rpn_serialize.c

typedef enum {
    CMD_INT = NODE_INT,
    CMD_NEG = NODE_NEG,
    CMD_NOT = NODE_NOT,
    CMD_ADD = NODE_ADD,
    CMD_SUB = NODE_SUB,
    CMD_RET = 128,
    CMD_SETREF,
    CMD_GETREF,
} Cmd;

// Can ensure termination by requiring data stream to be capped with 5 RETs.
Node *load(const uint8_t *data) {
    enum { MAX_STACK = 256 };
    Node *stack[MAX_STACK];
    Node *refs[256] = {0};
    Node **tos = stack;
    for (;;) {
        Cmd cmd = *(uint8_t *)data++;
        switch (cmd) {
        case CMD_INT:
            if (tos == stack + MAX_STACK) goto error;
            *tos++ = int_node(*(int32_t *)data);
            data += 4;
            break;
        case CMD_NEG:
        case CMD_NOT:
            if (tos <= stack) goto error;
            tos[-1] = unary_node(cmd, tos[-1]);
            break;
        case CMD_ADD:
        case CMD_SUB:
            if (tos <= stack + 1) goto error;
            tos[-2] = binary_node(cmd, tos[-2], tos[-1]);
            tos--;
            break;
        case CMD_GETREF:
            if (tos == stack + MAX_STACK) goto error;
            *tos++ = refs[*(uint8_t *)data++];
            break;
        case CMD_SETREF:
            if (tos <= stack) goto error;
            refs[*(uint8_t *)data++] = *--tos;
            break;
        case CMD_RET:
            if (tos != stack + 1) goto error;
            return tos[-1];
        default: error:
            // clean up nodes
            return 0;
        }
    }
}