+#define X 1
+#define R 5
+#define S 9
+#define I 13
+
+ir_swizzle *
+ir_swizzle::create(ir_rvalue *val, const char *str, unsigned vector_length)
+{
+ /* For each possible swizzle character, this table encodes the value in
+ * \c idx_map that represents the 0th element of the vector. For invalid
+ * swizzle characters (e.g., 'k'), a special value is used that will allow
+ * detection of errors.
+ */
+ static const unsigned char base_idx[26] = {
+ /* a b c d e f g h i j k l m */
+ R, R, I, I, I, I, R, I, I, I, I, I, I,
+ /* n o p q r s t u v w x y z */
+ I, I, S, S, R, S, S, I, I, X, X, X, X
+ };
+
+ /* Each valid swizzle character has an entry in the previous table. This
+ * table encodes the base index encoded in the previous table plus the actual
+ * index of the swizzle character. When processing swizzles, the first
+ * character in the string is indexed in the previous table. Each character
+ * in the string is indexed in this table, and the value found there has the
+ * value form the first table subtracted. The result must be on the range
+ * [0,3].
+ *
+ * For example, the string "wzyx" will get X from the first table. Each of
+ * the charcaters will get X+3, X+2, X+1, and X+0 from this table. After
+ * subtraction, the swizzle values are { 3, 2, 1, 0 }.
+ *
+ * The string "wzrg" will get X from the first table. Each of the characters
+ * will get X+3, X+2, R+0, and R+1 from this table. After subtraction, the
+ * swizzle values are { 3, 2, 4, 5 }. Since 4 and 5 are outside the range
+ * [0,3], the error is detected.
+ */
+ static const unsigned char idx_map[26] = {
+ /* a b c d e f g h i j k l m */
+ R+3, R+2, 0, 0, 0, 0, R+1, 0, 0, 0, 0, 0, 0,
+ /* n o p q r s t u v w x y z */
+ 0, 0, S+2, S+3, R+0, S+0, S+1, 0, 0, X+3, X+0, X+1, X+2
+ };
+
+ int swiz_idx[4] = { 0, 0, 0, 0 };
+ unsigned i;
+
+
+ /* Validate the first character in the swizzle string and look up the base
+ * index value as described above.
+ */
+ if ((str[0] < 'a') || (str[0] > 'z'))
+ return NULL;
+
+ const unsigned base = base_idx[str[0] - 'a'];
+
+
+ for (i = 0; (i < 4) && (str[i] != '\0'); i++) {
+ /* Validate the next character, and, as described above, convert it to a
+ * swizzle index.
+ */
+ if ((str[i] < 'a') || (str[i] > 'z'))
+ return NULL;
+
+ swiz_idx[i] = idx_map[str[i] - 'a'] - base;
+ if ((swiz_idx[i] < 0) || (swiz_idx[i] >= (int) vector_length))
+ return NULL;
+ }
+
+ if (str[i] != '\0')
+ return NULL;
+
+ return new ir_swizzle(val, swiz_idx[0], swiz_idx[1], swiz_idx[2],
+ swiz_idx[3], i);
+}
+
+#undef X
+#undef R
+#undef S
+#undef I
+