nanboxing.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <stdint.h>
#include <math.h> // for NaN
#include <assert.h>

union BoxedValue_t {
    uint64_t bits; // to inspect the raw bits
    double number; // to interpret the value as an IEEE float
};

typedef union BoxedValue_t BoxedValue;

#define MASK_SIGN 0x8000000000000000UL
#define MASK_EXP  0x7ff0000000000000UL
#define MASK_QNAN 0x0008000000000000UL
#define MASK_TAG  0x0007000000000000UL
#define MASK_PTR  0x0000ffffffffffffUL

#define TAG_NULL  0x0001000000000000UL
#define TAG_TRUE  0x0002000000000000UL
#define TAG_FALSE 0x0003000000000000UL

#define IS_NUMBER(v) (!isnan(v.number))
#define IS_OBJECT(v) (isnan(v.number) && (MASK_SIGN && (v.bits)))
#define AS_NUMBER(v) (v.number)
#define AS_OBJECT(v) ((void *)((v.bits) & MASK_PTR))

#define IS_TAGGED(v) (isnan(v.number) && (MASK_TAG & (v.bits)))

#define IS_NULL(v)  (IS_TAGGED(v) && (TAG_NULL & (v.bits)))
#define IS_TRUE(v)  (IS_TAGGED(v) && (TAG_TRUE & (v.bits)))
#define IS_FALSE(v) (IS_TAGGED(v) && (TAG_FALSE & (v.bits)))

#define QNAN  0xfff8000000000000
#define KNULL (QNAN | TAG_NULL) // `NULL` is reserved by C stdlib
#define TRUE  (QNAN | TAG_TRUE)
#define FALSE (QNAN | TAG_FALSE)

BoxedValue make_num(double value) {
    return (BoxedValue){.number = value};
}

BoxedValue make_imm(uint64_t value) {
    return (BoxedValue){.bits = value};
}

BoxedValue make_ptr(void *ptr) {
    BoxedValue value;
    value.bits = QNAN | (uint64_t)(ptr);
    return value;
}

typedef BoxedValue Value;


BoxedValue add_numbers(BoxedValue a, BoxedValue b) {
    if (IS_NUMBER(a) && IS_NUMBER(b)) {
        double sum = AS_NUMBER(a) + AS_NUMBER(b);
        return make_num(sum);
    }
    return make_imm(KNULL);
}

int main() {
    Value null = make_imm(KNULL);
    assert(IS_NULL(null));

    Value vtrue = make_imm(TRUE);
    assert(IS_TRUE(vtrue));

    Value vdbl = make_num(5.564);
    assert(IS_NUMBER(vdbl) && (AS_NUMBER(vdbl) == 5.564));

    Value a = make_num(6), b = make_num(7);
    assert(AS_NUMBER(add_numbers(a, b)) == 13);

    const char *sa = "test string";
    Value vptr = make_ptr((void *)sa);
    assert(IS_OBJECT(vptr));
    assert(((char *)AS_OBJECT(vptr)) == sa);

    printf("tests passed.\n");
    return 0;
}