Refactor xl.GetObject and erasureReadFile. (#2211)

* XL: Refactor xl.GetObject and erasureReadFile. erasureReadFile() responsible for just erasure coding, it takes ordered disks and checkSum slice.

* move getOrderedPartsMetadata and getOrderedDisks to xl-v1-utils.go

* Review fixes.

xl-v1-utils.go

@@ -142,3 +142,25 @@ func unionChecksumInfos(cur []checkSumInfo, updated []checkSumInfo, curPartName
 	}
 	return finalChecksums
 }
+
+// Return ordered partsMetadata depeinding on distribution.
+func getOrderedPartsMetadata(distribution []int, partsMetadata []xlMetaV1) (orderedPartsMetadata []xlMetaV1) {
+	orderedPartsMetadata = make([]xlMetaV1, len(partsMetadata))
+	for index := range partsMetadata {
+		blockIndex := distribution[index]
+		orderedPartsMetadata[blockIndex-1] = partsMetadata[index]
+	}
+	return orderedPartsMetadata
+}
+
+// getOrderedDisks - get ordered disks from erasure distribution.
+// returns ordered slice of disks from their actual distribution.
+func getOrderedDisks(distribution []int, disks []StorageAPI) (orderedDisks []StorageAPI) {
+	orderedDisks = make([]StorageAPI, len(disks))
+	// From disks gets ordered disks.
+	for index := range disks {
+		blockIndex := distribution[index]
+		orderedDisks[blockIndex-1] = disks[index]
+	}
+	return orderedDisks
+}