create an index for the log file

src/wal/index.go

@@ -0,0 +1,53 @@
+package wal
+
+import (
+	"sort"
+)
+
+type indexEntry struct {
+	startRequestNumber uint32 // first request number in the block
+	size               uint32 // number of requests in the block
+	startOffset        uint64 // the offset of the first request
+}
+
+type index struct {
+	entries []*indexEntry
+}
+
+func (self *index) addEntry(entry *indexEntry) {
+	self.entries = append(self.entries, entry)
+}
+
+func (self *index) findOffsetBlock(requestNumber uint32) func(int) bool {
+	return func(i int) bool {
+		// The returned function must satisfy `f(i) => f(i+1)`, meaning if
+		// for index i f returns true, then it must return true for every
+		// index greater than i. sort.Search will return the smallest
+		// index satisfying f
+		if self.entries[i].startRequestNumber > requestNumber {
+			return true
+		}
+
+		return false
+	}
+}
+
+func (self *index) requestOffset(requestNumber uint32) uint64 {
+	numberOfEntries := len(self.entries)
+	index := sort.Search(numberOfEntries, self.findOffsetBlock(requestNumber))
+	if index != numberOfEntries {
+		return self.entries[index-1].startOffset
+	}
+
+	if requestNumber >= self.entries[0].startRequestNumber {
+		return self.entries[0].startOffset
+	}
+
+	lastEntry := self.entries[len(self.entries)-1]
+	lastRequestNumber := lastEntry.startRequestNumber + lastEntry.size
+	if requestNumber > lastRequestNumber {
+		return lastEntry.startOffset
+	}
+
+	return 0
+}