[Zucchini] Update definition of Label from README.

Labels are not assigned to targets anymore, so the definition is now meaningless.
Instead, target affinity is defined.
This should allow us to finally close tracking bug.

Bug: 760614
Change-Id: Id705837bd502ae0e6333b9fc8cab250bc590ff11
Reviewed-on: https://chromium-review.googlesource.com/1119388
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573461}

components/zucchini/README.md

@@ -70,6 +70,12 @@ on its location, they are more likely to get modified from an old version of a
 binary to a newer version. This is why "naive" patching does not do well on
 binaries.
 
+**Target Key**: An alternative representation of a Target for a fixed pool, as its
+index in the sorted list of Target offsets. Keys are useful since:
+  * Their numerical index are smaller than offsets, allowing more efficient
+  storage of target correction data in patch.
+  * They simplify association from Targets to Labels.
+
 **Disassembler**: Architecture specific data and operations, used to extract and
 correct references in a binary.
 
@@ -127,18 +133,23 @@ bytewise difference to apply.
 3. are not part of any larger region from a different equivalence.
 Not all targets are necessarily associated with another target.
 
-**Label**: An (offset, index) pair, where |offset| is a target, and |index| is
-an integer used to uniquely identify |offset| in its corresponding pool of
-targets. Labels are created for each Reference in "old" and "new" binary as part
+**Target Affinity**: Level of confidence in the association between two targets.
+The affinity between targets that are potentially associated is measured based
+on surrounding content, as well as reference type.
+
+**Label**: An integer assigned for each Target in "old" and "new" binary as part
 of generating a patch, and used to alias targets when searching for similar
-regions that will form equivalences. Labels are created such that associated
-targets in old and new binaries share the same |index|, and such that indices in
-a pool are tightly packed. For example, suppose "old" Labels are:
-  - (0x1111, 0), (0x3333, 4), (0x5555, 1), (0x7777, 3)
-and given the following association of targets between "old" and "new":
-  - 0x1111 <=> 0x6666,  0x3333 <=> 0x2222.
-then we could assign indices for "new" Labels as:
-  - (0x2222, 4}, (0x4444, 8), (0x6666, 0), (0x8888, 2)
+regions that will form equivalences. Labels are assigned such that
+associated targets in old and new binaries share the same Label. Unmatched
+Targets have a Label of 0. For example, given
+  * "Old" targets = [0x1111, 0x3333, 0x5555, 0x7777],
+  * "New" targets = [0x2222, 0x4444, 0x6666, 0x8888],
+to represent matchings 0x1111 <=> 0x6666,  0x3333 <=> 0x2222, we'd assign
+  * Label 1 to 0x1111 (in "old") and 0x6666 (in "new"),
+  * Label 2 to 0x3333 (in "old") and 0x2222 (in "new").
+ Represented as arrays indexed over Target Keys, we'd have:
+  * "Old" labels = [1, 2, 0 ,0],
+  * "New" labels = [2, 0, 1, 0].
 
 **Encoded Image**: The result of projecting the content of an image to scalar
 values that describe content on a higher level of abstraction, masking away