Fix handling of tokens that contain quotes surrounded by spaces in CifDict::ToString

* `A "A" A` must be quoted as `'A "A" A', B 'B' B` must be quoted as `"B 'B' B"`.
* `C "C" 'C' C` must be quoted as `\n;C "C" 'C' C\n;`.
* Previously, `CifDict` didn't quote these correctly and e.g. `A "A" A` would become three tokens because of the bad early return on the first space in the token.

This makes `CifDict::ToString` ~15% faster as the trivial check allows us to skip the extra newline check for most tokens.

Reported in https://github.com/google-deepmind/alphafold3/issues/471.

PiperOrigin-RevId: 791587762
Change-Id: If6616263e570dff534633bd0302afd23471f946f

src/alphafold3/parsers/cpp/cif_dict_lib.cc

@@ -150,19 +150,39 @@ absl::StatusOr<std::vector<absl::string_view>> TokenizeInternal(
   return tokens;
 }
 
+bool IsTrivialToken(const absl::string_view value) {
+  // Returns true if the token doesn't need any quoting and isn't multiline.
+  if (value.empty()) {
+    return false;
+  }
+
+  return std::all_of(value.begin(), value.end(), [](char c) {
+    return absl::ascii_isalnum(c) || c == '.' || c == '?' || c == '-';
+  });
+}
+
+bool IsMultiLineToken(const absl::string_view value) {
+  // A token is multiline if it has a newline or both single and double quotes.
+  bool has_single_quotes = false;
+  bool has_double_quotes = false;
+  for (const char c : value) {
+    if (c == '\n') {
+      return true;
+    } else if (c == '\'') {
+      has_single_quotes = true;
+    } else if (c == '"') {
+      has_double_quotes = true;
+    }
+  }
+  return has_single_quotes && has_double_quotes;
+}
+
 absl::string_view GetEscapeQuote(const absl::string_view value) {
   // Empty values should not happen, but if so, they should be quoted.
   if (value.empty()) {
     return "\"";
   }
 
-  // Shortcut for the most common cases where no quoting needed.
-  if (std::all_of(value.begin(), value.end(), [](char c) {
-        return absl::ascii_isalnum(c) || c == '.' || c == '?' || c == '-';
-      })) {
-    return "";
-  }
-
   // The value must not start with one of these CIF keywords.
   if (absl::StartsWithIgnoreCase(value, "data_") ||
       absl::StartsWithIgnoreCase(value, "loop_") ||
@@ -179,14 +199,27 @@ absl::string_view GetEscapeQuote(const absl::string_view value) {
     return "\"";
   }
 
-  // No quotes or whitespace allowed inside.
+  // No quotes or whitespace allowed inside. Rare case when both double and
+  // single quotes are present is handled by IsMultiLineToken.
+  bool use_double_quote = true;
+  bool use_single_quote = true;
+  bool needs_quote = false;
   for (const char c : value) {
-    if (c == '"') {
-      return "'";
-    } else if (c == '\'' || c == ' ' || c == '\t') {
-      return "\"";
+    if (c == ' ' || c == '\t') {
+      needs_quote = true;
+    } else if (c == '"') {
+      needs_quote = true;
+      use_double_quote = false;
+    } else if (c == '\'') {
+      needs_quote = true;
+      use_single_quote = false;
     }
   }
+  if (needs_quote && use_double_quote) {
+    return "\"";
+  } else if (needs_quote && use_single_quote) {
+    return "'";
+  }
   return "";
 }
 
@@ -254,7 +287,11 @@ class Column {
     int max_value_length = 0;
     for (size_t i = 0; i < values->size(); ++i) {
       absl::string_view value = (*values)[i];
-      if (absl::StrContains(value, '\n')) {
+      if (IsTrivialToken(value)) {
+        // Shortcut for the most common cases where no quoting/multiline needed.
+        max_value_length = std::max<int>(max_value_length, value.size());
+        continue;
+      } else if (IsMultiLineToken(value)) {
         values_with_newlines_.insert(i);
       } else {
         absl::string_view quote = GetEscapeQuote(value);