SplitBit-Emulator/Source/Emulator/voyager.c

// voyager.c

// The Segan Voyager
// A SplitBit with a screen and a speaker attached
// Written by Anachronaut
//
// ---- What this is ----
//
// The same machine SplitBit runs, presented through a window instead of a terminal. Every
// instruction, every device and every cycle is in machine.c and shared; this file opens a
// window, gives the machine a slice of time per frame, and shows what came out.
//
// THAT ORDER MATTERS AND IS THE WHOLE DESIGN. The devices belong to the machine and advance
// on emulated cycles, so the same program produces the same frames and the same samples
// whether or not anybody is looking. Raylib presents; it does not decide. Which is what
// lets a test suite with no display hold this binary to the same behaviour as the other
// one.
//
// The window shows what the video device produced and decides nothing about it. Render is a
// pure function of video memory, so the same program draws the same picture whether or not
// anybody is watching - which is what lets a suite with no display check a screen.

#include "machine.h"
#include "video.h"
#include "io.h"
#include "utility.h"
#include "raylib.h"
#include <stdio.h>
#include <string.h>
#include <getopt.h>

// The window opens at the largest screen the device can produce, doubled, because a 640 by
// 400 window is small on a modern display and a 320 by 200 one is a postage stamp.
#define SCREEN_SCALE  2

// ---- Running without a window ----
//
// Taken out of the arguments here rather than in the shared parser, because it is a fact
// about this front end and the shared parser should not learn about a window that only one
// binary has. Everything else on the command line means exactly what it means to SplitBit.
//
// It exists so the suite can run this binary at all: a test machine has no display, and a
// front end that could only be exercised by a person looking at it would be a front end
// nothing checks. Headless, Voyager must print byte for byte what SplitBit prints, and
// Tests/voyager.sh holds it to that.
static int takeHeadless(int *argc, char *argv[]) {
    int headless = 0;
    int out = 0;
    for (int i = 0; i < *argc; i++) {
        if (strcmp(argv[i], "--headless") == 0) {
            headless = 1;
            continue;
        }
        argv[out++] = argv[i];
    }
    argv[out] = NULL;
    *argc = out;
    return headless;
}

// ---- The window, kept in one place ----
//
// Both the frame loop and the input hook have to be able to present, because a machine
// waiting for a key is still a machine somebody is looking at. A window that froze while a
// program asked a question would look broken every time it asked one.
static Texture2D screenTexture;
static int windowOpen = 0;

// ---- Keys are kept until they are asked for ----
//
// RAYLIB CLEARS ITS CHARACTER QUEUE ON EVERY POLL, and a poll happens inside EndDrawing, so
// a key survives exactly one frame unless something takes it. That is fine for a game that
// reads input every frame and wrong for everything else: Snake looks about ten times a
// second, so five keys in six were being thrown away by the next present before it ever
// glanced at them. The shell worked the whole time, because a blocking read presents and
// then looks immediately.
//
// So the window keeps its own queue, drained from Raylib at every present and emptied only
// when the console actually takes a byte. That is what the machine already promises - Snake's
// own comment says "the console keeps the next key until it is asked for" - and it makes the
// console's timing nobody else's business.
#define KEY_QUEUE 64
static unsigned char keyQueue[KEY_QUEUE];
static int keyHead = 0;
static int keyTail = 0;

static void keyPush(unsigned char byte) {
    const int next = (keyTail + 1) % KEY_QUEUE;
    if (next == keyHead) {
        // Full, so the oldest goes. Somebody leaning on the keyboard while a program ignores
        // it should not be able to push out what they typed most recently.
        keyHead = (keyHead + 1) % KEY_QUEUE;
    }
    keyQueue[keyTail] = byte;
    keyTail = next;
}

static int keyTake(void) {
    if (keyHead == keyTail) {
        return CONSOLE_NOTHING_YET;
    }
    const int byte = keyQueue[keyHead];
    keyHead = (keyHead + 1) % KEY_QUEUE;
    return byte;
}

// Everything Raylib has, taken before it can throw any of it away.
static void drainKeyboard(void) {
    int character;
    while ((character = GetCharPressed()) > 0) {
        if (character < 128) {
            keyPush((unsigned char)character);
        }
    }
    int key;
    while ((key = GetKeyPressed()) > 0) {
        // Only the keys a character queue does not carry, because they are not characters.
        // Everything else has already arrived above, and taking it again would double it.
        switch (key) {
            case KEY_ENTER: case KEY_KP_ENTER: keyPush('\n'); break;
            case KEY_BACKSPACE:                keyPush(0x08); break;
            case KEY_TAB:                      keyPush('\t'); break;
            case KEY_ESCAPE:                   keyPush(0x1B); break;
            default:                           break;
        }
    }
}

// ---- The reset button ----
//
// EMULATOR MAGIC, AND KNOWN TO BE. There is no reset line on this machine yet and no keyboard
// controller to assert one: the window reaches in and pokes the same flag a program pokes
// through the machine port. When those are designed, a keyboard controller will have to see
// this gesture and pull reset REGARDLESS OF WHAT THE CPU IS DOING - which is the property
// that matters and the one a port write cannot have, since a port write needs a program
// willing and able to make it.
//
// The shape of that is already visible here. A reset is normally noticed between
// instructions, and a halted machine runs none - so the window asks every frame rather than
// leaving it to the machine to notice, which is what real hardware would do with a line.
//
// ON REAL HARDWARE THIS IS NOT A KEY AT ALL. A Voyager has a button on the case, and what a
// window has instead of a case is a gesture. So the gesture wants two properties a single
// key does not have.
//
// It must not be a key SOFTWARE MIGHT WANT. A machine with a keyboard has function keys on
// it, and something will eventually have a use for F12 - which is where this was, and which
// would have meant taking it away again later.
//
// And it must not be reachable BY ACCIDENT. Restarting the machine throws away everything in
// memory, and a single key that does that sits one mistake away from losing work. Three keys
// together are not pressed by mistake.
//
// Control, Shift and R. It was Control, Alt and Delete, which has meant this since 1981 and
// is the one gesture nobody has to be told the meaning of - AND WHICH CANNOT BE USED.
//
// It is a secure attention key. Every serious operating system reserves it so that it always
// reaches the system and never an application, precisely so that a program cannot imitate a
// login screen; on Windows an application cannot see it at all without a kernel driver, and
// on Linux the desktop takes it. That is not an oversight to work around - it is the same
// guarantee a reset button wants, being enforced one layer further down, and there is no
// call this program can make that would win the argument.
//
// So the gesture has to be one the host has no opinion about. Control and Shift with a
// letter is about as free as a combination gets: it is not window management, not a virtual
// terminal switch, and not a shortcut any desktop claims by default.
//
// If a platform does send a character for it, nothing comes of that either - whatever
// arrives is in memory that is about to be thrown away.
//
// What it does is what writing MACHINE_RESET does: the machine starts the way it started, so
// the boot chain runs again and finds whatever the disk now says to run. Which is what makes
// a bare metal program escapable - Once puts a demo in front of the next start and deletes
// the request before jumping, so a demo that has taken the whole machine is one gesture from
// the system coming back, rather than closing the window and opening it again.
static void checkResetButton(void) {
    const int control = IsKeyDown(KEY_LEFT_CONTROL) || IsKeyDown(KEY_RIGHT_CONTROL);
    const int shift = IsKeyDown(KEY_LEFT_SHIFT) || IsKeyDown(KEY_RIGHT_SHIFT);
    if (control && shift && IsKeyPressed(KEY_R)) {
        requestReset();
    }
}

static void presentFrame(void) {
    // The device turns video memory into pixels; this puts them on the glass. Everything
    // that decides what the screen looks like is in the machine, where the suite can
    // reach it.
    videoRender();
    int width, height;
    const uint8_t *frame = videoPixels(&width, &height);
    if (width > 0 && height > 0) {
        UpdateTextureRec(screenTexture, (Rectangle){ 0, 0, (float)width, (float)height },
                         frame);
    }
    BeginDrawing();
    // Clearly not the screen. What is left over when the window's shape does not match the
    // picture's is a bezel, and it should look like one rather than like more screen.
    ClearBackground((Color){ 40, 40, 40, 255 });
    if (width > 0 && height > 0) {
        // ---- Filling the window, in whole pixels ----
        //
        // The largest whole-number scale that still fits. Whole numbers because a 320 by 200
        // picture stretched by 2.7 is a picture with some rows twice as tall as their
        // neighbours, which on eight pixel glyphs is the difference between text and mush.
        //
        // The two modes are exactly a factor of two apart and the window opens at twice the
        // larger, so both fill it exactly: 320 by 200 at four, and 640 by 400 at two.
        // Changing mode therefore changes how sharp the screen is and not how big it is.
        const int windowWidth = GetScreenWidth();
        const int windowHeight = GetScreenHeight();
        int scale = windowWidth / width;
        const int fits = windowHeight / height;
        if (fits < scale) scale = fits;
        if (scale < 1) scale = 1;
        const int drawnWidth = width * scale;
        const int drawnHeight = height * scale;
        Rectangle from = { 0, 0, (float)width, (float)height };
        Rectangle to = {
            (float)((windowWidth - drawnWidth) / 2),
            (float)((windowHeight - drawnHeight) / 2),
            (float)drawnWidth, (float)drawnHeight
        };
        DrawTexturePro(screenTexture, from, to, (Vector2){ 0, 0 }, 0.0f, WHITE);
    }
    EndDrawing();
    // EndDrawing has just polled, which is the one moment Raylib's queues hold anything.
    drainKeyboard();
    checkResetButton();
}

// What the console asks while it is waiting. Presenting from in here is what keeps the
// window answering, and EndDrawing paces it, so waiting for a key costs a frame rather
// than a spin.
static int voyagerKey(int mayWait) {
    if (!windowOpen) {
        return CONSOLE_GONE;
    }
    // ---- The button has to reach a machine that is waiting ----
    //
    // A reset is acted on between instructions, and a machine blocked on a key is part way
    // through one - so pressing the button while a program sits waiting would set the flag
    // and nothing would ever come along to notice it. Which is precisely the moment a reset
    // button earns its keep: a program that is stuck is the one you want to get out of.
    //
    // So the wait ends. The console treats that as the end of input, which it is for the
    // machine that is about to stop existing, and the reset puts the console's input back.
    //
    // ASKED OF THE REQUEST ITSELF rather than remembered here. A flag of its own outlived
    // the reset it belonged to: a program that never read the console - picture.bin, say,
    // which draws and halts - left it set, and the NEXT machine's first read came back as
    // the end of input. CosmOS booted and stopped immediately, having been told there was
    // nobody there. There is one fact and it lives in one place.
    if (resetIsPending()) {
        return CONSOLE_GONE;
    }
    // Whatever is already waiting, however long ago it was typed. This is the answer to
    // both questions, and asking it first is what makes a program that polls rarely see
    // every key rather than one in six.
    const int waiting = keyTake();
    if (waiting != CONSOLE_NOTHING_YET) {
        return waiting;
    }
    if (!mayWait) {
        // A poll is a poll. Presenting here would charge a frame for every glance, and a
        // program that looks in a loop would run at the frame rate.
        return CONSOLE_NOTHING_YET;
    }
    if (WindowShouldClose()) {
        windowOpen = 0;
        return CONSOLE_GONE;
    }
    // Presenting is what keeps the window answering while the machine waits, and EndDrawing
    // paces it, so waiting for a key costs a frame rather than a spin. It drains the
    // keyboard on the way out, so anything just typed is here now.
    presentFrame();
    return keyTake();
}



int main(int argc, char *argv[]) {
    int headless = takeHeadless(&argc, argv);

    EmulatorOptions options;
    uint8_t result = parseOptions(argc, argv, &options);
    if (result == OPTIONS_HELP) {
        printf("  --headless        Run with no window, which is how the tests run it.\n");
        return 0;
    } else if (result == OPTIONS_ERROR) {
        return 1;
    }
    char *programFile = NULL;
    if (optind < argc) {
        programFile = argv[optind];
        optind++;
    }
    if (optind < argc) {
        fprintf(stderr, "Error: Unexpected argument: %s\n", argv[optind]);
        return 1;
    }

    Machine machine;
    uint8_t started = machineStart(&machine, &options, programFile);
    if (started == MACHINE_NOTHING_TO_RUN) {
        fprintf(stderr, "Error: No boot image and no disk, so there is nothing to run.\n");
        printHelp(argv[0]);
        return 1;
    } else if (started != MACHINE_OK) {
        return 1;
    }

    if (headless) {
        // The same three lines SplitBit runs, and deliberately so: a headless Voyager is
        // not a reduced machine, it is the machine with nobody watching.
        while (machineRunning(&machine)) {
            machineRunSlice(&machine);
        }
    } else {
        // Resizable, because how big somebody wants a screen is not the machine's business.
        // The picture is rescaled to whatever the window becomes, in whole pixels.
        //
        // And presented in step with the display. Without the hint the frame limiter sleeps
        // towards sixty a second on its own clock, which beats against a screen refreshing on
        // its own - some frames shown twice, some skipped, and the machine handed an uneven
        // number of cycles each time because it takes them from the wall clock. The target
        // stays as well, for a driver that ignores the hint.
        SetConfigFlags(FLAG_WINDOW_RESIZABLE | FLAG_VSYNC_HINT);
        InitWindow(VIDEO_MAX_WIDTH * SCREEN_SCALE, VIDEO_MAX_HEIGHT * SCREEN_SCALE,
                   "Segan Voyager");
        SetTargetFPS(60);
        // ---- Escape is a byte, not a way out ----
        //
        // Raylib closes a window on Escape unless it is told not to, and this machine sends
        // Escape to the console like any other key. So a program reading keys could be
        // ended by one of them, taking whatever was in memory with it - which is a poor way
        // to find out that a default was left as it was found.
        SetExitKey(KEY_NULL);
        // One texture, updated in place. Making a new one every frame would be a new
        // allocation sixty times a second for a picture that is the same size every time.
        Image blank = GenImageColor(VIDEO_MAX_WIDTH, VIDEO_MAX_HEIGHT, BLACK);
        ImageFormat(&blank, PIXELFORMAT_UNCOMPRESSED_R8G8B8);
        screenTexture = LoadTextureFromImage(blank);
        UnloadImage(blank);
        windowOpen = 1;
        // The keyboard becomes the console's input, in place of a standard input the window
        // does not have.
        consoleSetInputHook(voyagerKey);
        // ---- A slice a frame ----
        //
        // The machine gets its turn, then the window gets its turn. Closing the window
        // stops the machine, and the machine halting leaves the window up so that whatever
        // it drew is still there to look at - a program that ends should not take its
        // output off the screen with it.
        // A slice, then a frame. The machine halting leaves the window up so that whatever
        // it drew is still there to look at - a program that ends should not take its output
        // off the screen with it.
        while (windowOpen && !WindowShouldClose()) {
            // Before the running check, not after it: a machine that has stopped is the one
            // worth restarting, and it is the one that cannot notice a reset by itself.
            machineTakeReset(&machine);
            if (machineRunning(&machine)) {
                machineRunSlice(&machine);
            }
            presentFrame();
        }
        windowOpen = 0;
        // Taken back before the machine stops, so nothing can ask a window that has gone.
        consoleSetInputHook(NULL);
        UnloadTexture(screenTexture);
        CloseWindow();
    }

    machineStop(&machine);
    return machineReport(&machine);
}