From 9a50128ac12ab834fd97699c7ca543fc040ea8f1 Mon Sep 17 00:00:00 2001
From: Nick Bowler <nbowler@draconx.ca>
Date: Wed, 14 Jun 2023 20:34:56 -0400
Subject: [PATCH] libcdecl: Tweak invalid character error from scanner.

By adjusting how we format the error message, the format string is
changed to be completely identical to a format string used by the
parser error reporting, which avoids some wasted code space in
the library.

Furthermore, make some tweaks to the invalid character pretty-printing
which seems to let GCC generate a bit more compact code.
---
 src/scan.l       | 87 +++++++++++++++++++++++++++++++++++-------------
 tests/general.at | 25 +++++++++++++-
 2 files changed, 88 insertions(+), 24 deletions(-)

diff --git a/src/scan.l b/src/scan.l
index 5610dc3..0a4db93 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -53,6 +53,67 @@
 	} \
 	strcpy(yylval->strval, yytext); \
 } while(0)
+
+static char *to_octal(char *dst, unsigned val)
+{
+	unsigned i;
+
+	for (i = 0; i < 3; i++) {
+		*dst++ = '0' + ((val >> 6) & 7u);
+		val <<= 3;
+	}
+
+	return dst;
+}
+
+/*
+ * Convert a single character to a C-style character constant, including quote
+ * characters.  At most 7 bytes are written to the buffer for the longest
+ * octal encoding, e.g., '\177'
+ */
+static void to_readable_ch(char *dst, char c)
+{
+	unsigned char uc = c;
+	unsigned i;
+	char esc;
+
+	/*
+	 * The 7 standard C control characters are contiguous in ASCII,
+	 * permitting a simple and compact lookup table; separating their
+	 * handling from backslash and quote characters hopefully allows
+	 * the compiler to recognize that.
+	 */
+	switch (c) {
+	case '\a': i = 0; break;
+	case '\b': i = 1; break;
+	case '\t': i = 2; break;
+	case '\n': i = 3; break;
+	case '\v': i = 4; break;
+	case '\f': i = 5; break;
+	case '\r': i = 6; break;
+	default:   i = 7; break;
+	}
+	esc = "abtnvfr"[i];
+
+	/* Otherwise printable characters that should still be escaped. */
+	switch (c) {
+	case '\\': case '\'': esc = c; break;
+	}
+
+	*dst++ = '\'';
+	if (esc) {
+		*dst++ = '\\';
+		*dst++ = esc;
+	} else if (isprint(uc)) {
+		*dst++ = c;
+	} else {
+		*dst++ = '\\';
+		dst = to_octal(dst, uc);
+	}
+	*dst++ = '\'';
+	*dst++ = 0;
+}
+
 %}
 
 %s ENGLISH
@@ -142,29 +203,9 @@ INTEGER 0x[[:xdigit:]]+|0[0-7]+|[[:digit:]]+
 
 [[:space:]]+
 . {
-	char buf[5] = { yytext[0] };
-	unsigned char c = buf[0];
-
-	if (!isprint(c) || c == '\\' || c == '\'') {
-		/* Encode nonprinting characters with C-style escapes */
-		buf[0] = '\\';
-		switch (c) {
-		case '\a': buf[1] = 'a'; break;
-		case '\b': buf[1] = 'b'; break;
-		case '\f': buf[1] = 'f'; break;
-		case '\n': buf[1] = 'n'; break;
-		case '\r': buf[1] = 'r'; break;
-		case '\t': buf[1] = 't'; break;
-		case '\v': buf[1] = 'v'; break;
-		case '\\': buf[1] = '\\'; break;
-		case '\'': buf[1] = '\''; break;
-		default:
-			buf[1] = '0' + ((c >> 6) & 3);
-			buf[2] = '0' + ((c >> 3) & 7);
-			buf[3] = '0' + ((c >> 0) & 7);
-		}
-	}
+	char buf[8];
 
-	cdecl__err(CDECL_ENOPARSE, _("syntax error, unexpected '%s'"), buf);
+	to_readable_ch(buf, yytext[0]);
+	cdecl__err(CDECL_ENOPARSE, _("syntax error, unexpected %s"), buf);
 	return T_LEX_ERROR;
 }
diff --git a/tests/general.at b/tests/general.at
index 899c7e4..5141902 100644
--- a/tests/general.at
+++ b/tests/general.at
@@ -97,7 +97,7 @@ AT_SETUP([cdecl99 command error messages])
 # This will only get the start of progname if it includes spaces;
 # so we won't worry too hard about the exact format later.
 AT_CHECK([LC_ALL=C cdecl99 --help], [0], [stdout])
-progname=`$AWK 'NR == 1 { print $2; }' stdout`
+progname=`$AWK 'NR == 1 { print $2; }' stdout`dnl'
 
 # every line is erroneous
 AT_DATA([input],
@@ -117,3 +117,26 @@ AT_CHECK([LC_ALL=C cdecl99 --file=input || exit 42], [42], [], [stderr])
 AT_CHECK([$AWK -v progname="$progname" -f check.awk stderr])
 
 AT_CLEANUP
+
+AT_SETUP([cdecl99 invalid character error messages])
+
+$AWK -f- >test.dat </dev/null <<'EOF'
+BEGIN {
+  print "explain \1";
+  print "explain \377";
+  print "explain \a";
+  print "explain \b";
+  print "explain ?";
+}
+EOF
+
+AT_CHECK([LC_ALL=C cdecl99 -f test.dat || exit 42], [42], [], [stderr])
+AT_CHECK([$AWK '{ print $NF; }' stderr], [0],
+[['\001'
+'\377'
+'\a'
+'\b'
+'?'
+]])
+
+AT_CLEANUP
-- 
2.43.2