Factor out count_preceding_backslashes

Now that we have multiple clients of count_preceding_backslashes, factor
it out from fish_indent into wcstringutil.h, and then use the shared
implementation.

src/fish_indent.cpp

@@ -46,6 +46,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include "parse_constants.h"
 #include "parse_util.h"
 #include "print_help.h"
+#include "wcstringutil.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 // The number of spaces per indent isn't supposed to be configurable.
@@ -87,17 +88,6 @@ namespace {
 template <bool B, typename T = void>
 using enable_if_t = typename std::enable_if<B, T>::type;
 
-/// \return the number of escaping backslashes before a character.
-/// \p idx may be "one past the end."
-size_t count_preceding_backslashes(const wcstring &text, size_t idx) {
-    assert(idx <= text.size() && "Out of bounds");
-    size_t backslashes = 0;
-    while (backslashes < idx && text.at(idx - backslashes - 1) == L'\\') {
-        backslashes++;
-    }
-    return backslashes;
-}
-
 /// \return whether a character at a given index is escaped.
 /// A character is escaped if it has an odd number of backslashes.
 bool char_is_escaped(const wcstring &text, size_t idx) {

src/reader.cpp

@@ -3227,28 +3227,11 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
 
                 // Historical behavior is to trim trailing spaces.
                 // However, escaped spaces ('\ ') should not be trimmed (#7661)
-                while (!text.empty() && text.back() == L' ') {
-                    // This can be done by counting pre-trailing '\'
-                    // If there's an odd number, this must be an excaped space
-                  
-                    // first check if the trailing space can be escaped at all
-                    //   (making the common case fast)
-                    if(text.length() == 1 || text[text.length()-2] != L'\\') {
-                        text.pop_back();
-                    } else {
-                        // We've found one backslash so far, count the rest
-                        size_t num_pretrailing_backslashes = 1;
-                        for(size_t i_offset=text.length()-2;
-                            i_offset > 0 && text[i_offset-1] == L'\\'; --i_offset)
-                            num_pretrailing_backslashes++;
-                        if(num_pretrailing_backslashes%2 == 0) {
-                            // even; this space is not escaped and should be removed
-                            text.pop_back();
-                        }
-                        // Because the above accounts for the final character,
-                        // we're done here. Break so there's not an infinite loop
-                        break;
-                    }
+                // This can be done by counting pre-trailing '\'
+                // If there's an odd number, this must be an escaped space.
+                while (!text.empty() && text.back() == L' ' &&
+                       count_preceding_backslashes(text, text.size() - 1) % 2 == 0) {
+                    text.pop_back();
                 }
 
                 if (history && !conf.in_silent_mode) {

src/wcstringutil.cpp

@@ -74,6 +74,15 @@ wcstring wcstolower(wcstring input) {
     return result;
 }
 
+size_t count_preceding_backslashes(const wcstring &text, size_t idx) {
+    assert(idx <= text.size() && "Out of bounds");
+    size_t backslashes = 0;
+    while (backslashes < idx && text.at(idx - backslashes - 1) == L'\\') {
+        backslashes++;
+    }
+    return backslashes;
+}
+
 bool string_prefixes_string(const wchar_t *proposed_prefix, const wcstring &value) {
     return string_prefixes_string(proposed_prefix, value.c_str());
 }

src/wcstringutil.h

@@ -220,6 +220,10 @@ wcstring trim(wcstring input, const wchar_t *any_of);
 /// Converts a string to lowercase.
 wcstring wcstolower(wcstring input);
 
+/// \return the number of escaping backslashes before a character.
+/// \p idx may be "one past the end."
+size_t count_preceding_backslashes(const wcstring &text, size_t idx);
+
 // Out-of-line helper for wcs2string_callback.
 void wcs2string_bad_char(wchar_t);