StrCatChainW implementation

[Huw Campbell]

Hello

Please see my implementation of this function, which  has not been seen in
wine previously. Explorer.exe calls it, so it's kind of important.

I ran a lot of tests on the edge cases: including: same ends, 0 ends, end
beyond the length...

Huw
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diff --git a/dlls/shlwapi/shlwapi.spec b/dlls/shlwapi/shlwapi.spec
index b7aac08..b47380f 100644
--- a/dlls/shlwapi/shlwapi.spec
+++ b/dlls/shlwapi/shlwapi.spec
@@ -760,6 +760,7 @@
 @ stdcall StrCatBuffA (str str long)
 @ stdcall StrCatBuffW (wstr wstr long)
 @ stdcall StrCatW (ptr wstr)
+@ stdcall StrCatChainW (ptr long long wstr)
 @ stdcall StrChrA (str long)
 @ stdcall StrChrIA (str long)
 @ stdcall StrChrIW (wstr long)
diff --git a/dlls/shlwapi/string.c b/dlls/shlwapi/string.c
index 1a4ad14..fd4b145 100644
--- a/dlls/shlwapi/string.c
+++ b/dlls/shlwapi/string.c
@@ -459,6 +459,50 @@ LPWSTR WINAPI StrCatW(LPWSTR lpszStr, LPCWSTR lpszSrc)
 }
 
 /*************************************************************************
+ * StrCatChainW	[SHLWAPI.@]
+ *
+ * Concatenates two Unicode strings.
+ *
+ * PARAMS
+ *  pszDst [O] Initial string
+ *  cchDst     Length of destination buffer
+ *  ichAt      Offset from the destination buffer to begin concatenation
+ *  pszSrc [I] String to concatenate
+ *
+ * RETURNS
+ *  The offset from the beginning of pszDst to the terminating NULL.
+ */
+
+DWORD WINAPI StrCatChainW(PWSTR pszDst, DWORD cchDst, DWORD ichAt, PCWSTR pszSrc)
+{
+  LPWSTR d = pszDst;
+  LPCWSTR s = pszSrc;
+  DWORD mark;
+
+  TRACE("(%s,%i,%i,%s)\n", debugstr_w(pszDst), cchDst, ichAt, debugstr_w(pszSrc));
+
+  if (ichAt == -1)
+    mark = strlenW(pszDst);
+  else
+    mark = ichAt;
+  if (mark > cchDst || cchDst == 0)
+    return mark;
+  if (mark == cchDst)
+    mark--;
+  d = d + mark;
+  if (pszSrc)
+  {
+    while ((mark < cchDst-1) && *s)
+    {
+      mark++;
+      *d++ = *s++;
+    }
+  }
+  *d = 0;
+  return mark;
+}
+
+/*************************************************************************
  * StrCpyW	[SHLWAPI.@]
  *
  * Copy a string to another string.