add locating data inside the ec files

weed/storage/erasure_coding/ec_locate.go

@@ -0,0 +1,68 @@
+package erasure_coding
+
+type Interval struct {
+	blockIndex       int
+	innerBlockOffset int64
+	size             uint32
+	isLargeBlock     bool
+}
+
+func locateData(largeBlockLength, smallBlockLength int64, datSize int64, offset int64, size uint32) (intervals []Interval) {
+	blockIndex, isLargeBlock, innerBlockOffset := locateOffset(largeBlockLength, smallBlockLength, datSize, offset)
+
+	nLargeBlockRows := int(datSize / (largeBlockLength * DataShardsCount))
+
+	for size > 0 {
+		interval := Interval{
+			blockIndex:       blockIndex,
+			innerBlockOffset: innerBlockOffset,
+			isLargeBlock:     isLargeBlock,
+		}
+
+		blockRemaining := largeBlockLength - innerBlockOffset
+		if !isLargeBlock {
+			blockRemaining = smallBlockLength - innerBlockOffset
+		}
+
+		if int64(size) <= blockRemaining {
+			interval.size = size
+			intervals = append(intervals, interval)
+			return
+		}
+		interval.size = uint32(blockRemaining)
+		intervals = append(intervals, interval)
+
+		size -= interval.size
+		blockIndex += 1
+		if isLargeBlock && blockIndex == nLargeBlockRows*DataShardsCount {
+			isLargeBlock = false
+			blockIndex = 0
+		}
+		innerBlockOffset = 0
+
+	}
+	return
+}
+
+func locateOffset(largeBlockLength, smallBlockLength int64, datSize int64, offset int64) (blockIndex int, isLargeBlock bool, innerBlockOffset int64) {
+	largeRowSize := largeBlockLength * DataShardsCount
+	nLargeBlockRows := datSize / (largeBlockLength * DataShardsCount)
+
+	// if offset is within the large block area
+	if offset < nLargeBlockRows*largeRowSize {
+		isLargeBlock = true
+		blockIndex, innerBlockOffset = locateOffsetWithinBlocks(largeBlockLength, offset)
+		return
+	}
+
+	isLargeBlock = false
+	offset -= nLargeBlockRows * largeRowSize
+	blockIndex, innerBlockOffset = locateOffsetWithinBlocks(smallBlockLength, offset)
+	return
+}
+
+func locateOffsetWithinBlocks(blockLength int64, offset int64) (blockIndex int, innerBlockOffset int64) {
+	blockIndex = int(offset / blockLength)
+	innerBlockOffset = offset % blockLength
+	return
+}