From 4519681dcd20c967abbe68795911c77c317fab4f Mon Sep 17 00:00:00 2001
From: "jan.nijtmans" <nijtmans@users.sourceforge.net>
Date: Mon, 23 Apr 2018 23:23:00 +0000
Subject: Bug-fix in Tcl_UtfAtIndex (for TCL_UTF_MAX=4 only). With test-case
 (in "string totitle") demonstrating the bug.

---
 generic/tclUtf.c  |  8 ++++++++
 tests/string.test | 11 +++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/generic/tclUtf.c b/generic/tclUtf.c
index 0d88d36..c08464b 100644
--- a/generic/tclUtf.c
+++ b/generic/tclUtf.c
@@ -762,10 +762,18 @@ Tcl_UtfAtIndex(
     register int index)		/* The position of the desired character. */
 {
     Tcl_UniChar ch = 0;
+    int len = 1;
 
     while (index-- > 0) {
+	len = TclUtfToUniChar(src, &ch);
+	src += len;
+    }
+#if TCL_UTF_MAX == 4
+     if (!len) {
+	/* Index points at character following High Surrogate */
 	src += TclUtfToUniChar(src, &ch);
     }
+#endif
     return src;
 }
 
diff --git a/tests/string.test b/tests/string.test
index d69fda4..868fc25 100644
--- a/tests/string.test
+++ b/tests/string.test
@@ -24,7 +24,7 @@ catch [list package require -exact Tcltest [info patchlevel]]
 
 testConstraint testobj [expr {[info commands testobj] != {}}]
 testConstraint testindexobj [expr {[info commands testindexobj] != {}}]
-testConstraint fullutf [expr {[format %c 0x010000] != "\ufffd"}]
+testConstraint tip389 [expr {[string length \U010000] == 2}]
 
 # Used for constraining memory leak tests
 testConstraint memory [llength [info commands memory]]
@@ -291,6 +291,9 @@ test string-5.19 {string index, bytearray object out of bounds} {
 test string-5.20 {string index, bytearray object out of bounds} {
     string index [binary format I* {0x50515253 0x52}] 20
 } {}
+test string-5.21 {string index, surrogates, bug [11ae2be95dac9417]} tip389 {
+    list [string index a\U100000b 1] [string index a\U100000b 2] [string index a\U100000b 3]
+} [list \U100000 {} b]
 
 
 proc largest_int {} {
@@ -1280,7 +1283,7 @@ test string-12.22 {string range, shimmering binary/index} {
     binary scan $s a* x
     string range $s $s end
 } 000000001
-test string-12.23 {string range, surrogates, bug [11ae2be95dac9417]} fullutf {
+test string-12.23 {string range, surrogates, bug [11ae2be95dac9417]} tip389 {
     list [string range a\U100000b 1 1] [string range a\U100000b 2 2] [string range a\U100000b 3 3]
 } [list \U100000 {} b]
 
@@ -1477,6 +1480,10 @@ test string-17.7 {string totitle, unicode} {
 test string-17.8 {string totitle, compiled} {
     lindex [string totitle [list aa bb [list cc]]] 0
 } Aa
+test string-17.9 {string totitle, surrogates, bug [11ae2be95dac9417]} tip389 {
+    list [string totitle a\U118c0c 1 1] [string totitle a\U118c0c 2 2] \
+	[string totitle a\U118c0c 3 3]
+} [list a\U118a0c a\U118c0C a\U118c0C]
 
 test string-18.1 {string trim} {
     list [catch {string trim} msg] $msg
-- 
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