From 24d57ad4ffaf7d8caef8902d37021866e73f8dbf Mon Sep 17 00:00:00 2001
From: dgp <dgp@users.sourceforge.net>
Date: Sat, 15 Nov 2014 21:46:19 +0000
Subject: Add to Tcl_ExternalToUtf() a capability to impose a limit on the
 number of chars produce in the encoding result.  When the flag
 TCL_ENCODING_CHAR_LIMIT is set and dstCharsPtr is not NULL, then the initial
 value of *dstCharsPtr is taken as the max number of chars to produce. The
 limit is imposed in a way that does not require the assistance of the
 encoding's driver procs, but the flag is passed on to them in case they can
 do better when they know they should.  No callers updated yet. No drivers
 updated yet.

One difficulty is that this necessarily imposes a pre-translation limit,
and the I/O system has a history of wanting to impose only a post-translation
limit.
---
 generic/tclEncoding.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/generic/tclEncoding.c b/generic/tclEncoding.c
index 0446816..f33e0e6 100644
--- a/generic/tclEncoding.c
+++ b/generic/tclEncoding.c
@@ -1203,8 +1203,10 @@ Tcl_ExternalToUtf(
 				 * output buffer. */
 {
     const Encoding *encodingPtr;
-    int result, srcRead, dstWrote, dstChars;
+    int result, srcRead, dstWrote, dstChars = 0;
     int noTerminate = flags & TCL_ENCODING_NO_TERMINATE;
+    int charLimited = (flags & TCL_ENCODING_CHAR_LIMIT) && dstCharsPtr;
+    int maxChars = INT_MAX;
     Tcl_EncodingState state;
 
     if (encoding == NULL) {
@@ -1229,6 +1231,9 @@ Tcl_ExternalToUtf(
     }
     if (dstCharsPtr == NULL) {
 	dstCharsPtr = &dstChars;
+	flags &= ~TCL_ENCODING_CHAR_LIMIT;
+    } else if (charLimited) {
+	maxChars = *dstCharsPtr;
     }
 
     if (!noTerminate) {
@@ -1241,9 +1246,20 @@ Tcl_ExternalToUtf(
 
 	dstLen--;
     }
-    result = encodingPtr->toUtfProc(encodingPtr->clientData, src, srcLen,
-	    flags, statePtr, dst, dstLen, srcReadPtr, dstWrotePtr,
-	    dstCharsPtr);
+    do {
+	int savedFlags = flags;
+	Tcl_EncodingState savedState = *statePtr;
+
+	result = encodingPtr->toUtfProc(encodingPtr->clientData, src, srcLen,
+		flags, statePtr, dst, dstLen, srcReadPtr, dstWrotePtr,
+		dstCharsPtr);
+	if (*dstCharsPtr <= maxChars) {
+	    break;
+	}
+	dstLen = Tcl_UtfAtIndex(dst, maxChars) - 1 - dst + TCL_UTF_MAX;
+	flags = savedFlags;
+	*statePtr = savedState;
+    } while (1);
     if (!noTerminate) {
 	/* ...and then append it */
 
-- 
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