#include <multiboot.h>
#include <acpi.h>

struct kernel_context {
    struct rsdp *rsdp;
    struct xsdp *xsdp;

    struct rsdt *rsdt;
};

struct kernel_context ctx = {0};

int strncmp( const char * s1, const char * s2, int n )
{
    while ( n && *s1 && ( *s1 == *s2 ) )
    {
        ++s1;
        ++s2;
        --n;
    }
    if ( n == 0 )
    {
        return 0;
    }
    else
    {
        return ( *(unsigned char *)s1 - *(unsigned char *)s2 );
    }
}

void *findFACP(void *RootSDT)
{
    struct rsdt *rsdt = (struct rsdt *) RootSDT;
    int entries = (rsdt->h.Length - sizeof(rsdt->h)) / 4;
 
    for (int i = 0; i < entries; i++)
    {
        struct ACPISDTHeader *h = (struct ACPISDTHeader *) rsdt->other_sdt[i];
        if (!strncmp(h->Signature, "FACP", 4))
            return (void *) h;
    }
 
    // No FACP found
    return 0;
}

void kmain(struct multiboot_info *info) {
    // Check if the bootloader gave us the upper and lower memory
    if (!(info->flags & 0x1)) goto halt;

    char *vmem = (char *)0xb8000;

    struct rsdp *found_rsdp = acpi_search_for_rsdp();
    if (!found_rsdp) goto halt;
    if (found_rsdp->Revision == ACPI_VER_1) {
        // ACPI v1.0 is being used
        if (!acpi_validate_rsdp_checksum(found_rsdp)) goto halt;
        ctx.rsdp = found_rsdp;
        ctx.xsdp = 0;

        if (!acpi_validate_sdt_checksum((struct ACPISDTHeader *)found_rsdp->RsdtAddress)) 
            goto halt;

        ctx.rsdt = (struct rsdt*)found_rsdp->RsdtAddress;

    } else if (found_rsdp->Revision == ACPI_VER_OTHER) {
        // ACPI v2.0+ is being used
        // not supported yet....
        goto halt;
    } else goto halt;

    void *fadt = findFACP(ctx.rsdt);
    if (!fadt) goto halt;
    char *next_sig = (char *)fadt;
    vmem[0] = next_sig[0];
    vmem[2] = next_sig[1];
    vmem[4] = next_sig[2];
    vmem[6] = next_sig[3];
halt:
    for (;;) {}
}