vim-patch:8.2.0899: assert_equalfile() does not give a hint about the difference

Problem:    Assert_equalfile() does not give a hint about the difference.
Solution:   Display the last seen text.
https://github.com/vim/vim/commit/30cc44a97f0ba1349e1a522dab22b11f47888183

2 files changed, 42 insertions, 6 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 12c904bc1b..0a83c3a586 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -5708,6 +5708,9 @@ int assert_equalfile(typval_T *argvars)
 
   IObuff[0] = NUL;
   FILE *const fd1 = os_fopen(fname1, READBIN);
+  char line1[200];
+  char line2[200];
+  ptrdiff_t lineidx = 0;
   if (fd1 == NULL) {
     snprintf((char *)IObuff, IOSIZE, (char *)e_notread, fname1);
   } else {
@@ -5716,6 +5719,7 @@ int assert_equalfile(typval_T *argvars)
       fclose(fd1);
       snprintf((char *)IObuff, IOSIZE, (char *)e_notread, fname2);
     } else {
+      int64_t linecount = 1;
       for (int64_t count = 0; ; count++) {
         const int c1 = fgetc(fd1);
         const int c2 = fgetc(fd2);
@@ -5727,10 +5731,24 @@ int assert_equalfile(typval_T *argvars)
         } else if (c2 == EOF) {
           STRCPY(IObuff, "second file is shorter");
           break;
-        } else if (c1 != c2) {
-          snprintf((char *)IObuff, IOSIZE,
-                   "difference at byte %" PRId64, count);
-          break;
+        } else {
+          line1[lineidx] = c1;
+          line2[lineidx] = c2;
+          lineidx++;
+          if (c1 != c2) {
+            snprintf((char *)IObuff, IOSIZE,
+                     "difference at byte %" PRId64 ", line %" PRId64,
+                     count, linecount);
+            break;
+          }
+        }
+        if (c1 == NL) {
+          linecount++;
+          lineidx = 0;
+        } else if (lineidx + 2 == (ptrdiff_t)sizeof(line1)) {
+          memmove(line1, line1 + 100, lineidx - 100);
+          memmove(line2, line2 + 100, lineidx - 100);
+          lineidx -= 100;
         }
       }
       fclose(fd1);
@@ -5746,6 +5764,17 @@ int assert_equalfile(typval_T *argvars)
       ga_concat(&ga, (char_u *)": ");
     }
     ga_concat(&ga, IObuff);
+    if (lineidx > 0) {
+      line1[lineidx] = NUL;
+      line2[lineidx] = NUL;
+      ga_concat(&ga, (char_u *)" after \"");
+      ga_concat(&ga, (char_u *)line1);
+      if (STRCMP(line1, line2) != 0) {
+        ga_concat(&ga, (char_u *)"\" vs \"");
+        ga_concat(&ga, (char_u *)line2);
+      }
+      ga_concat(&ga, (char_u *)"\"");
+    }
     assert_error(&ga);
     ga_clear(&ga);
     return 1;
diff --git a/src/nvim/testdir/test_assert.vim b/src/nvim/testdir/test_assert.vim
index 86993e0acc..b4f7478807 100644
--- a/src/nvim/testdir/test_assert.vim
+++ b/src/nvim/testdir/test_assert.vim
@@ -28,11 +28,18 @@ func Test_assert_equalfile()
   call writefile(['1234X89'], 'Xone')
   call writefile(['1234Y89'], 'Xtwo')
   call assert_equal(1, assert_equalfile('Xone', 'Xtwo'))
-  call assert_match("difference at byte 4", v:errors[0])
+  call assert_match('difference at byte 4, line 1 after "1234X" vs "1234Y"', v:errors[0])
+  call remove(v:errors, 0)
+
+  call writefile([repeat('x', 234) .. 'X'], 'Xone')
+  call writefile([repeat('x', 234) .. 'Y'], 'Xtwo')
+  call assert_equal(1, assert_equalfile('Xone', 'Xtwo'))
+  let xes = repeat('x', 134)
+  call assert_match('difference at byte 234, line 1 after "' .. xes .. 'X" vs "' .. xes .. 'Y"', v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_equalfile('Xone', 'Xtwo', 'a message'))
-  call assert_match("a message: difference at byte 4", v:errors[0])
+  call assert_match("a message: difference at byte 234, line 1 after", v:errors[0])
   call remove(v:errors, 0)
 
   call delete('Xone')