From 6e091776f8ef0f5248aa3ee72c75b95a198d97e8 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 5 Jul 2023 11:01:01 -0700
Subject: [PATCH] =?UTF-8?q?diff:=20don=E2=80=99t=20backspace=20before=20fi?=
 =?UTF-8?q?rst=20column?=
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* src/util.c (output_1_line): When expanding tabs, treat backspace
before column 1 as no-op, since that’s what most devices do.
* tests/expand-tabs: New test.
* tests/Makefile.am (TESTS): Add it.
---
 src/util.c        |  7 +++++--
 tests/Makefile.am |  1 +
 tests/expand-tabs | 19 +++++++++++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100755 tests/expand-tabs

diff --git a/src/util.c b/src/util.c
index 053184e..c1c8660 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1397,12 +1397,15 @@ output_1_line (char const *base, char const *limit, char const *flag_format,
               break;
 
             case '\b':
-              column--;
-	      if (column < 0)
+	      if (0 < column)
+		column--;
+	      else if (0 < tab)
 		{
 		  tab--;
 		  column = tab_size - 1;
 		}
+	      else
+		continue;
               putc (c, out);
               break;
 
diff --git a/tests/Makefile.am b/tests/Makefile.am
index db757bd..5eb725a 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -10,6 +10,7 @@ TESTS = \
   colliding-file-names \
   diff3 \
   excess-slash \
+  expand-tabs \
   help-version	\
   ifdef \
   invalid-re	\
diff --git a/tests/expand-tabs b/tests/expand-tabs
new file mode 100755
index 0000000..ae4c0fa
--- /dev/null
+++ b/tests/expand-tabs
@@ -0,0 +1,19 @@
+#!/bin/sh
+# Test tab expansion.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+cat >exp <<'EOF' || framework_failure_
+0a1
+>         x
+EOF
+
+for p in '\b\tx\n' '\b        x\n' '\b \tx\n'; do
+  printf "$p" | returns_ 1 diff -t /dev/null - >out || fail=1
+done
+
+compare exp out || fail=1
+
+Exit $fail