SchOS/kernel/dev/vt.c

// SPDX-License-Identifier: Apache-2.0
//
// Virtual terminals. See include/vt.h for why this exists at all; the short
// version is that switching needs a backing store, and the console drew
// straight into the framebuffer.
//
// The rule this file is built on: **a terminal does not know whether it is
// visible.** Writes land in the grid, the cursor moves, the grid scrolls --
// all of it unconditionally. Drawing is a separate question asked once, at
// the bottom, and answered by comparing against vt_active. Every alternative
// spreads "am I on screen" through the escape parser, and then a terminal
// that was switched away mid-sequence comes back wrong.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "posix.h"
#include "theme.h"
#include "gfx.h"
#include "vt.h"

int vt_active = 0;

// The grids. Static rather than allocated: there is no contiguous multi-page
// allocator, a grid is 72 KB, and six of them are a fixed cost the kernel can
// pay once at link time instead of failing to pay at run time.
static struct vtcell vt_storage[NVT][VT_MAXCOLS * VT_MAXROWS];
static struct vt     vts[NVT];
static int           vt_ready;

// The sixteen ANSI colours, then the two theme defaults. Moved here from
// console.c, where they were the only palette because there was only one
// terminal to paint.
static const uint32 palette[VT_NCOLOUR] = {
  0x000000, 0xAA0000, 0x00AA00, 0xAA5500,   // black red green yellow(brown)
  0x0000AA, 0xAA00AA, 0x00AAAA, 0xAAAAAA,   // blue magenta cyan white
  0x555555, 0xFF5555, 0x55FF55, 0xFFFF55,   // bright versions
  0x5555FF, 0xFF55FF, 0x55FFFF, 0xFFFFFF,
  SCH_FG,                                    // VT_FG_DEFAULT
  SCH_BG,                                    // VT_BG_DEFAULT
};

uint32
vt_rgb(uint8 idx)
{
  return palette[idx < VT_NCOLOUR ? idx : VT_FG_DEFAULT];
}

static struct vtcell *
cell_at(struct vt *v, int col, int row)
{
  return &v->cells[row * v->cols + col];
}

static int
is_active(struct vt *v)
{
  return vt_ready && v->index == vt_active;
}

// Paint one cell from the grid. The single place that turns stored state into
// pixels, so anything that changes a cell can simply call this and not think
// about whether the terminal is visible.
static void
show(struct vt *v, int col, int row)
{
  if (!is_active(v))
    return;
  struct vtcell *k = cell_at(v, col, row);
  // Reverse video swaps the two colours at draw time rather than at store
  // time, so turning it off restores what was underneath instead of leaving
  // the swapped pair behind.
  if (k->attr & VT_ATTR_REVERSE)
    gfx_draw_cell(col, row, k->ch, vt_rgb(k->bg), vt_rgb(k->fg));
  else
    gfx_draw_cell(col, row, k->ch, vt_rgb(k->fg), vt_rgb(k->bg));
}

static void
blank_row(struct vt *v, int row)
{
  for (int c = 0; c < v->cols; c++) {
    struct vtcell *k = cell_at(v, c, row);
    k->ch = ' ';
    k->fg = v->fg;
    k->bg = v->bg;
    k->attr = 0;
  }
}

// ---------------------------------------------------------------------------
// Setup
// ---------------------------------------------------------------------------

// The termios every terminal starts with. This was a single file-static in
// console.c called `tio`; it is per-terminal now because ICANON and ECHO
// describe a terminal, and a program that turns off echo on tty2 has said
// nothing whatever about tty1.
static void
termios_defaults(struct termios *t)
{
  memset(t, 0, sizeof(*t));
  t->c_iflag = ICRNL;
  t->c_oflag = OPOST | ONLCR;
  t->c_cflag = CS8 | CREAD;
  t->c_lflag = ISIG | ICANON | ECHO | ECHOE;
  t->c_cc[VINTR]  = 'C'  & 0x1f;
  t->c_cc[VQUIT]  = '\\' & 0x1f;
  t->c_cc[VERASE] = '\x7f';
  t->c_cc[VKILL]  = 'U'  & 0x1f;
  t->c_cc[VEOF]   = 'D'  & 0x1f;
  t->c_cc[VSUSP]  = 'Z'  & 0x1f;
  t->c_cc[VWERASE]= 'W'  & 0x1f;
  t->c_cc[VMIN]   = 1;
  t->c_ispeed = 15;
  t->c_ospeed = 15;
}

// The screen size, clamped to what a grid can hold.
//
// 80x24 when there is no framebuffer yet -- which is the usual case when
// vt_init runs, because the display driver is found on the PCI bus and the
// terminals are set up before the bus is walked. vt_resize() corrects it the
// moment a scanout arrives.
static void
measure(int *out_cols, int *out_rows)
{
  int cols = gfx_cols(), rows = gfx_rows();
  if (cols <= 0) cols = 80;
  if (rows <= 0) rows = 24;
  if (cols > VT_MAXCOLS) cols = VT_MAXCOLS;
  if (rows > VT_MAXROWS) rows = VT_MAXROWS;
  *out_cols = cols;
  *out_rows = rows;
}

void
vt_init(void)
{
  int cols, rows;
  measure(&cols, &rows);

  for (int i = 0; i < NVT; i++) {
    struct vt *v = &vts[i];
    memset(v, 0, sizeof(*v));
    v->index = i;
    v->cols  = cols;
    v->rows  = rows;
    v->cells = vt_storage[i];
    v->fg    = VT_FG_DEFAULT;
    v->bg    = VT_BG_DEFAULT;
    v->esc_state = VE_NORMAL;
    // pgid 1, which is init's, so the first shell on each terminal already
    // sees itself as the foreground job rather than stopping with SIGTTIN.
    v->fg_pgrp = 1;
    termios_defaults(&v->tio);
    for (int r = 0; r < rows; r++)
      blank_row(v, r);
  }

  vt_active = 0;
  vt_ready  = 1;
}

// The real screen has arrived. Re-measure and re-lay the grids.
//
// This exists because of the order the kernel comes up in: terminals are
// initialised before the PCI bus is walked, so the display driver attaches its
// scanout *after* vt_init has already sized every grid from a framebuffer that
// was not there yet. Without this the grids stayed 80x24 on a 160x50 screen --
// text stopped two thirds of the way down, `ls` formatted for a width the grid
// did not have and split words across the wrap, and zsh's cursor tracking was
// wrong on every line.
//
// Content is not preserved. Reflowing a grid into a different width is a real
// problem and there is nothing here worth reflowing: this runs during boot,
// before console_takeover(), when every grid is still blank.
void
vt_resize(void)
{
  int cols, rows;

  if (!vt_ready)
    return;                      // vt_init has not run; it will measure itself
  measure(&cols, &rows);
  if (cols == vts[0].cols && rows == vts[0].rows)
    return;                      // same screen, nothing to do

  for (int i = 0; i < NVT; i++) {
    struct vt *v = &vts[i];
    v->cols = cols;
    v->rows = rows;
    if (v->cx >= cols) v->cx = cols - 1;
    if (v->cy >= rows) v->cy = rows - 1;
    for (int r = 0; r < rows; r++)
      blank_row(v, r);
  }
  vt_repaint(vt_current());
}

struct vt *
vt_get(int i)
{
  if (i < 0)    i = 0;
  if (i >= NVT) i = NVT - 1;
  return &vts[i];
}

struct vt *
vt_current(void)
{
  return &vts[vt_active];
}

// ---------------------------------------------------------------------------
// Output
// ---------------------------------------------------------------------------

void
vt_repaint(struct vt *v)
{
  if (!is_active(v))
    return;
  for (int r = 0; r < v->rows; r++)
    for (int c = 0; c < v->cols; c++) {
      struct vtcell *k = cell_at(v, c, r);
      if (k->attr & VT_ATTR_REVERSE)
        gfx_draw_cell(c, r, k->ch, vt_rgb(k->bg), vt_rgb(k->fg));
      else
        gfx_draw_cell(c, r, k->ch, vt_rgb(k->fg), vt_rgb(k->bg));
    }
}

void
vt_scroll(struct vt *v)
{
  // The grid moves whether or not anyone is watching. If they are, the
  // framebuffer is scrolled with a memmove rather than repainted cell by
  // cell -- a full repaint of a 160x50 screen is a million pixel writes, and
  // a shell printing a long file would spend all its time in this function.
  memmove(v->cells, cell_at(v, 0, 1),
          (uint64)(v->rows - 1) * v->cols * sizeof(struct vtcell));
  blank_row(v, v->rows - 1);

  if (is_active(v)) {
    gfx_scroll_up(vt_rgb(v->bg));
    // The exposed row was blanked in v->bg; gfx_scroll_up painted it the same
    // colour, so there is nothing further to draw.
  }

  if (v->cy > 0)
    v->cy--;
}

void
vt_cursor_set(struct vt *v, int col, int row)
{
  // Clamped rather than validated: an escape sequence is untrusted input from
  // any program that can write to the terminal, and "CUP to row 9999" should
  // land on the last row rather than off the end of the grid.
  if (col < 0) col = 0;
  if (row < 0) row = 0;
  if (col >= v->cols) col = v->cols - 1;
  if (row >= v->rows) row = v->rows - 1;
  v->cx = col;
  v->cy = row;
  v->wrap_pending = 0;     // an explicit move is not a wrap
}

void
vt_cursor_get(struct vt *v, int *col, int *row)
{
  if (col) *col = v->cx;
  if (row) *row = v->cy;
}

static void
put_blank_at(struct vt *v, int col, int row)
{
  struct vtcell *k = cell_at(v, col, row);
  k->ch = ' ';
  k->fg = v->fg;
  k->bg = v->bg;
  k->attr = 0;
  show(v, col, row);
}

// Take the deferred wrap, if one is owed. Called before anything that needs
// the cursor to be at a real position.
static void
wrap_now(struct vt *v)
{
  if (!v->wrap_pending)
    return;
  v->wrap_pending = 0;
  v->cx = 0;
  v->cy++;
  while (v->cy >= v->rows)
    vt_scroll(v);
}

void
vt_putc(struct vt *v, int glyph)
{
  switch (glyph) {
  case '\n':
    v->wrap_pending = 0;
    v->cx = 0;
    v->cy++;
    break;

  case '\r':
    v->wrap_pending = 0;
    v->cx = 0;
    break;

  case '\b':
  case 0x7f:
    v->wrap_pending = 0;
    if (v->cx > 0) {
      v->cx--;
      put_blank_at(v, v->cx, v->cy);
    }
    break;

  case '\t':
    // To the next multiple of eight, blanking as it goes so a tab overwrites
    // rather than skipping over stale characters. Without a case here '\t'
    // reaches the font as glyph 9, which in CP437 is a hollow circle: pressing
    // Tab printed a circle.
    wrap_now(v);
    do {
      put_blank_at(v, v->cx, v->cy);
      v->cx++;
    } while (v->cx % 8 != 0 && v->cx < v->cols);
    if (v->cx >= v->cols) { v->cx = v->cols - 1; v->wrap_pending = 1; }
    break;

  default: {
    // A C0 control character that got this far has no handled meaning, and a
    // terminal never *draws* one. The font has a picture for every byte below
    // 0x20 and none of those pictures is what the sender meant: zsh emits
    // 0x0F to leave alt-charset mode, because that is what the `linux`
    // terminfo says rmacs is, and CP437 glyph 0x0F is a sun. It appeared in
    // the prompt on every line.
    if (glyph < 0x20)
      return;
    if (glyph < 0 || glyph > 255)
      glyph = '?';
    wrap_now(v);                     // the previous glyph filled the line
    struct vtcell *k = cell_at(v, v->cx, v->cy);
    k->ch = (uint8)glyph;
    k->fg = v->fg;
    k->bg = v->bg;
    k->attr = (uint8)v->attr;
    show(v, v->cx, v->cy);
    // Filling the last column does not move to the next line. It owes a wrap,
    // and pays it when the next glyph arrives -- which is what makes a line
    // that is exactly as wide as the screen take one row instead of two, and
    // what lets a program write to the bottom-right cell without scrolling.
    if (v->cx + 1 >= v->cols)
      v->wrap_pending = 1;
    else
      v->cx++;
    break;
  }
  }

  while (v->cy >= v->rows)
    vt_scroll(v);
}

// Fill a run of cells with the current background, wrapping at the right edge.
static void
erase_run(struct vt *v, int col, int row, int count)
{
  while (count-- > 0 && row < v->rows) {
    put_blank_at(v, col, row);
    if (++col >= v->cols) { col = 0; row++; }
  }
}

// ED -- erase in display. 0: cursor to end, 1: start to cursor, 2: all.
void
vt_erase_display(struct vt *v, int mode)
{
  switch (mode) {
  case 0:  erase_run(v, v->cx, v->cy, (v->rows - v->cy) * v->cols - v->cx); break;
  case 1:  erase_run(v, 0, 0, v->cy * v->cols + v->cx + 1);                 break;
  default: vt_clear(v);                                                     break;
  }
}

// EL -- erase in line. The same three modes, bounded to the current row.
void
vt_erase_line(struct vt *v, int mode)
{
  switch (mode) {
  case 0:  erase_run(v, v->cx, v->cy, v->cols - v->cx); break;
  case 1:  erase_run(v, 0, v->cy, v->cx + 1);           break;
  default: erase_run(v, 0, v->cy, v->cols);             break;
  }
}

void
vt_clear(struct vt *v)
{
  for (int r = 0; r < v->rows; r++)
    blank_row(v, r);
  v->cx = v->cy = 0;
  v->wrap_pending = 0;
  vt_repaint(v);
}


// ---------------------------------------------------------------------------
// The escape parser
//
// Moved here from console.c along with the state it needs. It was a set of
// file-statics there -- one parser for one screen -- and a parser is per
// terminal for the same reason a cursor is: a program halfway through a CSI
// sequence on tty2 has said nothing about tty1.
//
// The serial line does NOT go through any of this. On the other end of it is
// a real terminal emulator that understands ANSI far better than this ever
// will, so escapes are passed to it verbatim and interpreted only for the
// framebuffer. Two consumers, two right answers.
// ---------------------------------------------------------------------------

// ---------------------------------------------------------------------------
// Unicode -> glyph
//
// The font's upper half is CP437, not Latin-1, so a codepoint cannot be used
// as an index. It looks like it can -- font.h's own comments say so, labelling
// each glyph with the Unicode character of the same number -- but the bitmaps
// disagree: index 0xE9 draws a capital theta, and rendering U+00E9 there turns
// "cafe" into "caf" followed by a theta. Every accented character came out as
// an unrelated symbol, consistently enough to look almost deliberate.
//
// So: a table. Entry i covers codepoint 0x80 + i, and 0 means "this font has
// no glyph for that". The C1 range (0x80-0x9F) has no printable characters at
// all, hence the leading run of zeros.
// ---------------------------------------------------------------------------
static const uchar latin1_to_cp437[128] = {
  /* 0x80-0x9F: C1 controls, nothing to draw */
  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,
  0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,
  /* 0xA0 */ 0x20, 0xAD, 0x9B, 0x9C, 0,    0x9D, 0,    0x15,
  /* 0xA8 */ 0,    0,    0xA6, 0xAE, 0xAA, 0x2D, 0,    0,
  /* 0xB0 */ 0xF8, 0xF1, 0xFD, 0,    0,    0xE6, 0x14, 0xFA,
  /* 0xB8 */ 0,    0,    0xA7, 0xAF, 0xAC, 0xAB, 0,    0xA8,
  /* 0xC0 */ 0,    0,    0,    0,    0x8E, 0x8F, 0x92, 0x80,
  /* 0xC8 */ 0,    0x90, 0,    0,    0,    0,    0,    0,
  /* 0xD0 */ 0,    0xA5, 0,    0,    0,    0,    0x99, 0,
  /* 0xD8 */ 0,    0,    0,    0,    0x9A, 0,    0,    0xE1,
  /* 0xE0 */ 0x85, 0xA0, 0x83, 0,    0x84, 0x86, 0x91, 0x87,
  /* 0xE8 */ 0x8A, 0x82, 0x88, 0x89, 0x8D, 0xA1, 0x8C, 0x8B,
  /* 0xF0 */ 0,    0xA4, 0x95, 0xA2, 0x93, 0,    0x94, 0xF6,
  /* 0xF8 */ 0,    0x97, 0xA3, 0x96, 0x81, 0,    0,    0x98,
};

// The handful of characters above U+00FF that CP437 does have. Box drawing
// and block elements matter because they are what every text-mode program
// draws frames and progress bars out of.
struct cp_pair { uint32 cp; uchar glyph; };
static const struct cp_pair cp437_extra[] = {
  { 0x2022, 0x07 },                                   /* bullet */
  { 0x2500, 0xC4 }, { 0x2502, 0xB3 },                 /* light horizontal/vertical */
  { 0x250C, 0xDA }, { 0x2510, 0xBF },
  { 0x2514, 0xC0 }, { 0x2518, 0xD9 },
  { 0x251C, 0xC3 }, { 0x2524, 0xB4 },
  { 0x252C, 0xC2 }, { 0x2534, 0xC1 }, { 0x253C, 0xC5 },
  { 0x2550, 0xCD }, { 0x2551, 0xBA },                 /* double */
  { 0x2554, 0xC9 }, { 0x2557, 0xBB },
  { 0x255A, 0xC8 }, { 0x255D, 0xBC },
  { 0x2580, 0xDF }, { 0x2584, 0xDC }, { 0x2588, 0xDB },
  { 0x258C, 0xDD }, { 0x2590, 0xDE },
  { 0x2591, 0xB0 }, { 0x2592, 0xB1 }, { 0x2593, 0xB2 },
  { 0x25A0, 0xFE },                                   /* black square */
  { 0x221A, 0xFB }, { 0x221E, 0xEC },                 /* root, infinity */
  { 0x2260, 0,    }, { 0x2264, 0xF3 }, { 0x2265, 0xF2 },
  { 0x2248, 0xF7 }, { 0x2261, 0xF0 },
};

// Returns the glyph to draw for a codepoint, or 0 if there is none.
static int
glyph_for(uint32 cp)
{
  if (cp < 0x80)
    return (int)cp;                       // ASCII is CP437's lower half
  if (cp < 0x100)
    return latin1_to_cp437[cp - 0x80];

  for (unsigned i = 0; i < NELEM(cp437_extra); i++)
    if (cp437_extra[i].cp == cp)
      return cp437_extra[i].glyph;
  return 0;
}


// SGR. Simpler than it was, because a cell stores a palette index rather than
// an RGB value: "bright" is +8 and "default" is a named index, where before
// both had to be recovered by searching the palette for a matching colour.
static void
vt_sgr(struct vt *v)
{
  // "ESC[m" with no parameters means ESC[0m -- reset -- and ECMA-48 says so
  // for every CSI: an omitted parameter takes its default, and SGR's is 0.
  //
  // This loop used to start at a count of zero and therefore do nothing, so a
  // bare reset was silently dropped. Programs emit it constantly: fastfetch
  // writes "\x1b[32m1%\x1b[m", and with the reset ignored everything after
  // the first coloured character stayed green forever. Worse with background
  // colours -- its palette swatch sets a white background and resets, so the
  // whole console went white-on-white and stayed there, which looks like a
  // corrupted framebuffer rather than a missing branch.
  int nparm = v->esc_nparm ? v->esc_nparm : 1;   // esc_parm[0] is already 0

  for (int i = 0; i < nparm; i++) {
    int p = v->esc_parm[i];

    if (p == 0) {
      v->fg = VT_FG_DEFAULT;
      v->bg = VT_BG_DEFAULT;
      v->bold = 0;
      v->attr = 0;
    } else if (p == 1) {
      v->bold = 1;
      if (v->fg < 8) v->fg += 8;             // brighten a colour already chosen
    } else if (p == 7) {
      v->attr |= VT_ATTR_REVERSE;
    } else if (p == 27) {
      v->attr &= ~VT_ATTR_REVERSE;
    } else if (p == 22) {
      v->bold = 0;
      if (v->fg >= 8 && v->fg < 16) v->fg -= 8;
    } else if (p >= 30 && p <= 37) {
      v->fg = (uint8)((p - 30) + (v->bold ? 8 : 0));
    } else if (p == 39) {
      v->fg = VT_FG_DEFAULT;
    } else if (p >= 40 && p <= 47) {
      v->bg = (uint8)(p - 40);
    } else if (p == 49) {
      v->bg = VT_BG_DEFAULT;
    } else if (p >= 90 && p <= 97) {
      v->fg = (uint8)((p - 90) + 8);
    } else if (p >= 100 && p <= 107) {
      v->bg = (uint8)((p - 100) + 8);
    }
  }
}

// One final byte of a CSI sequence, with esc_parm[] already filled in.
static void
vt_csi(struct vt *v, int final)
{
  int col = v->cx, row = v->cy;
  int n  = v->esc_nparm > 0 ? v->esc_parm[0] : 0;
  int n1 = n ? n : 1;                        // most sequences default to 1

  switch (final) {
  case 'A': vt_cursor_set(v, col, row - n1); break;
  case 'B': vt_cursor_set(v, col, row + n1); break;
  case 'C': vt_cursor_set(v, col + n1, row); break;
  case 'D': vt_cursor_set(v, col - n1, row); break;
  case 'E': vt_cursor_set(v, 0, row + n1);   break;
  case 'F': vt_cursor_set(v, 0, row - n1);   break;
  case 'G': vt_cursor_set(v, n1 - 1, row);   break;
  case 'd': vt_cursor_set(v, col, n1 - 1);   break;

  case 'H': case 'f': {
    int r = v->esc_nparm > 0 && v->esc_parm[0] ? v->esc_parm[0] : 1;
    int c = v->esc_nparm > 1 && v->esc_parm[1] ? v->esc_parm[1] : 1;
    vt_cursor_set(v, c - 1, r - 1);          // ANSI counts from 1
    break;
  }

  case 'J': vt_erase_display(v, n); break;
  case 'K': vt_erase_line(v, n);    break;

  case 'm': vt_sgr(v); break;

  case 's': v->saved_cx = col; v->saved_cy = row;      break;
  case 'u': vt_cursor_set(v, v->saved_cx, v->saved_cy); break;

  // Cursor visibility (DECTCEM) and the other private modes. There is no
  // hardware cursor being drawn, so there is nothing to show or hide --
  // swallowing them silently is correct, and far better than printing
  // "?25l" into the middle of someone's output.
  case 'h': case 'l': break;

  default: break;                            // unknown: drop it, do not print
  }
}

// Feed one output byte to a terminal. The serial side has already had the
// raw byte; this is the framebuffer's interpretation of it.
void
vt_feed(struct vt *v, int c)
{
  switch (v->esc_state) {
  case VE_ESC:
    if (c == '[')      { v->esc_state = VE_CSI;
                         v->esc_nparm = 0; v->esc_private = 0;
                         for (int i = 0; i < VT_MAXPARM; i++) v->esc_parm[i] = 0;
                         return; }
    if (c == ']')      { v->esc_state = VE_OSC; return; }
    v->esc_state = VE_NORMAL;
    return;

  case VE_OSC:
    // Consume to BEL or to ESC \. A terminal that does not follow OSC 8 must
    // still swallow it, or the URL is printed as text -- which is what
    // "Disk (]8;;file:///\]8;;\): 4.6 MiB" was: fastfetch wrapping a path in
    // a hyperlink, and this console spelling it out.
    if (c == 0x07)     { v->esc_state = VE_NORMAL; return; }
    if (c == 0x1b)     { v->esc_state = VE_OSC_ESC; return; }
    return;

  case VE_OSC_ESC:
    v->esc_state = (c == '\\') ? VE_NORMAL : VE_OSC;
    return;

  case VE_CSI:
    if (c == '?') { v->esc_private = 1; return; }
    if (c >= '0' && c <= '9') {
      if (v->esc_nparm == 0) v->esc_nparm = 1;
      if (v->esc_nparm <= VT_MAXPARM)
        v->esc_parm[v->esc_nparm - 1] = v->esc_parm[v->esc_nparm - 1] * 10 + (c - '0');
      return;
    }
    if (c == ';') {
      if (v->esc_nparm < VT_MAXPARM) v->esc_nparm++;
      return;
    }
    v->esc_state = VE_NORMAL;
    if (!v->esc_private)
      vt_csi(v, c);
    return;
  }

  // VE_NORMAL
  if (c == 0x1b) { v->esc_state = VE_ESC; return; }

  // UTF-8 continuation. A codepoint the font cannot draw becomes '?' -- the
  // one thing it must not become is a *different* character, which is exactly
  // what indexing the font by codepoint did.
  if (v->u8_need > 0) {
    if ((c & 0xC0) != 0x80) {                // malformed: abandon the sequence
      v->u8_need = 0;
      vt_putc(v, '?');
      return;
    }
    v->u8_cp = (v->u8_cp << 6) | (c & 0x3F);
    if (--v->u8_need > 0)
      return;
    int g = glyph_for(v->u8_cp);
    vt_putc(v, g ? g : '?');
    return;
  }

  if (c >= 0xF0)      { v->u8_cp = c & 0x07; v->u8_need = 3; return; }
  else if (c >= 0xE0) { v->u8_cp = c & 0x0F; v->u8_need = 2; return; }
  else if (c >= 0xC0) { v->u8_cp = c & 0x1F; v->u8_need = 1; return; }
  else if (c >= 0x80) { vt_putc(v, '?'); return; }   // stray continuation

  vt_putc(v, c);
}

// ---------------------------------------------------------------------------
// Switching
// ---------------------------------------------------------------------------

int
vt_switch(int i)
{
  if (i < 0 || i >= NVT)
    return -1;
  if (!vt_ready)
    return -1;
  if (i == vt_active)
    return 0;

  vt_active = i;

  // Repaint from the grid. Nothing was saved on the way out because nothing
  // needed to be: the outgoing terminal's grid has been current all along,
  // which is the whole reason the grid exists.
  vt_repaint(&vts[i]);
  return 0;
}

int
vt_index(struct vt *v)
{
  return v ? v->index : 0;
}

// May the kernel act on the switch chord?
//
// Not if the terminal on screen is in VT_PROCESS: a program has said it will
// handle switching itself, and the whole point of saying so is that the chord
// belongs to it now. This is the documented Unix answer to "my application
// wants that key", and it is the one a display server will use -- Schcom, on
// the terminal it owns, so that Ctrl+Alt+Shift+F2 reaches its own compositor
// bindings instead of yanking the screen out from under it.
//
// What this does *not* do yet: deliver relsig/acqsig and wait for VT_RELDISP.
// Linux signals the owning process on every switch request and blocks until it
// answers, which is how a display server gets to release the GPU first. There
// is nothing here that needs to release anything, so the kernel simply stops
// switching rather than negotiating. When Schcom needs the handshake, this is
// where it goes, and struct vt_mode already carries the fields for it.
int
vt_kernel_switches(void)
{
  return vt_current()->mode != VT_PROCESS;
}

// Step to the previous or next terminal that is actually in use.
//
// Skipping the idle ones matters: six terminals with two shells on them means
// four blank screens between the things you wanted, and cycling past them is
// worse than not having the key. "In use" is `opened`, the same count
// VT_GETSTATE reports, so what the key does and what `chvt` says are the same
// fact.
//
// If nothing else is open this stays put rather than wrapping to a blank one.
int
vt_cycle(int dir)
{
  if (!vt_ready)
    return -1;

  for (int step = 1; step <= NVT; step++) {
    int i = vt_active + dir * step;
    while (i < 0)    i += NVT;
    while (i >= NVT) i -= NVT;
    if (i == vt_active)
      break;
    if (vts[i].opened > 0)
      return vt_switch(i);
  }
  return 0;                      // nowhere else to go; not an error
}