// video.h
// The Voyager's video device.
// Written by Anachronaut

#ifndef VIDEO_H
#define VIDEO_H

#include <stdint.h>

// ---- What this is ----
//
// A tile engine. The CPU writes cell indices and the device expands them into pixels, which
// is the difference between a screen costing 2,000 bytes a frame and 64,000 - and at a
// megahertz that is the difference between a screen and no screen at all.
//
// It follows that COLOUR DEPTH IS FREE AT FRAME TIME. The map is the same size whether the
// tiles behind it are one bit deep or eight, because the depth lives in tile memory, which
// is written once when a program loads and not sixty times a second. So the tiles are eight
// bits: an 8x8 cell is 64 pixels and each one picks independently out of 256 colours, with
// no per-cell limit of the kind that made a Spectrum two and C64 multicolour four.
//
// ---- The device brings memory ----
//
// One bank, registered the way the disk's buffer is, so it costs a program nothing in Data
// Memory and keeps what is in it between frames. A program blits the region that changed
// and the rest stays as it was, which is the whole reason this is a bank rather than a
// window onto a port.

#define VIDEO_MEMORY_BYTES 0x10000

// Tile memory: 256 tiles of 8x8, one byte a pixel.
#define VIDEO_TILE_BASE    0x0000
#define VIDEO_TILE_BYTES   64
#define VIDEO_TILE_COUNT   256

// ---- The map, one page a row ----
//
// A row is padded to exactly 256 bytes whether the mode uses all of it or not, and that is
// not waste, it is arithmetic. THE MACHINE HAS NO MULTIPLY. On a 40 column screen every
// cursor move would otherwise need row times 40 in software, which is a tax on the most
// common operation in the whole system. At a page a row the address needs no arithmetic at
// all: the row number IS the high byte and the doubled column IS the low byte.
//
// It also frees the geometry from having to be a power of two, which is what lets the
// pixel resolution be whatever looks right.
#define VIDEO_MAP_BASE     0x4000
#define VIDEO_MAP_STRIDE   256
#define VIDEO_MAP_ROWS     128
#define VIDEO_MAP_COLUMNS  (VIDEO_MAP_STRIDE / 2)

// Two bytes to a cell: which tile, and how to colour it.
#define VIDEO_CELL_BYTES   2

// ---- The palette ----
//
// Four bytes an entry rather than three, for the same reason a map row is a page: entry n
// begins at n times four, which is a shift. Three would need a multiply the machine does
// not have. The fourth byte is unused and reads as whatever was put there.
#define VIDEO_PALETTE_BASE  0xC000
#define VIDEO_PALETTE_BYTES 4
#define VIDEO_PALETTE_SIZE  256

// ---- Modes ----
//
// Both are 8x8 cells over the same engine; only how many of them differ. The pixel count
// costs the CPU nothing, because it only ever writes the map - which is why the larger mode
// is affordable at all.
#define VIDEO_MODE_40x25 0
#define VIDEO_MODE_80x50 1
#define VIDEO_MODE_COUNT 2

#define VIDEO_CELL_PIXELS 8
#define VIDEO_MAX_WIDTH   (80 * VIDEO_CELL_PIXELS)
#define VIDEO_MAX_HEIGHT  (50 * VIDEO_CELL_PIXELS)

// ---- Ports ----
//
// Sixteen, like the controller, and it interrupts on its base the way the disk established.
// Nothing interrupts yet; the frame interrupt is the next rung.
#define VIDEO_STATUS   0x30
#define VIDEO_MODE     0x31
#define VIDEO_COLUMNS  0x32
#define VIDEO_ROWS     0x33
#define VIDEO_SCROLL   0x34

void videoReset(void);

uint8_t *videoMemory(uint32_t *capacity);

uint8_t videoWrite(uint8_t value, uint8_t port);
uint8_t videoRead(uint8_t port);

// Turns what is in video memory into pixels. A pure function of that memory, so the same
// contents give the same picture with nobody watching - which is what lets the suite check
// a screen on a machine that has no display.
void videoRender(void);

// The pixels the last render produced, three bytes each, red then green then blue.
const uint8_t *videoPixels(int *width, int *height);

// Renders and writes a binary PPM. Returns 0 if it worked.
int videoWriteImage(const char *path);

#endif // VIDEO_H