Add QUtil::utf8_to_ascii

ChangeLog

@@ -1,3 +1,9 @@
+2019-01-03  Jay Berkenbilt  <ejb@ql.org>
+
+	* Add method QUtil::utf8_to_ascii, which returns an ASCII string
+	for a UTF-8 string, replacing out-of-range characters with a
+	specified substitute.
+
 2019-01-02  Jay Berkenbilt  <ejb@ql.org>
 
 	* Add method QPDFObjectHandle::getResourceNames that returns a set

include/qpdf/QUtil.hh

@@ -152,6 +152,12 @@ namespace QUtil
     QPDF_DLL
     std::string toUTF16(unsigned long uval);
 
+    // Convert a UTF-8 encoded string to ASCII by replacing all
+    // characters outside of ascii with the given unknown_char.
+    QPDF_DLL
+    std::string utf8_to_ascii(
+        std::string const& utf8, char unknown_char = '?');
+
     // If secure random number generation is supported on your
     // platform and qpdf was not compiled with insecure random number
     // generation, this returns a cryptographically secure random

libqpdf/QUtil.cc

@@ -892,3 +892,26 @@ QUtil::parse_numrange(char const* range, int max)
     }
     return result;
 }
+
+std::string
+QUtil::utf8_to_ascii(std::string const& utf8, char unknown_char)
+{
+    std::string ascii_value;
+    for (size_t i = 0; i < utf8.length(); ++i)
+    {
+        unsigned char ch = static_cast<unsigned char>(utf8.at(i));
+        if (ch < 128)
+        {
+            ascii_value.append(1, ch);
+        }
+        else if ((ch & 0xc0) == 0x80)
+        {
+            // Ignore subsequent byte of UTF-8 encoded character
+        }
+        else
+        {
+            ascii_value.append(1, unknown_char);
+        }
+    }
+    return ascii_value;
+}

libtests/qtest/qutil/qutil.out

@@ -47,6 +47,10 @@ HAGOOGAMAGOOGLE: 0
 0xdead -> ff fd
 0x7fffffff -> ff fd
 0x80000000 -> ff fd
+---- utf8_to_ascii
+Does π have fingers?
+Does ? have fingers?
+Does * have fingers?
 ---- whoami
 quack1
 quack2

libtests/qutil.cc

@@ -220,6 +220,17 @@ void to_utf16_test()
     print_utf16(0x80000000UL);
 }
 
+void utf8_to_ascii_test()
+{
+    char const* input = "Does \317\200 have fingers?";
+    std::cout << input
+              << std::endl
+              << QUtil::utf8_to_ascii(input)
+              << std::endl
+              << QUtil::utf8_to_ascii(input, '*')
+              << std::endl;
+}
+
 void print_whoami(char const* str)
 {
     PointerHolder<char> dup(true, QUtil::copy_string(str));
@@ -328,6 +339,8 @@ int main(int argc, char* argv[])
 	to_utf8_test();
 	std::cout << "---- utf16" << std::endl;
 	to_utf16_test();
+	std::cout << "---- utf8_to_ascii" << std::endl;
+        utf8_to_ascii_test();
 	std::cout << "---- whoami" << std::endl;
 	get_whoami_test();
 	std::cout << "---- file" << std::endl;