FEAT/FIX: Picking the right PICK

In Rebol2, picking value with index 0 was always returning null. In R3-alpha it was changed that zero index was working like index -1... more at this blog http://www.rebol.net/cgi-bin/r3blog.r?view=0173

I decided to revert this change and let it working as before (and so be compatible with Red too)

So now it works for example like:

>> pick tail "123" -1
== #"3"
>> pick tail "123" 0
== none

and it is also consistent with AT positioning:

>> at tail "123" -1
== "3"

Related issues:
metaeducation/rebol-issues#608
metaeducation/rebol-issues#609
metaeducation/rebol-issues#748
metaeducation/rebol-issues#857
metaeducation/rebol-issues#2117

and also:
metaeducation/rebol-issues#613

src/core/t-block.c

@@ -540,7 +540,10 @@ static struct {
 	*/
 
 	if (IS_INTEGER(pvs->select)) {
-		n = Int32(pvs->select) + VAL_INDEX(pvs->value) - 1;
+		REBINT i = Int32(pvs->select);
+		if (i == 0) return PE_NONE; // like in case: path/0
+		if (i < 0) i++;
+		n = i + VAL_INDEX(pvs->value) - 1;
 	}
 	else if (IS_WORD(pvs->select)) {
 		n = Find_Word(VAL_SERIES(pvs->value), VAL_INDEX(pvs->value), VAL_WORD_CANON(pvs->select));
@@ -573,6 +576,8 @@ static struct {
 	REBINT n = 0;
 
 	n = Get_Num_Arg(selector);
+	if (n == 0) return 0;
+	if (n < 0) n++;
 	n += VAL_INDEX(block) - 1;
 	if (n < 0 || (REBCNT)n >= VAL_TAIL(block)) return 0;
 	return VAL_BLK_SKIP(block, n);

src/core/t-string.c

@@ -354,7 +354,10 @@ static REBSER *make_binary(REBVAL *arg, REBOOL make)
 	REBSER *ser = VAL_SERIES(data);
 
 	if (IS_INTEGER(pvs->select)) {
-		n = Int32(pvs->select) + VAL_INDEX(data) - 1;
+		i = Int32(pvs->select);
+		if (i == 0) return PE_NONE; // like in case: path/0
+		if (i < 0) i++;
+		n = i + VAL_INDEX(data) - 1;
 	}
 	else return PE_BAD_SELECT;
 
@@ -542,8 +545,9 @@ static REBSER *make_binary(REBVAL *arg, REBOOL make)
 	case A_PICK:
 	case A_POKE:
 		len = Get_Num_Arg(arg); // Position
-		//if (len > 0) index--;
-		if (REB_I32_SUB_OF(len, 1, &len)
+		if (len < 0) REB_I32_ADD_OF(index, 1, &index);
+		if (len == 0
+			|| REB_I32_SUB_OF(len, 1, &len)
 			|| REB_I32_ADD_OF(index, len, &index)
 			|| index < 0 || index >= tail) {
 			if (action == A_PICK) goto is_none;

src/core/t-vector.c

@@ -468,10 +468,13 @@ void Set_Vector_Row(REBSER *ser, REBVAL *blk)
 	REBINT bits;
 	REBYTE *vp;
 	REBI64 i;
-	REBDEC f;
+	REBDEC f = 0.0;
 
-	if (IS_INTEGER(pvs->select) || IS_DECIMAL(pvs->select))
+	if (IS_INTEGER(pvs->select) || IS_DECIMAL(pvs->select)) {
 		n = Int32(pvs->select);
+		if (n == 0) return PE_NONE;
+		if (n < 0) n++;
+	}
 	else return PE_BAD_SELECT;
 
 	n += VAL_INDEX(pvs->value);