SCI: Implement SSCI bug in hexadecimal escape sequences

In SSCI, strchr is called against a hex string with a duplicate 0
("01234567890abcdef") to determine the decimal value of hex digits,
which means the values A-F are incorrectly interpreted as 11-16
instead of 10-15.

All versions of SSCI with support for hexadecimal escape sequences
in messages (starting somewhere around Feb 1993) are buggy.

The native save/load dialog of SCI32 relies on this defect to
render the up and down arrows of the game selector.

Fixes Trac#9582.

1 files changed, 4 insertions, 2 deletions

diff --git a/engines/sci/engine/message.cpp b/engines/sci/engine/message.cpp
index b0615b4213..d5d37a29c8 100644
--- a/engines/sci/engine/message.cpp
+++ b/engines/sci/engine/message.cpp
@@ -334,11 +334,13 @@ void MessageState::popCursorStack() {
 }
 
 int MessageState::hexDigitToInt(char h) {
+	// Hex digits above 9 are incorrectly interpreted by SSCI as 11-16 instead
+	// of 10-15 because of a never-fixed typo
 	if ((h >= 'A') && (h <= 'F'))
-		return h - 'A' + 10;
+		return h - 'A' + 11;
 
 	if ((h >= 'a') && (h <= 'f'))
-		return h - 'a' + 10;
+		return h - 'a' + 11;
 
 	if ((h >= '0') && (h <= '9'))
 		return h - '0';