From 069f7c8b21df80a80445ffc986f1e243c72bc1cb Mon Sep 17 00:00:00 2001
From: Simon Tatham <anakin@pobox.com>
Date: Sun, 5 Mar 2023 10:01:36 +0000
Subject: [PATCH] Fix behaviour of backspace in a 1-column terminal.

This is the first bug found as a direct result of writing that
terminal test program - I added some tests for things I expected to
work already, and some of them didn't, proving immediately that it was
a good idea!

If the terminal is one column wide, and you've printed a
character (hence, set the wrapnext flag), what should backspace do?
Surely it should behave like any other backspace with wrapnext set,
i.e. clear the wrapnext flag, returning the cursor's _logical_
position to the location of the most recently printed character. But
in fact it was anti-wrapping to the previous line, because I'd got the
cases in the wrong order in the if-else chain that forms the backspace
handler. So the handler for 'we're in column 0, wrapping time' was
coming before 'wrapnext is set, just clear it'.

Now wrapnext is checked _first_, before checking anything at all. Any
time we can just clear that, we should.
---
 terminal/terminal.c  |  7 +++---
 test/test_terminal.c | 51 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/terminal/terminal.c b/terminal/terminal.c
index 002d1528..5a113027 100644
--- a/terminal/terminal.c
+++ b/terminal/terminal.c
@@ -3997,7 +3997,10 @@ static void term_out(Terminal *term, bool called_from_term_data)
                 break;
               }
               case '\b':              /* BS: Back space */
-                if (term->curs.x == 0 && (term->curs.y == 0 || !term->wrap)) {
+                if (term->wrapnext) {
+                    term->wrapnext = false;
+                } else if (term->curs.x == 0 &&
+                           (term->curs.y == 0 || !term->wrap)) {
                     /* do nothing */
                 } else if (term->curs.x == 0 && term->curs.y > 0) {
                     term->curs.x = term->cols - 1, term->curs.y--;
@@ -4021,8 +4024,6 @@ static void term_out(Terminal *term, bool called_from_term_data)
                     termline *ldata = scrlineptr(term->curs.y);
                     if (term->curs.x > 0 && (ldata->lattr & LATTR_WRAPPED2))
                         term->curs.x--;
-                } else if (term->wrapnext) {
-                    term->wrapnext = false;
                 } else {
                     term->curs.x--;
                 }
diff --git a/test/test_terminal.c b/test/test_terminal.c
index de83f6ad..5c377280 100644
--- a/test/test_terminal.c
+++ b/test/test_terminal.c
@@ -326,6 +326,57 @@ static void test_wrap(Mock *mk)
     IEQUAL(mk->term->curs.x, 78);
     IEQUAL(mk->term->curs.y, 0);
     IEQUAL(mk->term->wrapnext, 0);
+
+    /*
+     * Now test the special cases that arise when the terminal is only
+     * one column wide!
+     */
+
+    reset(mk);
+    term_size(mk->term, 24, 1, 0);
+    mk->term->curs.x = 0;
+    mk->term->curs.y = 0;
+    mk->term->wrap = true;
+    /* Printing a single-width character takes us into wrapnext immediately */
+    term_datapl(mk->term, PTRLEN_LITERAL("a"));
+    IEQUAL(mk->term->curs.x, 0);
+    IEQUAL(mk->term->curs.y, 0);
+    IEQUAL(mk->term->wrapnext, 1);
+    IEQUAL(get_lineattr(mk->term, 0), 0);
+    IEQUAL(get_termchar(mk->term, 0, 0).chr, CSET_ASCII | 'a');
+    /* Printing a second one wraps, and takes us _back_ to wrapnext */
+    term_datapl(mk->term, PTRLEN_LITERAL("b"));
+    IEQUAL(mk->term->curs.x, 0);
+    IEQUAL(mk->term->curs.y, 1);
+    IEQUAL(mk->term->wrapnext, 1);
+    IEQUAL(get_lineattr(mk->term, 0), LATTR_WRAPPED);
+    IEQUAL(get_termchar(mk->term, 0, 0).chr, CSET_ASCII | 'a');
+    IEQUAL(get_termchar(mk->term, 0, 1).chr, CSET_ASCII | 'b');
+    /* Backspacing once clears the wrapnext flag, putting us on the b */
+    term_datapl(mk->term, PTRLEN_LITERAL("\b"));
+    IEQUAL(mk->term->curs.x, 0);
+    IEQUAL(mk->term->curs.y, 1);
+    IEQUAL(mk->term->wrapnext, 0);
+    /* Backspacing again returns to the previous line, putting us on the a */
+    term_datapl(mk->term, PTRLEN_LITERAL("\b"));
+    IEQUAL(mk->term->curs.x, 0);
+    IEQUAL(mk->term->curs.y, 0);
+    IEQUAL(mk->term->wrapnext, 0);
+
+    /* And now try with a double-width character */
+    reset(mk);
+    term_size(mk->term, 24, 1, 0);
+    mk->term->curs.x = 0;
+    mk->term->curs.y = 0;
+    mk->term->wrap = true;
+    /* DW character won't fit at all, so it transforms into U+FFFD
+     * REPLACEMENT CHARACTER and then behaves like a SW char */
+    term_datapl(mk->term, PTRLEN_LITERAL("\xEA\xB0\x80"));
+    IEQUAL(mk->term->curs.x, 0);
+    IEQUAL(mk->term->curs.y, 0);
+    IEQUAL(mk->term->wrapnext, 1);
+    IEQUAL(get_lineattr(mk->term, 0), 0);
+    IEQUAL(get_termchar(mk->term, 0, 0).chr, 0xFFFD);
 }
 
 int main(void)