lens.c

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdint.h>

typedef struct {
    int baz;
} bar;

typedef struct {
    bar *bar;
    int baz;
} foo;

typedef struct lens {
    size_t *offsets;
    size_t size;
} lens;

lens idLens() {
    lens l = { NULL, 0 };
    return l;
}

lens offsetLens(size_t offset) {
    size_t *offsets = malloc(sizeof(size_t));
    lens l = { offsets, 1 };
    return l;
}

lens addOffsetLens(lens *l, size_t o) {
    if (l->size == 0) { return offsetLens(o); }
    size_t *offsets = malloc(l->size * sizeof(size_t));
    for (int i = 0; i < l->size; i++) {
        offsets[i] = l->offsets[i];
    }
    offsets[l->size - 1] += o;
    lens l2 = { offsets, l->size };
    return l2;
}

lens composeLens(lens *l1, lens *l2) {
    size_t *offsets = malloc((l1->size + l2->size) * sizeof(size_t));
    if (l1->offsets != NULL) {
        for (int i = 0; i < l1->size; i++) {
            offsets[i] = l1->offsets[i];
        }
    }
    if (l2->offsets != NULL) {
        for (int i = 0; i < l2->size; i++) {
            offsets[l1->size + i] = l2->offsets[i];
        }
    }
    lens l = { offsets, l1->size + l2->size };
    return l;
}

void* get(void *s, lens *l) {
    void* p = s;
    for (int i = 0; i < l->size; i++) {
        intptr_t f = ((intptr_t*)p)[l->offsets[i]];
        p = (void*) f;
    }
    return p;
}

void set_int(void *s, lens *l, int v) {
    if (l->size == 0) *(int*)s = v;
    void *p = s;
    for (int i = 0; i < l->size - 1; i++) {
        intptr_t f = ((intptr_t*)p)[l->offsets[i]];
        p = (void*) f;
    }
    ((int*)p)[l->offsets[l->size - 1]] = v;
}

static lens _foo_bar;
static lens _bar_baz;
static lens _foo_bar_baz;

int main() {
    _foo_bar = offsetLens(offsetof(foo, bar));
    _bar_baz = offsetLens(offsetof(bar, baz));
    _foo_bar_baz = composeLens(&_foo_bar, &_bar_baz);
    bar b = { 42 };
    foo f = { &b, 666 };
    printf("f.bar->baz = %d\n", (int)(get(&f, &_foo_bar_baz)));
    set_int(&f, &_foo_bar_baz, 666);
    printf("f.bar->baz = %d\n", (int)(get(&f, &_foo_bar_baz)));
    return 0;
}