Coverage.start

static VALUE
rb_coverage_start(int argc, VALUE *argv, VALUE klass)
{
    VALUE coverages, opt;
    int mode;

    rb_scan_args(argc, argv, "01", &opt);

    if (argc == 0) {
        mode = 0; /* compatible mode */
    }
    else if (opt == ID2SYM(rb_intern("all"))) {
        mode = COVERAGE_TARGET_LINES | COVERAGE_TARGET_BRANCHES | COVERAGE_TARGET_METHODS;
    }
    else {
        mode = 0;
        opt = rb_convert_type(opt, T_HASH, "Hash", "to_hash");

        if (RTEST(rb_hash_lookup(opt, ID2SYM(rb_intern("lines")))))
            mode |= COVERAGE_TARGET_LINES;
        if (RTEST(rb_hash_lookup(opt, ID2SYM(rb_intern("branches")))))
            mode |= COVERAGE_TARGET_BRANCHES;
        if (RTEST(rb_hash_lookup(opt, ID2SYM(rb_intern("methods")))))
            mode |= COVERAGE_TARGET_METHODS;
        if (RTEST(rb_hash_lookup(opt, ID2SYM(rb_intern("oneshot_lines"))))) {
            if (mode & COVERAGE_TARGET_LINES)
                rb_raise(rb_eRuntimeError, "cannot enable lines and oneshot_lines simultaneously");
            mode |= COVERAGE_TARGET_LINES;
            mode |= COVERAGE_TARGET_ONESHOT_LINES;
        }
    }

    if (mode & COVERAGE_TARGET_METHODS) {
        me2counter = rb_ident_hash_new();
    }
    else {
        me2counter = Qnil;
    }

    coverages = rb_get_coverages();
    if (!RTEST(coverages)) {
        coverages = rb_hash_new();
        rb_obj_hide(coverages);
        current_mode = mode;
        if (mode == 0) mode = COVERAGE_TARGET_LINES;
        rb_set_coverages(coverages, mode, me2counter);
    }
    else if (current_mode != mode) {
        rb_raise(rb_eRuntimeError, "cannot change the measuring target during coverage measurement");
    }
    return Qnil;
}