From 3041443d1989b6196dc5b868f4f7a3f467c431b1 Mon Sep 17 00:00:00 2001
From: Brian Cavalier <brian@hovercraftstudios.com>
Date: Fri, 11 Jul 2014 12:15:01 -0400
Subject: [PATCH] Tweak patch context docs. Clean up jiff/lib/context, add
 jsdoc

---
 README.md      | 11 ++++++-----
 lib/context.js | 31 ++++++++++++++++++++++++-------
 2 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/README.md b/README.md
index 132a3b6..38672f3 100644
--- a/README.md
+++ b/README.md
@@ -108,18 +108,19 @@ As of v0.2, `jiff.diff` and `jiff.patch` support [patch contexts](http://en.wiki
 
 Using patch contexts can greatly improve patch accuracy for arrays, at the cost of increasing the size of patches.
 
-Patch contexts are supported via a pair of closely related functions: `makeContext` and `findContext`.
+Patch contexts are entirely opt-in. To use them, you must provide a pair of closely related functions: `makeContext` and `findContext`.  An API for creating default `makeContext` and `findContext` functions is provided in [`jiff/lib/context`](#jifflibcontext), or you can implement your own.
 
-If you supply the optional `makeContext` function to `jiff.diff`, it will be used to generated a context for each change to an array.
+When you supply the optional `makeContext` function to `jiff.diff`, it will be used to generated a context for each change to an array.
 
-Likewise, if you supply the optional `findContext` function to `jiff.patch` (or `jiff.patchInPlace`), it will be used to find adjusted array indices where patches should actually be applied.
+Likewise, when you supply the optional `findContext` function to `jiff.patch` (or `jiff.patchInPlace`), it will be used to find adjusted array indices where patches should actually be applied.
 
-The context is opaque, and jiff will not attempt to inspect or use it: `jiff.diff` will simply add whatever is returned by `makeContext` to patch operations, and `jiff.patch` will simply hand it to `findContext` when it sees a context in a patch operation.
+The context is opaque, and jiff itself will not attempt to inspect or interpret it: `jiff.diff` will simply add whatever is returned by `makeContext` to patch operations, and `jiff.patch` will simply hand it to `findContext` when it sees a context in a patch operation.
 
-An API for creating default `makeContext` and `findContext` functions is provided in [`jiff/lib/context`](#jifflibcontext).
 
 ## Experimental APIs
 
+These APIs are still considered experimental, signatures may change.
+
 ### jiff/lib/context
 
 ```js
diff --git a/lib/context.js b/lib/context.js
index c71b0dd..7f34419 100644
--- a/lib/context.js
+++ b/lib/context.js
@@ -3,21 +3,39 @@ exports.matchContext = matchContext;
 exports.makeContextFinder = makeContextFinder;
 exports.makeContext = makeContext;
 
+/**
+ * Creates a findContext function that expects a context
+ * { before: [...], after: [...] } containing some number of items before
+ * and after the change.  Uses the provided equals function to compare
+ * array itesm and find the best fit position in the array to
+ * apply the patch.  Uses a similar algorithm to GNU patch.
+ * @param {function(a:*, b:*):boolean} equals return truthy if a and b are equal
+ * @returns {Function} a findContext function that can be passed to jiff.patch
+ */
 function makeContextFinder(equals) {
 	return function(index, array, context) {
 		return findPosition(equals, index, array, context);
 	};
 }
 
+/**
+ * Creates a makeContext function that will generate patch contexts
+ * { before: [...], after: [...] } containing `size` number of items
+ * before and after the change.
+ * @param {number} size max number of items before/after the change to include
+ * @returns {Function} a makeContext function that can be passed to jiff.diff
+ */
 function makeContext(size) {
 	return function (index, array) {
 		return {
 			before: array.slice(Math.max(0, index - size), index),
-			after: array.slice(Math.min(array.length, index), index + size)
+			after: array.slice(Math.min(array.length, index + 1), index + size + 1)
 		};
 	};
 }
 
+// TODO: Include removed items in the patch context when patch is a remove
+
 function findPosition (equals, start, array, context) {
 	var index;
 	var before = context.before;
@@ -27,9 +45,9 @@ function findPosition (equals, start, array, context) {
 	var amax = after.length;
 
 	while(amax > 0 || bmax < blen) {
-		index = findPositionWith(equals, array,
+		index = findPositionWith(equals, array, start,
 			before.slice(bmax),
-			after.slice(0, amax), start);
+			after.slice(0, amax));
 
 		if(index >= 0) {
 			return index;
@@ -42,16 +60,15 @@ function findPosition (equals, start, array, context) {
 	return start;
 }
 
-function findPositionWith(equals, array, before, after, start) {
+function findPositionWith(equals, array, start, before, after, patch) {
 	var blen = before.length;
 	var b = start-blen;
 
-	var bspan = blen;
 	var found = false;
 	var i = b;
 
 	while(i >= 0 && !found) {
-		found = matchContext(equals, array, i, i+blen, before, after);
+		found = matchContext(equals, array, i, i+blen+1, before, after);
 		if(found) {
 			return i + blen;
 		}
@@ -61,7 +78,7 @@ function findPositionWith(equals, array, before, after, start) {
 
 	i = start;
 	while(i < array.length && !found) {
-		found = matchContext(equals, array, i-bspan, i, before, after);
+		found = matchContext(equals, array, i-blen, i+1, before, after);
 		if(found) {
 			return i;
 		}