Make string difference more readable

With this patch:
```
    assertEquals "deadbeef" "deadbuff"
```
will output like this
```
    ASSERT:
    expected:<deadbeef>
    ---------------^
    but was :<deadbuff>
```

set env variable SHUNIT_DIFF_ALIGN=[always/never/auto]

shunit2

@@ -49,6 +49,8 @@ SHUNIT_CMD_TPUT=${SHUNIT_CMD_TPUT:-${__SHUNIT_CMD_TPUT}}
 
 # Enable color output. Options are 'auto', 'always', or 'never'.
 SHUNIT_COLOR=${SHUNIT_COLOR:-auto}
+# Enable difference alignment output. Options are 'auto', 'always', or 'never'.
+SHUNIT_DIFF_ALIGN=${SHUNIT_DIFF_ALIGN:-never}
 
 #
 # Internal constants.
@@ -647,6 +649,35 @@ fail() {
 # shellcheck disable=SC2016,SC2034
 _FAIL_='eval fail --lineno "${LINENO:-}"'
 
+_shunit_different() {
+    expected="$1"
+    actually="$2"
+    len1=${#expected}
+    len2=${#actually}
+    min_len=$len1
+    if ${__SHUNIT_BUILTIN} [ "$min_len" -gt "$len2" ]; then
+      min_len=$len2;
+    fi
+
+    i=1;
+    while ${__SHUNIT_BUILTIN} [ "$i" -le "$min_len" ];
+    do
+      charA=$(echo "$expected"|cut -c$i);
+      charB=$(echo "$actually"|cut -c$i);
+      if ${__SHUNIT_BUILTIN} [ "$charA" = "$charB" ]; then
+        printf "%c" "-";
+      else
+        printf "%c" "^";
+        break;
+      fi
+      i=$((i + 1));
+    done
+    if ${__SHUNIT_BUILTIN} [ "$i" -gt "$min_len" ]; then
+      printf "%c" "^";
+    fi
+
+}
+
 # Records a test failure, stating two values were not equal.
 #
 # Args:
@@ -675,9 +706,14 @@ failNotEquals() {
   shunit_actual_=$2
 
   shunit_message_=${shunit_message_%% }
+  if ${__SHUNIT_BUILTIN} [ ${__shunit_diff_align} -eq ${SHUNIT_TRUE} ]; then
+    diff_position_=$(_shunit_different "${shunit_expected_}" "${shunit_actual_})")
+    _shunit_assertFail "${shunit_message_:+${shunit_message_} }\\nexpected:<${shunit_expected_}>\\n----------${diff_position_}\\n but was:<${shunit_actual_}>"
+    return
+  fi
   _shunit_assertFail "${shunit_message_:+${shunit_message_} }expected:<${shunit_expected_}> but was:<${shunit_actual_}>"
 
-  unset shunit_message_ shunit_expected_ shunit_actual_
+  unset shunit_message_ shunit_expected_ shunit_actual_ diff_position_
   return ${SHUNIT_FALSE}
 }
 # shellcheck disable=SC2016,SC2034
@@ -1043,6 +1079,26 @@ _shunit_cleanup() {
   unset _shunit_name_ _shunit_signal_
 }
 
+# configureDiff difference alignment.
+#
+# Args:
+#   alignment: string: align state (one of `always`, `auto`, or `never`).
+_shunit_configureDiff() {
+  _shunit_diff_conf_=${SHUNIT_FALSE}
+  case $1 in
+    'always') _shunit_diff_conf_=${SHUNIT_TRUE} ;;
+    'auto')
+      if which cut|grep cut >/dev/null 2>&1; then
+        _shunit_diff_conf_=${SHUNIT_TRUE}
+      fi
+      ;;
+    'never'|'none') _shunit_diff_conf_=${SHUNIT_FALSE} ;;
+    *) _shunit_fatal "unrecognized difference alignment option '$1'" ;;
+  esac
+  __shunit_diff_align=${_shunit_diff_conf_}
+  unset _shunit_diff_conf_
+}
+
 # configureColor based on user color preference.
 #
 # Args:
@@ -1329,6 +1385,9 @@ fi
 # Configure default output coloring behavior.
 _shunit_configureColor "${SHUNIT_COLOR}"
 
+# Configure default difference alignment behavior.
+_shunit_configureDiff "${SHUNIT_DIFF_ALIGN}"
+
 # Execute the oneTimeSetUp function (if it exists).
 if ! oneTimeSetUp; then
   _shunit_fatal "oneTimeSetUp() returned non-zero return code."

shunit2_misc_test.sh

@@ -58,6 +58,8 @@ EOF
 testIssue7() {
   # Disable coloring so 'ASSERT:' lines can be matched correctly.
   _shunit_configureColor 'none'
+  # Disable difference alignment so 'ASSERT:' lines can be matched correctly.
+  _shunit_configureDiff 'none'
 
   # Ignoring errors with `|| :` as we only care about the message in this test.
   ( assertEquals 'Some message.' 1 2 >"${stdoutF}" 2>"${stderrF}" ) || :
@@ -165,6 +167,34 @@ EOF
   fi
 }
 
+testIssue89() {
+  tempdiff="${SHUNIT_TMPDIR}/difference.diff"
+  _shunit_configureColor none
+  # test new aligned difference
+  _shunit_configureDiff always
+  ( assertEquals "deadbeef" "deadbuff" >"${stdoutF}" 2>"${stderrF}" ) || :
+  diff -u "${stdoutF}" - >"${tempdiff}" 2>&1 <<EOF
+ASSERT:
+expected:<deadbeef>
+---------------^
+ but was:<deadbuff>
+EOF
+  rtrn=$?
+  [ "${rtrn}" -eq "${SHUNIT_TRUE}" ] || cat "${tempdiff}" >&2
+  rm "${tempdiff}"
+  # test does it breaks original diff implementation
+  _shunit_configureColor none
+  _shunit_configureDiff none
+  ( assertEquals "deadbeef" "deadbuff" >"${stdoutF}" 2>"${stderrF}" ) || :
+  diff -u "${stdoutF}" - >"${tempdiff}" 2>&1 <<EOF
+ASSERT:expected:<deadbeef> but was:<deadbuff>
+EOF
+  rtrn=$?
+  [ "${rtrn}" -eq "${SHUNIT_TRUE}" ] || cat "${tempdiff}" >&2
+  rm "${tempdiff}"
+}
+
+
 # Demonstrate that asserts are no longer executed in subshells.
 # https://github.com/kward/shunit2/issues/123
 #