(x) do { memcpy(_stack, &x, sizeof(x)); _stack += (sizeof(x) + 7) / 8; } while (0)
#define POP(x) do { _stack -= (sizeof(x) + 7) / 8; memcpy(&x, _stack, sizeof(x)); } while (0)

#if __GNUC__
// Use computed gotos on GCC-compatible compilers (including Clang). 
#define JOIN(x, y) x##y
#define LABEL(name) JOIN(label, name)
#define BEGIN(name) CALL(name); goto _end;
#define END RETURN; _end: ;
#define FUNCTION(name) RETURN; name:
#define CALL(name) do { *_stack++ = (uint64_t)&&LABEL(__LINE__); goto name; LABEL(__LINE__): ; } while (0)
#define RETURN goto *(void *)*--_stack
#else
// Use Duff's Device as a fallback on other compilers.
#define BEGIN(name) uint32_t _case = 0; *_stack++ = 0; goto name; _dispatch: switch(_case) { case 0: goto _end;
#define END RETURN; _end: ; }
#define FUNCTION(name) RETURN; name:
#define CALL(name) do { *_stack++ = __LINE__; goto name; case __LINE__: ; } while (0)
#define RETURN do { _case = *--_stack; goto _dispatch; } while (0)
#endif

void parse(char *input, char *input_end) {
    char ch;
    uintptr_t nesting;
    char stack[1024];
    STACK(stack, sizeof(stack));
    BEGIN(parse_root)
    FUNCTION(parse_root)
    while (input != input_end) {
        if (*input++ == '@')
            CALL(parse_tag);
    }
    FUNCTION(parse_tag)
    while (input != input_end && !word_terminator[*input])
        input++;
    if (input != input_end) {
        ch = *input;
        if (ch == '{') {
            input++;
            nesting = 0;
            while (input != input_end) {
                ch = *input++;
                if (ch == '@') {
                    PUSH(nesting);
                    CALL(parse_tag);
                    POP(nesting);
                } else if (ch == '{') {
                    nesting++;
                } else if (ch == '}') {
                    if (nesting == 0)
                        break;
                    nesting--;
                }
            }
        } else if (ch == '=') {
            input++;
            while (input != input_end && !word_terminator[*input])
                input++;
        } else if (ch == ':') {
            input++;
            while (input != input_end && *input != '\n' && *input != '\r') {
                ch = *input++;
                if (ch == '@')
                    CALL(parse_tag);
            }
        }
    }
    END
}