1 files changed, 180 insertions, 0 deletions
diff --git a/swarm/storage/feeds/lookup/lookup.go b/swarm/storage/feeds/lookup/lookup.go
new file mode 100644
index 000000000..a5154d261
--- /dev/null
+++ b/swarm/storage/feeds/lookup/lookup.go
@@ -0,0 +1,180 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+/*
+Package lookup defines Feed lookup algorithms and provides tools to place updates
+so they can be found
+*/
+package lookup
+
+const maxuint64 = ^uint64(0)
+
+// LowestLevel establishes the frequency resolution of the lookup algorithm as a power of 2.
+const LowestLevel uint8 = 0 // default is 0 (1 second)
+
+// HighestLevel sets the lowest frequency the algorithm will operate at, as a power of 2.
+// 25 -> 2^25 equals to roughly one year.
+const HighestLevel = 25 // default is 25 (~1 year)
+
+// DefaultLevel sets what level will be chosen to search when there is no hint
+const DefaultLevel = HighestLevel
+
+//Algorithm is the function signature of a lookup algorithm
+type Algorithm func(now uint64, hint Epoch, read ReadFunc) (value interface{}, err error)
+
+// Lookup finds the update with the highest timestamp that is smaller or equal than 'now'
+// It takes a hint which should be the epoch where the last known update was
+// If you don't know in what epoch the last update happened, simply submit lookup.NoClue
+// read() will be called on each lookup attempt
+// Returns an error only if read() returns an error
+// Returns nil if an update was not found
+var Lookup Algorithm = FluzCapacitorAlgorithm
+
+// ReadFunc is a handler called by Lookup each time it attempts to find a value
+// It should return <nil> if a value is not found
+// It should return <nil> if a value is found, but its timestamp is higher than "now"
+// It should only return an error in case the handler wants to stop the
+// lookup process entirely.
+type ReadFunc func(epoch Epoch, now uint64) (interface{}, error)
+
+// NoClue is a hint that can be provided when the Lookup caller does not have
+// a clue about where the last update may be
+var NoClue = Epoch{}
+
+// getBaseTime returns the epoch base time of the given
+// time and level
+func getBaseTime(t uint64, level uint8) uint64 {
+	return t & (maxuint64 << level)
+}
+
+// Hint creates a hint based only on the last known update time
+func Hint(last uint64) Epoch {
+	return Epoch{
+		Time:  last,
+		Level: DefaultLevel,
+	}
+}
+
+// GetNextLevel returns the frequency level a next update should be placed at, provided where
+// the last update was and what time it is now.
+// This is the first nonzero bit of the XOR of 'last' and 'now', counting from the highest significant bit
+// but limited to not return a level that is smaller than the last-1
+func GetNextLevel(last Epoch, now uint64) uint8 {
+	// First XOR the last epoch base time with the current clock.
+	// This will set all the common most significant bits to zero.
+	mix := (last.Base() ^ now)
+
+	// Then, make sure we stop the below loop before one level below the current, by setting
+	// that level's bit to 1.
+	// If the next level is lower than the current one, it must be exactly level-1 and not lower.
+	mix |= (1 << (last.Level - 1))
+
+	// if the last update was more than 2^highestLevel seconds ago, choose the highest level
+	if mix > (maxuint64 >> (64 - HighestLevel - 1)) {
+		return HighestLevel
+	}
+
+	// set up a mask to scan for nonzero bits, starting at the highest level
+	mask := uint64(1 << (HighestLevel))
+
+	for i := uint8(HighestLevel); i > LowestLevel; i-- {
+		if mix&mask != 0 { // if we find a nonzero bit, this is the level the next update should be at.
+			return i
+		}
+		mask = mask >> 1 // move our bit one position to the right
+	}
+	return 0
+}
+
+// GetNextEpoch returns the epoch where the next update should be located
+// according to where the previous update was
+// and what time it is now.
+func GetNextEpoch(last Epoch, now uint64) Epoch {
+	if last == NoClue {
+		return GetFirstEpoch(now)
+	}
+	level := GetNextLevel(last, now)
+	return Epoch{
+		Level: level,
+		Time:  now,
+	}
+}
+
+// GetFirstEpoch returns the epoch where the first update should be located
+// based on what time it is now.
+func GetFirstEpoch(now uint64) Epoch {
+	return Epoch{Level: HighestLevel, Time: now}
+}
+
+var worstHint = Epoch{Time: 0, Level: 63}
+
+// FluzCapacitorAlgorithm works by narrowing the epoch search area if an update is found
+// going back and forth in time
+// First, it will attempt to find an update where it should be now if the hint was
+// really the last update. If that lookup fails, then the last update must be either the hint itself
+// or the epochs right below. If however, that lookup succeeds, then the update must be
+// that one or within the epochs right below.
+// see the guide for a more graphical representation
+func FluzCapacitorAlgorithm(now uint64, hint Epoch, read ReadFunc) (value interface{}, err error) {
+	var lastFound interface{}
+	var epoch Epoch
+	if hint == NoClue {
+		hint = worstHint
+	}
+
+	t := now
+
+	for {
+		epoch = GetNextEpoch(hint, t)
+		value, err = read(epoch, now)
+		if err != nil {
+			return nil, err
+		}
+		if value != nil {
+			lastFound = value
+			if epoch.Level == LowestLevel || epoch.Equals(hint) {
+				return value, nil
+			}
+			hint = epoch
+			continue
+		}
+		if epoch.Base() == hint.Base() {
+			if lastFound != nil {
+				return lastFound, nil
+			}
+			// we have reached the hint itself
+			if hint == worstHint {
+				return nil, nil
+			}
+			// check it out
+			value, err = read(hint, now)
+			if err != nil {
+				return nil, err
+			}
+			if value != nil {
+				return value, nil
+			}
+			// bad hint.
+			epoch = hint
+			hint = worstHint
+		}
+		base := epoch.Base()
+		if base == 0 {
+			return nil, nil
+		}
+		t = base - 1
+	}
+}