SchOS/include/vt.h

// SPDX-License-Identifier: GPL-2.0-or-later
//
// Virtual terminals.
//
// Until this existed there was one console. `/dev/tty` was an alias for
// `/dev/console` with the same major and minor, `tty_fg_pgrp` was a single
// global int, and TIOCSCTTY was `return 0`. One screen, one foreground
// process group, one line discipline, no way to have a second shell.
//
// The thing that makes switching possible is a backing store. The console
// drew straight into the framebuffer and scrolled by moving pixels, which
// works exactly as long as nothing else ever wants the screen: there was
// nowhere to put terminal 2 while terminal 1 was visible, and nothing to
// repaint terminal 1 from when it came back. So each VT owns a grid of cells
// and the framebuffer shows whichever grid is active.
//
// A VT that is not active is not a special case. Writes to it land in its
// grid, its cursor moves, it scrolls; the only thing it does not do is draw.
// That is the whole switching mechanism: vt_switch repaints, and everything
// else is written as though it were always on screen.

#ifndef VT_H
#define VT_H

#include "types.h"
#include "param.h"
#include "posix.h"

// Six, and Linux's numbering: /dev/tty1 through /dev/tty6, reached with
// Alt+1..Alt+6. A seventh is deliberately not bound -- that is where a
// display server goes, and Schcom will want it.
#define NVT 6

// A cell.
//
// `ch` is the glyph the font draws, not a codepoint: the console maps UTF-8
// to CP437 before it gets here, because that is what include/font.h contains.
// Storing the codepoint would mean re-running that mapping on every repaint
// and getting a different answer for anything the font cannot draw.
//
// The colours are palette indices rather than RGB. Sixteen ANSI colours plus
// the two theme defaults fit in a byte each and make a cell four bytes; the
// same cell holding two 32-bit colours would be twelve, and the grids are
// static.
struct vtcell {
  // A Unicode codepoint, not a byte.
  //
  // This was a uint8, which is to say the grid could hold 256 distinct glyphs
  // and every terminal was a CP437 terminal. A codepoint outside that was not
  // stored wrongly, it was *converted* -- `glyph_for()` mapped what it could
  // and turned the rest into `?`. Ą, Ę, Ś, Ω, Œ, Ə, ∞ and every other
  // character anybody actually types were question marks before they reached
  // the font, so no font could have fixed it.
  //
  // Four bytes a cell instead of one. A 1920x1080 screen at 9x18 is 12,720
  // cells, so the grid costs 100 KB instead of 50 KB per terminal.
  uint32 ch;
  uint8 fg;
  uint8 bg;
  uint8 attr;         // reserved: bold/underline/reverse, which do not change
  uint8 pad;          // the glyph and so can be added without touching a grid
};

#define VT_FG_DEFAULT  16       // SCH_FG from include/theme.h
#define VT_BG_DEFAULT  17       // SCH_BG
#define VT_NCOLOUR     18

// Cell attributes that do not change which glyph is drawn, only how.
#define VT_ATTR_REVERSE 0x01

// The largest console a grid can hold, which is 1920x1200 with the 8x16 font.
// A framebuffer wider or taller than this still works -- vt_init clamps the
// grid and the rest of the screen stays background -- but it is worth raising
// rather than living with, because the clamp is invisible until someone
// notices the bottom of their screen never has text on it.
#define VT_MAXCOLS 240
#define VT_MAXROWS 75

#define VT_INPUT_BUF 256
#define VT_MAXPARM   8

// Escape parser states. E_OSC and E_OSC_ESC exist for hyperlinks: a terminal
// that does not follow OSC 8 must still consume it, or the URL is printed as
// text. See the comment in kernel/core/console.c that this came from.
enum { VE_NORMAL, VE_ESC, VE_CSI, VE_OSC, VE_OSC_ESC };

struct vt {
  int  index;                       // 0-based; /dev/tty<index+1>
  int  cols, rows;

  struct vtcell *cells;             // cols*rows, row-major, into vt_storage

  int  cx, cy;                      // cursor, in cells
  // Deferred wrap, the rule every real terminal follows and none of them
  // document well: a glyph printed in the last column does NOT move the
  // cursor to the next line. It sets this instead, and the wrap happens when
  // the *next* glyph arrives. Anything that moves the cursor deliberately --
  // CR, LF, backspace, an escape sequence -- clears it.
  int  wrap_pending;
  int  saved_cx, saved_cy;          // DECSC/DECRC

  uint8 fg, bg;                     // current SGR colours
  int   bold;
  int   attr;                       // VT_ATTR_* applied to new cells

  // Escape parser.
  int  esc_state;
  int  esc_parm[VT_MAXPARM];
  int  esc_nparm;
  int  esc_private;

  // UTF-8 decoder.
  uint32 u8_cp;
  int    u8_need;

  // Line discipline. One per terminal, because ICANON and ECHO are properties
  // of a terminal and not of the machine.
  struct termios tio;

  struct {
    char buf[VT_INPUT_BUF];
    uint r, w, e;
    uint abort_read;
    uint eof;
  } in;

  // Job control. This was one global for the whole system; a terminal each
  // is the entire point of having more than one.
  int fg_pgrp;
  int sid;                          // session that owns this terminal, 0 = none

  int opened;                       // times it has been opened, for /dev/tty

  // VT_AUTO or VT_PROCESS. In VT_PROCESS a program has claimed switching and
  // the kernel stops acting on the chord -- see vt_kernel_switches().
  int mode;
};

// The active terminal, and the one the framebuffer is showing.
extern int vt_active;

void          vt_init(void);
void          vt_resize(void);            // a scanout arrived; re-measure
struct vt    *vt_get(int i);              // 0-based, clamped, never null
struct vt    *vt_current(void);
int           vt_switch(int i);           // 0-based; repaints. -1 if out of range
int           vt_cycle(int dir);          // -1 previous, +1 next, in-use only

// Output, per terminal. Drawing happens only if `v` is the active one.
void vt_feed(struct vt *v, int c);        // one output byte, through the parser
void vt_putc(struct vt *v, int glyph);    // one glyph, already mapped
void vt_cursor_set(struct vt *v, int col, int row);
void vt_cursor_get(struct vt *v, int *col, int *row);
void vt_erase_display(struct vt *v, int mode);
void vt_erase_line(struct vt *v, int mode);
void vt_clear(struct vt *v);
void vt_scroll(struct vt *v);
void vt_repaint(struct vt *v);            // grid -> framebuffer, if active

uint32 vt_rgb(uint8 idx);                 // palette index -> RGB
int    vt_index(struct vt *v);            // 0-based number of a terminal
int    vt_kernel_switches(void);          // 0 if a program owns switching

#endif // VT_H