lnwire: separate ChannelID into a distinct struct, add tests

This commit separates out the ChannelID into a new file, with
additional helper methods for conversion and formatting. With this
commit, the struct is now more general purpose and can be used in the
new routing package, database, and other related sub-systems.

lnwire/channel_id.go

@@ -0,0 +1,40 @@
+package lnwire
+
+// ChannelID represent the set of data which is needed to retrieve all
+// necessary data to validate the channel existence.
+type ChannelID struct {
+	// BlockHeight is the height of the block where funding transaction
+	// located.
+	//
+	// NOTE: This field is limited to 3 bytes.
+	BlockHeight uint32
+
+	// TxIndex is a position of funding transaction within a block.
+	//
+	// NOTE: This field is limited to 3 bytes.
+	TxIndex uint32
+
+	// TxPosition indicating transaction output which pays to the
+	// channel.
+	TxPosition uint16
+}
+
+// NewChanIDFromInt returns a new ChannelID which is the decoded version of the
+// compact channel ID encoded within the uint64. The format of the compact
+// channel ID is as follows: 3 bytes for the block height, 3 bytes for the
+// transaction index, and 2 bytes for the output index.
+func NewChanIDFromInt(chanID uint64) ChannelID {
+	return ChannelID{
+		BlockHeight: uint32(chanID >> 40),
+		TxIndex:     uint32(chanID>>16) & 0xFFFFFF,
+		TxPosition:  uint16(chanID),
+	}
+}
+
+// ToUint64 converts the ChannelID into a compact format encoded within a
+// uint64 (8 bytes).
+func (c *ChannelID) ToUint64() uint64 {
+	// TODO(roasbeef): explicit error on overflow?
+	return ((uint64(c.BlockHeight) << 40) | (uint64(c.TxIndex) << 16) |
+		(uint64(c.TxPosition)))
+}