path: root/src/screen.cpp

// High level library for handling the terminal screen.
//
// The screen library allows the interactive reader to write its output to screen efficiently by
// keeping an internal representation of the current screen contents and trying to find the most
// efficient way for transforming that to the desired screen content.
//
// IWYU pragma: no_include <cstddef>
#include "config.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#if HAVE_NCURSES_H
#include <ncurses.h>
#elif HAVE_NCURSES_CURSES_H
#include <ncurses/curses.h>
#else
#include <curses.h>
#endif
#if HAVE_TERM_H
#include <term.h>
#elif HAVE_NCURSES_TERM_H
#include <ncurses/term.h>
#endif
#include <assert.h>
#include <time.h>
#include <wchar.h>
#include <algorithm>
#include <string>
#include <vector>

#include "common.h"
#include "env.h"
#include "fallback.h"  // IWYU pragma: keep
#include "highlight.h"
#include "output.h"
#include "pager.h"
#include "screen.h"
#include "util.h"

/// The number of characters to indent new blocks.
#define INDENT_STEP 4u

/// The initial screen width.
#define SCREEN_WIDTH_UNINITIALIZED -1

/// A helper value for an invalid location.
#define INVALID_LOCATION (screen_data_t::cursor_t(-1, -1))

static void invalidate_soft_wrap(screen_t *scr);

/// Ugly kludge. The internal buffer used to store output of tputs. Since tputs external function
/// can only take an integer and not a pointer as parameter we need a static storage buffer.
typedef std::vector<char> data_buffer_t;
static data_buffer_t *s_writeb_buffer = 0;

static int s_writeb(char character);

/// Class to temporarily set s_writeb_buffer and the writer function in a scoped way.
class scoped_buffer_t {
    data_buffer_t *const old_buff;
    int (*const old_writer)(char);

   public:
    explicit scoped_buffer_t(data_buffer_t *buff)
        : old_buff(s_writeb_buffer), old_writer(output_get_writer()) {
        s_writeb_buffer = buff;
        output_set_writer(s_writeb);
    }

    ~scoped_buffer_t() {
        s_writeb_buffer = old_buff;
        output_set_writer(old_writer);
    }
};

/// Tests if the specified narrow character sequence is present at the specified position of the
/// specified wide character string. All of \c seq must match, but str may be longer than seq.
static size_t try_sequence(const char *seq, const wchar_t *str) {
    for (size_t i = 0;; i++) {
        if (!seq[i]) return i;
        if (seq[i] != str[i]) return 0;
    }

    return 0;
}

/// Returns the number of columns left until the next tab stop, given the current cursor postion.
static size_t next_tab_stop(size_t current_line_width) {
    // Assume tab stops every 8 characters if undefined.
    size_t tab_width = init_tabs > 0 ? (size_t)init_tabs : 8;
    return ((current_line_width / tab_width) + 1) * tab_width;
}

/// Like fish_wcwidth, but returns 0 for control characters instead of -1.
static int fish_wcwidth_min_0(wchar_t widechar) { return maxi(0, fish_wcwidth(widechar)); }

/// Whether we permit soft wrapping. If so, in some cases we don't explicitly move to the second
/// physical line on a wrapped logical line; instead we just output it.
static bool allow_soft_wrap(void) {
    // Should we be looking at eat_newline_glitch as well?
    return auto_right_margin;
}

/// Does this look like the escape sequence for setting a screen name.
static bool is_screen_name_escape_seq(const wchar_t *code, size_t *resulting_length) {
    bool found = false;
    if (code[1] == L'k') {
        const env_var_t term_name = env_get_string(L"TERM");
        if (!term_name.missing() && string_prefixes_string(L"screen", term_name)) {
            const wchar_t *const screen_name_end_sentinel = L"\x1b\\";
            const wchar_t *screen_name_end = wcsstr(&code[2], screen_name_end_sentinel);
            if (screen_name_end == NULL) {
                // Consider just <esc>k to be the code.
                *resulting_length = 2;
            } else {
                const wchar_t *escape_sequence_end =
                    screen_name_end + wcslen(screen_name_end_sentinel);
                *resulting_length = escape_sequence_end - code;
            }
            found = true;
        }
    }
    return found;
}

/// iTerm2 escape codes: CSI followed by ], terminated by either BEL or escape + backslash.
/// See https://code.google.com/p/iterm2/wiki/ProprietaryEscapeCodes.
static bool is_iterm2_escape_seq(const wchar_t *code, size_t *resulting_length) {
    bool found = false;
    if (code[1] == ']') {
        // Start at 2 to skip over <esc>].
        size_t cursor = 2;
        for (; code[cursor] != L'\0'; cursor++) {
            // Consume a sequence of characters up to <esc>\ or <bel>.
            if (code[cursor] == '\x07' || (code[cursor] == '\\' && code[cursor - 1] == '\x1b')) {
                found = true;
                break;
            }
        }
        if (found) {
            *resulting_length = cursor + 1;
        }
    }
    return found;
}

/// Generic VT100 one byte sequence: CSI followed by something in the range @ through _.
static bool is_single_byte_escape_seq(const wchar_t *code, size_t *resulting_length) {
    bool found = false;
    if (code[1] == L'[' && (code[2] >= L'@' && code[2] <= L'_')) {
        *resulting_length = 3;
        found = true;
    }
    return found;
}

/// Generic VT100 two byte sequence: <esc> followed by something in the range @ through _.
static bool is_two_byte_escape_seq(const wchar_t *code, size_t *resulting_length) {
    bool found = false;
    if (code[1] >= L'@' && code[1] <= L'_') {
        *resulting_length = 2;
        found = true;
    }
    return found;
}

/// Generic VT100 CSI-style sequence. <esc>, followed by zero or more ASCII characters NOT in
/// the range [@,_], followed by one character in that range.
static bool is_csi_style_escape_seq(const wchar_t *code, size_t *resulting_length) {
    bool found = false;
    if (code[1] == L'[') {
        // Start at 2 to skip over <esc>[
        size_t cursor = 2;
        for (; code[cursor] != L'\0'; cursor++) {
            // Consume a sequence of ASCII characters not in the range [@, ~].
            wchar_t widechar = code[cursor];

            // If we're not in ASCII, just stop.
            if (widechar > 127) break;

            // If we're the end character, then consume it and then stop.
            if (widechar >= L'@' && widechar <= L'~') {
                cursor++;
                break;
            }
        }
        // curs now indexes just beyond the end of the sequence (or at the terminating zero).
        found = true;
        *resulting_length = cursor;
    }
    return found;
}

/// Returns the number of characters in the escape code starting at 'code' (which should initially
/// contain \x1b).
size_t escape_code_length(const wchar_t *code) {
    assert(code != NULL);

    // The only escape codes we recognize start with \x1b.
    if (code[0] != L'\x1b') return 0;

    size_t resulting_length = 0;
    bool found = false;

    if (cur_term != NULL) {
        // Detect these terminfo color escapes with parameter value 0..7, all of which don't move
        // the cursor.
        char *const esc[] = {
            set_a_foreground, set_a_background, set_foreground, set_background,
        };

        for (size_t p = 0; p < sizeof esc / sizeof *esc && !found; p++) {
            if (!esc[p]) continue;

            for (size_t k = 0; k < 8; k++) {
                size_t len = try_sequence(tparm(esc[p], k), code);
                if (len) {
                    resulting_length = len;
                    found = true;
                    break;
                }
            }
        }
    }

    if (cur_term != NULL) {
        // Detect these semi-common terminfo escapes without any parameter values, all of which
        // don't move the cursor.
        char *const esc2[] = {enter_bold_mode,        exit_attribute_mode,   enter_underline_mode,
                              exit_underline_mode,    enter_standout_mode,   exit_standout_mode,
                              flash_screen,           enter_subscript_mode,  exit_subscript_mode,
                              enter_superscript_mode, exit_superscript_mode, enter_blink_mode,
                              enter_italics_mode,     exit_italics_mode,     enter_reverse_mode,
                              enter_shadow_mode,      exit_shadow_mode,      enter_standout_mode,
                              exit_standout_mode,     enter_secure_mode};

        for (size_t p = 0; p < sizeof esc2 / sizeof *esc2 && !found; p++) {
            if (!esc2[p]) continue;
            // Test both padded and unpadded version, just to be safe. Most versions of tparm don't
            // actually seem to do anything these days.
            size_t len = maxi(try_sequence(tparm(esc2[p]), code), try_sequence(esc2[p], code));
            if (len) {
                resulting_length = len;
                found = true;
            }
        }
    }
    
    if (!found) found = is_screen_name_escape_seq(code, &resulting_length);
    if (!found) found = is_iterm2_escape_seq(code, &resulting_length);
    if (!found) found = is_single_byte_escape_seq(code, &resulting_length);
    if (!found) found = is_csi_style_escape_seq(code, &resulting_length);
    if (!found) is_two_byte_escape_seq(code, &resulting_length);

    return resulting_length;
}

// Information about a prompt layout.
struct prompt_layout_t {
    // How many lines the prompt consumes.
    size_t line_count;
    // Width of the longest line.
    size_t max_line_width;
    // Width of the last line.
    size_t last_line_width;
};

/// Calculate layout information for the given prompt. Does some clever magic to detect common
/// escape sequences that may be embeded in a prompt, such as color codes.
static prompt_layout_t calc_prompt_layout(const wchar_t *prompt) {
    size_t current_line_width = 0;
    size_t j;

    prompt_layout_t prompt_layout = {};
    prompt_layout.line_count = 1;

    for (j = 0; prompt[j]; j++) {
        if (prompt[j] == L'\x1b') {
            // This is the start of an escape code. Skip over it if it's at least one character
            // long.
            size_t escape_len = escape_code_length(&prompt[j]);
            if (escape_len > 0) {
                j += escape_len - 1;
            }
        } else if (prompt[j] == L'\t') {
            current_line_width = next_tab_stop(current_line_width);
        } else if (prompt[j] == L'\n' || prompt[j] == L'\f') {
            // PCA: At least one prompt uses \f\r as a newline. It's unclear to me what this is
            // meant to do, but terminals seem to treat it as a newline so we do the same.
            current_line_width = 0;
            prompt_layout.line_count += 1;
        } else if (prompt[j] == L'\r') {
            current_line_width = 0;
        } else {
            // Ordinary decent character. Just add width. This returns -1 for a control character -
            // don't add that.
            current_line_width += fish_wcwidth_min_0(prompt[j]);
            prompt_layout.max_line_width = maxi(prompt_layout.max_line_width, current_line_width);
        }
    }
    prompt_layout.last_line_width = current_line_width;
    return prompt_layout;
}

static size_t calc_prompt_lines(const wcstring &prompt) {
    // Hack for the common case where there's no newline at all. I don't know if a newline can
    // appear in an escape sequence, so if we detect a newline we have to defer to
    // calc_prompt_width_and_lines.
    size_t result = 1;
    if (prompt.find(L'\n') != wcstring::npos || prompt.find(L'\f') != wcstring::npos) {
        result = calc_prompt_layout(prompt.c_str()).line_count;
    }
    return result;
}

/// Stat stdout and stderr and save result. This should be done before calling a function that may
/// cause output.
static void s_save_status(screen_t *s) {
// PCA Let's not do this futimes stuff, because sudo dumbly uses the tty's ctime as part of its
// tty_tickets feature. Disabling this should fix issue #122.
#if 0
    // This futimes call tries to trick the system into using st_mtime as a tampering flag. This of
    // course only works on systems where futimes is defined, but it should make the status saving
    // stuff failsafe.
    struct timeval t[] = {
        { time(0)-1, 0 },
        { time(0)-1, 0 }
    };

    // Don't check return value on these. We don't care if they fail, really.  This is all just to
    // make the prompt look ok, which is impossible to do 100% reliably. We try, at least.
    futimes(1, t);
    futimes(2, t);
#endif

    fstat(1, &s->prev_buff_1);
    fstat(2, &s->prev_buff_2);
}

/// Stat stdout and stderr and compare result to previous result in reader_save_status. Repaint if
/// modification time has changed.
///
/// Unfortunately, for some reason this call seems to give a lot of false positives, at least under
/// Linux.
static void s_check_status(screen_t *s) {
    fflush(stdout);
    fflush(stderr);
    if (!has_working_tty_timestamps) {
        // We can't reliably determine if the terminal has been written to behind our back so we
        // just assume that hasn't happened and hope for the best. This is important for multi-line
        // prompts to work correctly.
        return;
    }

    fstat(1, &s->post_buff_1);
    fstat(2, &s->post_buff_2);

    bool changed = (s->prev_buff_1.st_mtime != s->post_buff_1.st_mtime) ||
                   (s->prev_buff_2.st_mtime != s->post_buff_2.st_mtime);

#if defined HAVE_STRUCT_STAT_ST_MTIMESPEC_TV_NSEC
    changed = changed ||
              s->prev_buff_1.st_mtimespec.tv_nsec != s->post_buff_1.st_mtimespec.tv_nsec ||
              s->prev_buff_2.st_mtimespec.tv_nsec != s->post_buff_2.st_mtimespec.tv_nsec;
#elif defined HAVE_STRUCT_STAT_ST_MTIM_TV_NSEC
    changed = changed || s->prev_buff_1.st_mtim.tv_nsec != s->post_buff_1.st_mtim.tv_nsec ||
              s->prev_buff_2.st_mtim.tv_nsec != s->post_buff_2.st_mtim.tv_nsec;
#endif

    if (changed) {
        // Ok, someone has been messing with our screen. We will want to repaint. However, we do not
        // know where the cursor is. It is our best bet that we are still on the same line, so we
        // move to the beginning of the line, reset the modelled screen contents, and then set the
        // modeled cursor y-pos to its earlier value.
        int prev_line = s->actual.cursor.y;
        write_loop(STDOUT_FILENO, "\r", 1);
        s_reset(s, screen_reset_current_line_and_prompt);
        s->actual.cursor.y = prev_line;
    }
}

/// Appends a character to the end of the line that the output cursor is on. This function
/// automatically handles linebreaks and lines longer than the screen width.
static void s_desired_append_char(screen_t *s, wchar_t b, int c, int indent, size_t prompt_width) {
    int line_no = s->desired.cursor.y;

    if (b == L'\n') {
        int i;
        // Current line is definitely hard wrapped.
        s->desired.create_line(s->desired.line_count());
        s->desired.line(s->desired.cursor.y).is_soft_wrapped = false;
        s->desired.cursor.y++;
        s->desired.cursor.x = 0;
        for (i = 0; i < prompt_width + indent * INDENT_STEP; i++) {
            s_desired_append_char(s, L' ', 0, indent, prompt_width);
        }
    } else if (b == L'\r') {
        line_t &current = s->desired.line(line_no);
        current.clear();
        s->desired.cursor.x = 0;
    } else {
        int screen_width = common_get_width();
        int cw = fish_wcwidth_min_0(b);

        s->desired.create_line(line_no);

        // Check if we are at the end of the line. If so, continue on the next line.
        if ((s->desired.cursor.x + cw) > screen_width) {
            // Current line is soft wrapped (assuming we support it).
            s->desired.line(s->desired.cursor.y).is_soft_wrapped = true;
            // fprintf(stderr, "\n\n1 Soft wrapping %d\n\n", s->desired.cursor.y);

            line_no = (int)s->desired.line_count();
            s->desired.add_line();
            s->desired.cursor.y++;
            s->desired.cursor.x = 0;
        }

        line_t &line = s->desired.line(line_no);
        line.append(b, c);
        s->desired.cursor.x += cw;

        // Maybe wrap the cursor to the next line, even if the line itself did not wrap. This
        // avoids wonkiness in the last column.
        if (s->desired.cursor.x >= screen_width) {
            line.is_soft_wrapped = true;
            s->desired.cursor.x = 0;
            s->desired.cursor.y++;
        }
    }
}

/// The writeb function offered to tputs.
static int s_writeb(char c) {
    s_writeb_buffer->push_back(c);
    return 0;
}

/// Write the bytes needed to move screen cursor to the specified position to the specified buffer.
/// The actual_cursor field of the specified screen_t will be updated.
///
/// \param s the screen to operate on
/// \param b the buffer to send the output escape codes to
/// \param new_x the new x position
/// \param new_y the new y position
static void s_move(screen_t *s, data_buffer_t *b, int new_x, int new_y) {
    if (s->actual.cursor.x == new_x && s->actual.cursor.y == new_y) return;

    // If we are at the end of our window, then either the cursor stuck to the edge or it didn't. We
    // don't know! We can fix it up though.
    if (s->actual.cursor.x == common_get_width()) {
        // Either issue a cr to go back to the beginning of this line, or a nl to go to the
        // beginning of the next one, depending on what we think is more efficient.
        if (new_y <= s->actual.cursor.y) {
            b->push_back('\r');
        } else {
            b->push_back('\n');
            s->actual.cursor.y++;
        }
        // Either way we're not in the first column.
        s->actual.cursor.x = 0;
    }

    int i;
    int x_steps, y_steps;

    char *str;
    /*
      debug( 0, L"move from %d %d to %d %d",
      s->screen_cursor[0], s->screen_cursor[1],
      new_x, new_y );
    */
    scoped_buffer_t scoped_buffer(b);  //!OCLINT(has side effects)

    y_steps = new_y - s->actual.cursor.y;

    if (y_steps > 0 && (strcmp(cursor_down, "\n") == 0)) {
        // This is very strange - it seems some (all?) consoles use a simple newline as the cursor
        // down escape. This will of course move the cursor to the beginning of the line as well as
        // moving it down one step. The cursor_up does not have this behaviour...
        s->actual.cursor.x = 0;
    }

    if (y_steps < 0) {
        str = cursor_up;
    } else {
        str = cursor_down;
    }

    for (i = 0; i < abs(y_steps); i++) {
        writembs(str);
    }

    x_steps = new_x - s->actual.cursor.x;

    if (x_steps && new_x == 0) {
        b->push_back('\r');
        x_steps = 0;
    }

    char *multi_str = NULL;
    if (x_steps < 0) {
        str = cursor_left;
        multi_str = parm_left_cursor;
    } else {
        str = cursor_right;
        multi_str = parm_right_cursor;
    }

    // Use the bulk ('multi') output for cursor movement if it is supported and it would be shorter
    // Note that this is required to avoid some visual glitches in iTerm (issue #1448).
    bool use_multi =
        multi_str != NULL && multi_str[0] != '\0' && abs(x_steps) * strlen(str) > strlen(multi_str);
    if (use_multi) {
        char *multi_param = tparm(multi_str, abs(x_steps));
        writembs(multi_param);
    } else {
        for (i = 0; i < abs(x_steps); i++) {
            writembs(str);
        }
    }

    s->actual.cursor.x = new_x;
    s->actual.cursor.y = new_y;
}

/// Set the pen color for the terminal.
static void s_set_color(screen_t *s, data_buffer_t *b, highlight_spec_t c) {
    scoped_buffer_t scoped_buffer(b);  //!OCLINT(has side effects)

    unsigned int uc = (unsigned int)c;
    set_color(highlight_get_color(uc & 0xffff, false),
              highlight_get_color((uc >> 16) & 0xffff, true));
}

/// Convert a wide character to a multibyte string and append it to the buffer.
static void s_write_char(screen_t *s, data_buffer_t *b, wchar_t c) {
    scoped_buffer_t scoped_buffer(b);  //!OCLINT(has side effects)
    s->actual.cursor.x += fish_wcwidth_min_0(c);
    writech(c);
    if (s->actual.cursor.x == s->actual_width && allow_soft_wrap()) {
        s->soft_wrap_location.x = 0;
        s->soft_wrap_location.y = s->actual.cursor.y + 1;

        // Note that our cursor position may be a lie: Apple Terminal makes the right cursor stick
        // to the margin, while Ubuntu makes it "go off the end" (but still doesn't wrap). We rely
        // on s_move to fix this up.
    } else {
        invalidate_soft_wrap(s);
    }
}

/// Send the specified string through tputs and append the output to the specified buffer.
static void s_write_mbs(data_buffer_t *b, char *s) {
    scoped_buffer_t scoped_buffer(b);  //!OCLINT(has side effects)
    writembs(s);
}

/// Convert a wide string to a multibyte string and append it to the buffer.
static void s_write_str(data_buffer_t *b, const wchar_t *s) {
    scoped_buffer_t scoped_buffer(b);  //!OCLINT(has side effects)
    writestr(s);
}

/// Returns the length of the "shared prefix" of the two lines, which is the run of matching text
/// and colors. If the prefix ends on a combining character, do not include the previous character
/// in the prefix.
static size_t line_shared_prefix(const line_t &a, const line_t &b) {
    size_t idx, max = std::min(a.size(), b.size());
    for (idx = 0; idx < max; idx++) {
        wchar_t ac = a.char_at(idx), bc = b.char_at(idx);
        if (fish_wcwidth(ac) < 1 || fish_wcwidth(bc) < 1) {
            // Possible combining mark, return one index prior.
            if (idx > 0) idx--;
            break;
        }

        // We're done if the text or colors are different.
        if (ac != bc || a.color_at(idx) != b.color_at(idx)) break;
    }
    return idx;
}

// We are about to output one or more characters onto the screen at the given x, y. If we are at the
// end of previous line, and the previous line is marked as soft wrapping, then tweak the screen so
// we believe we are already in the target position. This lets the terminal take care of wrapping,
// which means that if you copy and paste the text, it won't have an embedded newline.
static bool perform_any_impending_soft_wrap(screen_t *scr, int x, int y) {
    if (x == scr->soft_wrap_location.x && y == scr->soft_wrap_location.y) {  //!OCLINT
        // We can soft wrap; but do we want to?
        if (scr->desired.line(y - 1).is_soft_wrapped && allow_soft_wrap()) {
            // Yes. Just update the actual cursor; that will cause us to elide emitting the commands
            // to move here, so we will just output on "one big line" (which the terminal soft
            // wraps.
            scr->actual.cursor = scr->soft_wrap_location;
        }
    }
    return false;
}

/// Make sure we don't soft wrap.
static void invalidate_soft_wrap(screen_t *scr) { scr->soft_wrap_location = INVALID_LOCATION; }

// Various code for testing term behavior.

#if 0
static bool test_stuff(screen_t *scr)
{
    data_buffer_t output;
    scoped_buffer_t scoped_buffer(&output);

    s_move(scr, &output, 0, 0);
    int screen_width = common_get_width();

    const wchar_t *left = L"left";
    const wchar_t *right = L"right";

    for (size_t idx = 0; idx < 80; idx++)
    {
        output.push_back('A');
    }

    if (! output.empty())
    {
        write_loop(STDOUT_FILENO, &output.at(0), output.size());
        output.clear();
    }

    sleep(5);

    for (size_t i=0; i < 1; i++)
    {
        writembs(cursor_left);
    }

    if (! output.empty())
    {
        write_loop(1, &output.at(0), output.size());
        output.clear();
    }



    while (1)
    {
        int c = getchar();
        if (c != EOF) break;
    }


    while (1)
    {
        int c = getchar();
        if (c != EOF) break;
    }
    puts("Bye");
    exit(0);
    while (1) sleep(10000);
    return true;
}
#endif

/// Update the screen to match the desired output.
static void s_update(screen_t *scr, const wchar_t *left_prompt, const wchar_t *right_prompt) {
    // if (test_stuff(scr)) return;
    const size_t left_prompt_width = calc_prompt_layout(left_prompt).last_line_width;
    const size_t right_prompt_width = calc_prompt_layout(right_prompt).last_line_width;

    int screen_width = common_get_width();

    // Figure out how many following lines we need to clear (probably 0).
    size_t actual_lines_before_reset = scr->actual_lines_before_reset;
    scr->actual_lines_before_reset = 0;

    data_buffer_t output;

    bool need_clear_lines = scr->need_clear_lines;
    bool need_clear_screen = scr->need_clear_screen;
    bool has_cleared_screen = false;

    if (scr->actual_width != screen_width) {
        // Ensure we don't issue a clear screen for the very first output, to avoid issue #402.
        if (scr->actual_width != SCREEN_WIDTH_UNINITIALIZED) {
            need_clear_screen = true;
            s_move(scr, &output, 0, 0);
            s_reset(scr, screen_reset_current_line_contents);

            need_clear_lines = need_clear_lines || scr->need_clear_lines;
            need_clear_screen = need_clear_screen || scr->need_clear_screen;
        }
        scr->actual_width = screen_width;
    }

    scr->need_clear_lines = false;
    scr->need_clear_screen = false;

    // Determine how many lines have stuff on them; we need to clear lines with stuff that we don't
    // want.
    const size_t lines_with_stuff = maxi(actual_lines_before_reset, scr->actual.line_count());
#if 0
    if (lines_with_stuff > scr->desired.line_count()) {
        // There are lines that we output to previously that will need to be cleared.
        need_clear_lines = true;
    }
#endif

    if (wcscmp(left_prompt, scr->actual_left_prompt.c_str())) {
        s_move(scr, &output, 0, 0);
        s_write_str(&output, left_prompt);
        scr->actual_left_prompt = left_prompt;
        scr->actual.cursor.x = (int)left_prompt_width;
    }

    for (size_t i = 0; i < scr->desired.line_count(); i++) {
        const line_t &o_line = scr->desired.line(i);
        line_t &s_line = scr->actual.create_line(i);
        size_t start_pos = i == 0 ? left_prompt_width : 0;
        int current_width = 0;

        // If this is the last line, maybe we should clear the screen.
        const bool should_clear_screen_this_line =
            need_clear_screen && i + 1 == scr->desired.line_count() && clr_eos != NULL;

        // Note that skip_remaining is a width, not a character count.
        size_t skip_remaining = start_pos;

        if (!should_clear_screen_this_line) {
            // Compute how much we should skip. At a minimum we skip over the prompt. But also skip
            // over the shared prefix of what we want to output now, and what we output before, to
            // avoid repeatedly outputting it.
            const size_t shared_prefix = line_shared_prefix(o_line, s_line);
            if (shared_prefix > 0) {
                int prefix_width = fish_wcswidth(&o_line.text.at(0), shared_prefix);
                if (prefix_width > skip_remaining) skip_remaining = prefix_width;
            }

            // If we're soft wrapped, and if we're going to change the first character of the next
            // line, don't skip over the last two characters so that we maintain soft-wrapping.
            if (o_line.is_soft_wrapped && i + 1 < scr->desired.line_count()) {
                bool next_line_will_change = true;
                if (i + 1 < scr->actual.line_count()) {  //!OCLINT
                    if (line_shared_prefix(scr->desired.line(i + 1), scr->actual.line(i + 1)) > 0) {
                        next_line_will_change = false;
                    }
                }
                if (next_line_will_change) {
                    skip_remaining = mini(skip_remaining, (size_t)(scr->actual_width - 2));
                }
            }
        }

        // Skip over skip_remaining width worth of characters.
        size_t j = 0;
        for (; j < o_line.size(); j++) {
            int width = fish_wcwidth_min_0(o_line.char_at(j));
            if (skip_remaining < width) break;
            skip_remaining -= width;
            current_width += width;
        }

        // Skip over zero-width characters (e.g. combining marks at the end of the prompt).
        for (; j < o_line.size(); j++) {
            int width = fish_wcwidth_min_0(o_line.char_at(j));
            if (width > 0) break;
        }

        // Now actually output stuff.
        for (; j < o_line.size(); j++) {
            // If we are about to output into the last column, clear the screen first. If we clear
            // the screen after we output into the last column, it can erase the last character due
            // to the sticky right cursor. If we clear the screen too early, we can defeat soft
            // wrapping.
            if (j + 1 == screen_width && should_clear_screen_this_line && !has_cleared_screen) {
                s_move(scr, &output, current_width, (int)i);
                s_write_mbs(&output, clr_eos);
                has_cleared_screen = true;
            }

            perform_any_impending_soft_wrap(scr, current_width, (int)i);
            s_move(scr, &output, current_width, (int)i);
            s_set_color(scr, &output, o_line.color_at(j));
            s_write_char(scr, &output, o_line.char_at(j));
            current_width += fish_wcwidth_min_0(o_line.char_at(j));
        }

        // Clear the screen if we have not done so yet.
        if (should_clear_screen_this_line && !has_cleared_screen) {
            s_move(scr, &output, current_width, (int)i);
            s_write_mbs(&output, clr_eos);
            has_cleared_screen = true;
        }

        bool clear_remainder = false;
        // Clear the remainder of the line if we need to clear and if we didn't write to the end of
        // the line. If we did write to the end of the line, the "sticky right edge" (as part of
        // auto_right_margin) means that we'll be clearing the last character we wrote!
        if (has_cleared_screen) {
            // Already cleared everything.
            clear_remainder = false;
        } else if (need_clear_lines && current_width < screen_width) {
            clear_remainder = true;
        } else if (right_prompt_width < scr->last_right_prompt_width) {
            clear_remainder = true;
        } else {
            int prev_width =
                s_line.text.empty() ? 0 : fish_wcswidth(&s_line.text.at(0), s_line.text.size());
            clear_remainder = prev_width > current_width;
        }
        if (clear_remainder) {
            s_set_color(scr, &output, 0xffffffff);
            s_move(scr, &output, current_width, (int)i);
            s_write_mbs(&output, clr_eol);
        }

        // Output any rprompt if this is the first line.
        if (i == 0 && right_prompt_width > 0) {
            s_move(scr, &output, (int)(screen_width - right_prompt_width), (int)i);
            s_set_color(scr, &output, 0xffffffff);
            s_write_str(&output, right_prompt);
            scr->actual.cursor.x += right_prompt_width;

            // We output in the last column. Some terms (Linux) push the cursor further right, past
            // the window. Others make it "stick." Since we don't really know which is which, issue
            // a cr so it goes back to the left.
            //
            // However, if the user is resizing the window smaller, then it's possible the cursor
            // wrapped. If so, then a cr will go to the beginning of the following line! So instead
            // issue a bunch of "move left" commands to get back onto the line, and then jump to the
            // front of it.
            s_move(scr, &output, scr->actual.cursor.x - (int)right_prompt_width,
                   scr->actual.cursor.y);
            s_write_str(&output, L"\r");
            scr->actual.cursor.x = 0;
        }
    }

    // Clear remaining lines (if any) if we haven't cleared the screen.
    if (!has_cleared_screen && scr->desired.line_count() < lines_with_stuff) {
        s_set_color(scr, &output, 0xffffffff);
        for (size_t i = scr->desired.line_count(); i < lines_with_stuff; i++) {
            s_move(scr, &output, 0, (int)i);
            s_write_mbs(&output, clr_eol);
        }
    }

    s_move(scr, &output, scr->desired.cursor.x, scr->desired.cursor.y);
    s_set_color(scr, &output, 0xffffffff);

    if (!output.empty()) {
        write_loop(STDOUT_FILENO, &output.at(0), output.size());
    }

    // We have now synced our actual screen against our desired screen. Note that this is a big
    // assignment!
    scr->actual = scr->desired;
    scr->last_right_prompt_width = right_prompt_width;
}

/// Returns true if we are using a dumb terminal.
static bool is_dumb(void) { return !cursor_up || !cursor_down || !cursor_left || !cursor_right; }

struct screen_layout_t {
    // The left prompt that we're going to use.
    wcstring left_prompt;
    // How much space to leave for it.
    size_t left_prompt_space;
    // The right prompt.
    wcstring right_prompt;
    // The autosuggestion.
    wcstring autosuggestion;
    // Whether the prompts get their own line or not.
    bool prompts_get_own_line;
};

// Given a vector whose indexes are offsets and whose values are the widths of the string if
// truncated at that offset, return the offset that fits in the given width. Returns
// width_by_offset.size() - 1 if they all fit. The first value in width_by_offset is assumed to be
// 0.
static size_t truncation_offset_for_width(const std::vector<size_t> &width_by_offset,
                                          size_t max_width) {
    assert(!width_by_offset.empty() && width_by_offset.at(0) == 0);
    size_t i;
    for (i = 1; i < width_by_offset.size(); i++) {
        if (width_by_offset.at(i) > max_width) break;
    }
    // i is the first index that did not fit; i-1 is therefore the last that did.
    return i - 1;
}

static screen_layout_t compute_layout(screen_t *s, size_t screen_width,
                                      const wcstring &left_prompt_str,
                                      const wcstring &right_prompt_str, const wcstring &commandline,
                                      const wcstring &autosuggestion_str, const int *indent) {
    screen_layout_t result = {};

    // Start by ensuring that the prompts themselves can fit.
    const wchar_t *left_prompt = left_prompt_str.c_str();
    const wchar_t *right_prompt = right_prompt_str.c_str();
    const wchar_t *autosuggestion = autosuggestion_str.c_str();

    prompt_layout_t left_prompt_layout = calc_prompt_layout(left_prompt);
    prompt_layout_t right_prompt_layout = calc_prompt_layout(right_prompt);

    size_t left_prompt_width = left_prompt_layout.last_line_width;
    size_t right_prompt_width = right_prompt_layout.last_line_width;

    if (left_prompt_layout.max_line_width > screen_width) {
        // If we have a multi-line prompt, see if the longest line fits; if not neuter the whole
        // left prompt.
        left_prompt = L"> ";
        left_prompt_width = 2;
    }

    if (left_prompt_width + right_prompt_width >= screen_width) {
        // Nix right_prompt.
        right_prompt = L"";
        right_prompt_width = 0;
    }

    if (left_prompt_width + right_prompt_width >= screen_width) {
        // Still doesn't fit, neuter left_prompt.
        left_prompt = L"> ";
        left_prompt_width = 2;
    }

    // Now we should definitely fit.
    assert(left_prompt_width + right_prompt_width < screen_width);

    // Convert commandline to a list of lines and their widths.
    wcstring_list_t command_lines(1);
    std::vector<size_t> line_widths(1);
    for (size_t i = 0; i < commandline.size(); i++) {
        wchar_t c = commandline.at(i);
        if (c == L'\n') {
            // Make a new line.
            command_lines.push_back(wcstring());
            line_widths.push_back(indent[i] * INDENT_STEP);
        } else {
            command_lines.back() += c;
            line_widths.back() += fish_wcwidth_min_0(c);
        }
    }
    const size_t first_command_line_width = line_widths.at(0);

    // If we have more than one line, ensure we have no autosuggestion.
    if (command_lines.size() > 1) {
        autosuggestion = L"";
    }

    // Compute the width of the autosuggestion at all possible truncation offsets.
    std::vector<size_t> autosuggest_truncated_widths;
    autosuggest_truncated_widths.reserve(1 + wcslen(autosuggestion));
    size_t autosuggest_total_width = 0;
    for (size_t i = 0; autosuggestion[i] != L'\0'; i++) {
        autosuggest_truncated_widths.push_back(autosuggest_total_width);
        autosuggest_total_width += fish_wcwidth_min_0(autosuggestion[i]);
    }

    // Here are the layouts we try in turn:
    //
    // 1. Left prompt visible, right prompt visible, command line visible, autosuggestion visible.
    //
    // 2. Left prompt visible, right prompt visible, command line visible, autosuggestion truncated
    // (possibly to zero).
    //
    // 3. Left prompt visible, right prompt hidden, command line visible, autosuggestion hidden.
    //
    // 4. Newline separator (left prompt visible, right prompt hidden, command line visible,
    // autosuggestion visible).
    //
    // A remark about layout #4: if we've pushed the command line to a new line, why can't we draw
    // the right prompt? The issue is resizing: if you resize the window smaller, then the right
    // prompt will wrap to the next line. This means that we can't go back to the line that we were
    // on, and things turn to chaos very quickly.
    bool done = false;

    // Case 1
    if (!done &&
        left_prompt_width + right_prompt_width + first_command_line_width +
                autosuggest_total_width <
            screen_width) {
        result.left_prompt = left_prompt;
        result.left_prompt_space = left_prompt_width;
        result.right_prompt = right_prompt;
        result.autosuggestion = autosuggestion;
        done = true;
    }

    // Case 2. Note that we require strict inequality so that there's always at least one space
    // between the left edge and the rprompt.
    if (!done && left_prompt_width + right_prompt_width + first_command_line_width < screen_width) {
        result.left_prompt = left_prompt;
        result.left_prompt_space = left_prompt_width;
        result.right_prompt = right_prompt;

        // Need at least two characters to show an autosuggestion.
        size_t available_autosuggest_space =
            screen_width - (left_prompt_width + right_prompt_width + first_command_line_width);
        if (autosuggest_total_width > 0 && available_autosuggest_space > 2) {
            size_t truncation_offset = truncation_offset_for_width(autosuggest_truncated_widths,
                                                                   available_autosuggest_space - 2);
            result.autosuggestion = wcstring(autosuggestion, truncation_offset);
            result.autosuggestion.push_back(ellipsis_char);
        }
        done = true;
    }

    // Case 3
    if (!done && left_prompt_width + first_command_line_width < screen_width) {
        result.left_prompt = left_prompt;
        result.left_prompt_space = left_prompt_width;
        done = true;
    }

    // Case 4
    if (!done) {
        result.left_prompt = left_prompt;
        result.left_prompt_space = left_prompt_width;
        // See remark about for why we can't use the right prompt here result.right_prompt =
        // right_prompt. If the command wraps, and the prompt is not short, place the command on its
        // own line. A short prompt is 33% or less of the terminal's width.
        const size_t prompt_percent_width = (100 * left_prompt_width) / screen_width;
        if (left_prompt_width + first_command_line_width + 1 > screen_width &&
            prompt_percent_width > 33) {
            result.prompts_get_own_line = true;
        }

        done = true;
    }

    assert(done);
    return result;
}

void s_write(screen_t *s, const wcstring &left_prompt, const wcstring &right_prompt,
             const wcstring &commandline, size_t explicit_len, const highlight_spec_t *colors,
             const int *indent, size_t cursor_pos, const page_rendering_t &pager,
             bool cursor_is_within_pager) {
    screen_data_t::cursor_t cursor_arr;

    CHECK(s, );
    CHECK(indent, );

    // Turn the command line into the explicit portion and the autosuggestion.
    const wcstring explicit_command_line = commandline.substr(0, explicit_len);
    const wcstring autosuggestion = commandline.substr(explicit_len);

    // If we are using a dumb terminal, don't try any fancy stuff, just print out the text.
    // right_prompt not supported.
    if (is_dumb()) {
        const std::string prompt_narrow = wcs2string(left_prompt);
        const std::string command_line_narrow = wcs2string(explicit_command_line);

        write_loop(STDOUT_FILENO, "\r", 1);
        write_loop(STDOUT_FILENO, prompt_narrow.c_str(), prompt_narrow.size());
        write_loop(STDOUT_FILENO, command_line_narrow.c_str(), command_line_narrow.size());

        return;
    }

    s_check_status(s);
    const size_t screen_width = common_get_width();

    // Completely ignore impossibly small screens.
    if (screen_width < 4) {
        return;
    }

    // Compute a layout.
    const screen_layout_t layout = compute_layout(s, screen_width, left_prompt, right_prompt,
                                                  explicit_command_line, autosuggestion, indent);

    // Determine whether, if we have an autosuggestion, it was truncated.
    s->autosuggestion_is_truncated =
        !autosuggestion.empty() && autosuggestion != layout.autosuggestion;

    // Clear the desired screen.
    s->desired.resize(0);
    s->desired.cursor.x = s->desired.cursor.y = 0;

    // Append spaces for the left prompt.
    for (size_t i = 0; i < layout.left_prompt_space; i++) {
        s_desired_append_char(s, L' ', 0, 0, layout.left_prompt_space);
    }

    // If overflowing, give the prompt its own line to improve the situation.
    size_t first_line_prompt_space = layout.left_prompt_space;
    if (layout.prompts_get_own_line) {
        s_desired_append_char(s, L'\n', 0, 0, 0);
        first_line_prompt_space = 0;
    }

    // Reconstruct the command line.
    wcstring effective_commandline = explicit_command_line + layout.autosuggestion;

    // Output the command line.
    size_t i;
    for (i = 0; i < effective_commandline.size(); i++) {
        // Grab the current cursor's x,y position if this character matches the cursor's offset.
        if (!cursor_is_within_pager && i == cursor_pos) {
            cursor_arr = s->desired.cursor;
        }
        s_desired_append_char(s, effective_commandline.at(i), colors[i], indent[i],
                              first_line_prompt_space);
    }

    // Cursor may have been at the end too.
    if (!cursor_is_within_pager && i == cursor_pos) {
        cursor_arr = s->desired.cursor;
    }

    // Now that we've output everything, set the cursor to the position that we saved in the loop
    // above.
    s->desired.cursor = cursor_arr;

    if (cursor_is_within_pager) {
        s->desired.cursor.x = (int)cursor_pos;
        s->desired.cursor.y = (int)s->desired.line_count();
    }

    // Append pager_data (none if empty).
    s->desired.append_lines(pager.screen_data);

    s_update(s, layout.left_prompt.c_str(), layout.right_prompt.c_str());
    s_save_status(s);
}

void s_reset(screen_t *s, screen_reset_mode_t mode) {
    CHECK(s, );

    bool abandon_line = false, repaint_prompt = false, clear_to_eos = false;
    switch (mode) {
        case screen_reset_current_line_contents: {
            break;
        }
        case screen_reset_current_line_and_prompt: {
            repaint_prompt = true;
            break;
        }
        case screen_reset_abandon_line: {
            abandon_line = true;
            repaint_prompt = true;
            break;
        }
        case screen_reset_abandon_line_and_clear_to_end_of_screen: {
            abandon_line = true;
            repaint_prompt = true;
            clear_to_eos = true;
            break;
        }
    }

    // If we're abandoning the line, we must also be repainting the prompt.
    assert(!abandon_line || repaint_prompt);

    // If we are not abandoning the line, we need to remember how many lines we had output to, so we
    // can clear the remaining lines in the next call to s_update. This prevents leaving junk
    // underneath the cursor when resizing a window wider such that it reduces our desired line
    // count.
    if (!abandon_line) {
        s->actual_lines_before_reset = maxi(s->actual_lines_before_reset, s->actual.line_count());
    }

    if (repaint_prompt && !abandon_line) {
        // If the prompt is multi-line, we need to move up to the prompt's initial line. We do this
        // by lying to ourselves and claiming that we're really below what we consider "line 0"
        // (which is the last line of the prompt). This will cause us to move up to try to get back
        // to line 0, but really we're getting back to the initial line of the prompt.
        const size_t prompt_line_count = calc_prompt_lines(s->actual_left_prompt);
        assert(prompt_line_count >= 1);
        s->actual.cursor.y += (prompt_line_count - 1);
    } else if (abandon_line) {
        s->actual.cursor.y = 0;
    }

    if (repaint_prompt) s->actual_left_prompt.clear();
    s->actual.resize(0);
    s->need_clear_lines = true;
    s->need_clear_screen = s->need_clear_screen || clear_to_eos;

    if (abandon_line) {
        // Do the PROMPT_SP hack.
        int screen_width = common_get_width();
        wcstring abandon_line_string;
        abandon_line_string.reserve(screen_width + 32);  // should be enough

        // Don't need to check for fish_wcwidth errors; this is done when setting up
        // omitted_newline_char in common.cpp.
        int non_space_width = fish_wcwidth(omitted_newline_char);
        if (screen_width >= non_space_width) {
            bool has_256_colors = output_get_color_support() & color_support_term256;
            if (has_256_colors) {
                // Draw the string in term256 gray.
                abandon_line_string.append(L"\x1b[38;5;245m");
            } else {
                // Draw in "bright black" (gray).
                abandon_line_string.append(
                    L"\x1b[0m"       // bright
                    L"\x1b[30;1m");  // black
            }
            abandon_line_string.push_back(omitted_newline_char);
            abandon_line_string.append(L"\x1b[0m");  // normal text ANSI escape sequence
            abandon_line_string.append(screen_width - non_space_width, L' ');
        }
        abandon_line_string.push_back(L'\r');
        // Now we are certainly on a new line. But we may have dropped the omitted newline char on
        // it. So append enough spaces to overwrite the omitted newline char, and then clear all the
        // spaces from the new line
        abandon_line_string.append(non_space_width, L' ');
        abandon_line_string.push_back(L'\r');
        abandon_line_string.append(L"\x1b[2K");

        const std::string narrow_abandon_line_string = wcs2string(abandon_line_string);
        write_loop(STDOUT_FILENO, narrow_abandon_line_string.c_str(),
                   narrow_abandon_line_string.size());
        s->actual.cursor.x = 0;
    }

    if (!abandon_line) {
        // This should prevent resetting the cursor position during the next repaint.
        write_loop(STDOUT_FILENO, "\r", 1);
        s->actual.cursor.x = 0;
    }

    fstat(1, &s->prev_buff_1);
    fstat(2, &s->prev_buff_2);
}

bool screen_force_clear_to_end() {
    bool result = false;
    if (clr_eos) {
        data_buffer_t output;
        s_write_mbs(&output, clr_eos);
        if (!output.empty()) {
            write_loop(STDOUT_FILENO, &output.at(0), output.size());
            result = true;
        }
    }
    return result;
}

screen_t::screen_t()
    : desired(),
      actual(),
      actual_left_prompt(),
      last_right_prompt_width(),
      actual_width(SCREEN_WIDTH_UNINITIALIZED),
      soft_wrap_location(INVALID_LOCATION),
      autosuggestion_is_truncated(false),
      need_clear_lines(false),
      need_clear_screen(false),
      actual_lines_before_reset(0),
      prev_buff_1(),
      prev_buff_2(),
      post_buff_1(),
      post_buff_2() {}