Psych::Emitter#start_document

static VALUE start_document(VALUE self, VALUE version, VALUE tags, VALUE imp)
{
    yaml_emitter_t * emitter;
    yaml_tag_directive_t * head = NULL;
    yaml_tag_directive_t * tail = NULL;
    yaml_event_t event;
    yaml_version_directive_t version_directive;
    TypedData_Get_Struct(self, yaml_emitter_t, &psych_emitter_type, emitter);


    Check_Type(version, T_ARRAY);

    if(RARRAY_LEN(version) > 0) {
        VALUE major = rb_ary_entry(version, (long)0);
        VALUE minor = rb_ary_entry(version, (long)1);

        version_directive.major = NUM2INT(major);
        version_directive.minor = NUM2INT(minor);
    }

    if(RTEST(tags)) {
        long i = 0;
        long len;
        rb_encoding * encoding = rb_utf8_encoding();

        Check_Type(tags, T_ARRAY);

        len = RARRAY_LEN(tags);
        head  = xcalloc((size_t)len, sizeof(yaml_tag_directive_t));
        tail  = head;

        for(i = 0; i < len && i < RARRAY_LEN(tags); i++) {
            VALUE tuple = RARRAY_AREF(tags, i);
            VALUE name;
            VALUE value;

            Check_Type(tuple, T_ARRAY);

            if(RARRAY_LEN(tuple) < 2) {
                xfree(head);
                rb_raise(rb_eRuntimeError, "tag tuple must be of length 2");
            }
            name  = RARRAY_AREF(tuple, 0);
            value = RARRAY_AREF(tuple, 1);
            StringValue(name);
            StringValue(value);
            name = rb_str_export_to_enc(name, encoding);
            value = rb_str_export_to_enc(value, encoding);

            tail->handle = (yaml_char_t *)StringValueCStr(name);
            tail->prefix = (yaml_char_t *)StringValueCStr(value);

            tail++;
        }
    }

    yaml_document_start_event_initialize(
            &event,
            (RARRAY_LEN(version) > 0) ? &version_directive : NULL,
            head,
            tail,
            imp ? 1 : 0
            );

    emit(emitter, &event);

    if(head) xfree(head);

    return self;
}