usp10: Add tests and fix for ScriptXtoCP .

dlls/usp10/tests/usp10.c

@@ -574,6 +574,25 @@ static void test_ScriptXtoX(void)
     ok(hr == S_OK, "ScriptXtoCP should return S_OK not %08x\n", hr);
     ok(piCP == 4, "iX=%d should return piCP=4 not %d\n", iX, piCP);
 
+    iX = 0;
+    cChars = 10;
+    cGlyphs = 10;
+    hr = ScriptXtoCP(iX, cChars, cGlyphs, pwLogClust, psva, piAdvance, &psa, &piCP, &piTrailing);
+    ok(hr == S_OK, "ScriptXtoCP should return S_OK not %08x\n", hr);
+    ok(piCP == 0, "iX=%d should return piCP=0 not %d\n", iX, piCP);
+    iX = 195;
+    cChars = 10;
+    cGlyphs = 10;
+    hr = ScriptXtoCP(iX, cChars, cGlyphs, pwLogClust, psva, piAdvance, &psa, &piCP, &piTrailing);
+    ok(hr == S_OK, "ScriptXtoCP should return S_OK not %08x\n", hr);
+    ok(piCP == 0, "iX=%d should return piCP=0 not %d\n", iX, piCP);
+    iX = 196;
+    cChars = 10;
+    cGlyphs = 10;
+    hr = ScriptXtoCP(iX, cChars, cGlyphs, pwLogClust, psva, piAdvance, &psa, &piCP, &piTrailing);
+    ok(hr == S_OK, "ScriptXtoCP should return S_OK not %08x\n", hr);
+    ok(piCP == 1, "iX=%d should return piCP=1 not %d\n", iX, piCP);
+
     iCP=5;
     fTrailing = FALSE;
     cChars = 10;

dlls/usp10/usp10.c

@@ -561,7 +561,7 @@ HRESULT WINAPI ScriptXtoCP(int iX,
                            int *piTrailing)
 {
     int item;
-    int iPosX = 1;
+    int iPosX;
     float fMaxPosX = 1;
     float fAvePosX;
     TRACE("(%d,%d,%d,%p,%p,%p,%p,%p,%p)\n",
@@ -585,8 +585,9 @@ HRESULT WINAPI ScriptXtoCP(int iX,
     }        
 
     fAvePosX = fMaxPosX / cGlyphs;
-    for (item = 0; item < cGlyphs  && iPosX < iX; item++)
-        iPosX = fAvePosX * (item +1);
+    iPosX = fAvePosX;
+    for (item = 1; item < cGlyphs  && iPosX < iX; item++)
+        iPosX += fAvePosX;
     if  (iPosX - iX > fAvePosX/2)
         *piTrailing = 0;
     else