#include <memory.h>
#include <i86.h>
#include <dos.h>

#include "dpmi.h"
#include "vesa.h"

char far *VGA = (char*)0xA0000;
long windowSizeBytes, bytesPerScanLine;
void *dosmem;
int currentWindowPosition;

#define MAX_COLORS (256)

void setupVESAHandling() {
  dosmem = dpmiAllocateDOSMemory(1024);
}

void teardownVESAHandling() {
  dpmiDeallocateDOSMemory();
}

void addDOSMemToRealModeRegs(struct dpmiRealModeRegs *rc) {
  rc->edi = (unsigned long)dosmem & 0xf;
  rc->es = (unsigned long)dosmem >> 4;
}

void vbeGetModeInfo(
  uint16_t mode,
  struct VbeModeInfo *modeInfo
) {
  struct dpmiRealModeRegs rc={0};

  _fmemset(dosmem, 0, sizeof(struct VbeModeInfo));

  rc.eax = 0x4f01;
  rc.ecx = mode;
  addDOSMemToRealModeRegs(&rc);

  dpmiInt(0x10, &rc);

  _fmemcpy(modeInfo, dosmem, sizeof(struct VbeModeInfo));
}

int vbeGetVBEInfo(
  struct VbeInfo far *p
) {
  struct dpmiRealModeRegs rc={0};

  // request VBE2 extended info
  _fmemset(p, 0, sizeof(struct VbeInfo));
  _fmemcpy(p->VbeSignature,"VBE2",4);
  _fmemcpy(dosmem, p, sizeof(struct VbeInfo));

  rc.eax = 0x4f00;
  addDOSMemToRealModeRegs(&rc);

  dpmiInt(0x10, &rc);

  _fmemcpy(p, dosmem, sizeof(struct VbeInfo));

  return 0;
}

void startSVGAMode(int mode, struct VbeModeInfo *vbeModeInfo) {
  /**
   * Side effects: sets some internal variables for window and scanline sizes
   */
  union REGS regs;

  vbeGetModeInfo(mode, vbeModeInfo);

  windowSizeBytes = 1024L * vbeModeInfo->WinSize;
  bytesPerScanLine = vbeModeInfo->BytesPerScanLine;

  regs.w.ax = 0x4f02;
  regs.w.bx = mode;
  int386(0x10, &regs, &regs);

  vbeSetWindow(0, 0);
}

void vbeSetWindow(int window, int position) {
  /**
   * Side effects: tracks current window position
   */
  union REGS regs;

  regs.w.ax = 0x4f05;
  regs.h.bh = 0;
  regs.h.bl = window;
  regs.w.dx = position;
  int386(0x10, &regs, &regs);

  currentWindowPosition = position;
}

void maybeVbeSetWindow(int window, int position) {
  if (currentWindowPosition == position) return;

  vbeSetWindow(window, position);
}

void setVESAColorPalette(
  struct VESAColor *colors
) {
  struct dpmiRealModeRegs rc={0};
  int i;

  _fmemcpy(dosmem, colors, MAX_COLORS * 4);

  // prep our real mode interrupt call
  rc.eax = 0x4f09;
  rc.ebx = 0;
  rc.ecx = MAX_COLORS;
  rc.edx = 0;
  addDOSMemToRealModeRegs(&rc);

  dpmiInt(0x10, &rc);
}

void setPixel(int x, int y, int color) {
  long absolutePosition = (long)y * bytesPerScanLine + x;
  long windowPosition = absolutePosition / windowSizeBytes;
  long windowOffset = absolutePosition % windowSizeBytes;

  /**
  * This will not shift window positions unless needed.
  */
  maybeVbeSetWindow(0, windowPosition);

  VGA[windowOffset] = color;
}