arena_format.c

// Writen by ChatGPT, you may want to read it before use.
// Example usage:

// fmt_arena_reset(); // once per frame before any usage
// Clay_BeginLayout();
// ...
// CLAY_TEXT(Clay_WrapStr(fmt_fmt ("%s (%d cores)", gpuName, numCores)), textCfg);  

#ifndef FORMAT_ARENA_INIT_CAP
#define FORMAT_ARENA_INIT_CAP 4096
#endif

typedef struct {
    char  *base;
    size_t cap;
    size_t off;
} fmt_arena_t;

static fmt_arena_t g_fmt_arena = { NULL, 0, 0 };

static void *fmt_arena_alloc(size_t n) {
    const size_t align = sizeof(void*);
    n = (n + (align - 1)) & ~(align - 1); // align

    if (!g_fmt_arena.base) {
        g_fmt_arena.cap  = FORMAT_ARENA_INIT_CAP;
        g_fmt_arena.base = (char*)malloc(g_fmt_arena.cap);
        if (!g_fmt_arena.base) return NULL;
        g_fmt_arena.off = 0;
    }
    if (g_fmt_arena.off + n > g_fmt_arena.cap) {
        size_t newcap = g_fmt_arena.cap;
        while (g_fmt_arena.off + n > newcap) {
            if (newcap > SIZE_MAX / 2) { newcap = SIZE_MAX; break; }
            newcap *= 2;
        }
        char *p = (char*)realloc(g_fmt_arena.base, newcap);
        if (!p) return NULL;
        g_fmt_arena.base = p;
        g_fmt_arena.cap  = newcap;
    }
    void *out = g_fmt_arena.base + g_fmt_arena.off;
    g_fmt_arena.off += n;
    return out;
}

void fmt_arena_reset(void) { g_fmt_arena.off = 0; }
void fmt_arena_free(void)  { free(g_fmt_arena.base); g_fmt_arena.base=NULL; g_fmt_arena.cap=g_fmt_arena.off=0; }

const char* fmt_int (int n)
{
    char tmp[32];
    int len = snprintf(tmp, sizeof(tmp), "%d", n);
    if (len < 0) return NULL;

    char *s = (char*)fmt_arena_alloc((size_t)len + 1);
    if (!s) return NULL;
    memcpy(s, tmp, (size_t)len + 1); // includes NUL
    return s;
}

#if defined(__GNUC__) || defined(__clang__)
__attribute__((format(printf,1,2)))
#endif
const char* fmt_fmt(const char* format, ...)
{
    va_list ap;
    va_start(ap, format);

    // First pass: compute required length (excluding NUL)
    va_list ap2;
    va_copy(ap2, ap);
#if defined(_MSC_VER)
    int len = _vscprintf(format, ap2);   // MSVC: length only
#else
    int len = vsnprintf(NULL, 0, format, ap2);
#endif
    va_end(ap2);

    if (len < 0) { va_end(ap); return NULL; }

    char *s = (char*)fmt_arena_alloc((size_t)len + 1);
    if (!s)      { va_end(ap); return NULL; }

    // Second pass: actually format
    vsnprintf(s, (size_t)len + 1, format, ap);
    va_end(ap);

    return s;  // valid until your arena is reset/freed
}