diff --git a/githooks/update b/githooks/update
index 242e7729f..d0c9ebf92 100755
--- a/githooks/update
+++ b/githooks/update
@@ -15,6 +15,7 @@ var crypto = require('crypto');
 var exec = require('child_process').exec;
 var sugar = require('sugar');
 
+var thisFile = __filename;
 var thisDir = __dirname;
 var rootDir = path.resolve(thisDir, '..');
 
@@ -27,7 +28,7 @@ function updateIndex () {
 	var indexContents = fs.readFileSync(path.resolve(rootDir, 'index.template.html'), { encoding: 'utf8' });
 
 	// add hashes to js and css files
-	console.log('Updating hashes...');
+	console.log("Updating hashes...");
 	indexContents = indexContents.replace(/(src|href)="\/(.*?)\?[a-z0-9]*?"/g, function (a, b, c) {
 		var hash = Math.random(); // just in case creating the hash fails
 		try {
@@ -38,10 +39,10 @@ function updateIndex () {
 
 		return b + '="/' + c + '?' + hash + '"';
 	});
-	console.log('Hashes updated.');
+	console.log("Hashes updated.");
 
 	// add news
-	console.log('Updating news...');
+	console.log("Updating news...");
 	exec('php ' + path.resolve(thisDir, 'news-data.php'), function (error, stdout, stderr) {
 		var newsData = [0, '[failed to retrieve news]'];
 		if (!error && !stderr) {
@@ -56,119 +57,191 @@ function updateIndex () {
 
 		indexContents = indexContents.replace(/<!-- newsid -->/g, newsData[0]);
 		indexContents = indexContents.replace(/<!-- news -->/g, newsData[1]);
+		console.log("News updated.");
 
+		console.log("Writing new `index.html` file...");
 		fs.writeFileSync(path.resolve(rootDir, 'index.html'), indexContents);
-		console.log('News updated.');
+		console.log("File `index.html` successfully updated.");
 	});
 }
 
+function updateLearnsets (callback) {
+	var reservedKeywords = ['return']; // `return` is the only ES3+ reserved keyword that (currently) raises conflicts
+	var numberRegExp = new RegExp('^[0-9]*');
+	var reservedRegExp = new RegExp('^(' + reservedKeywords.join('|') + ')$', 'g');
+	var alphabetize = function (a, b) {return a.localeCompare(b);};
+	var getLsetGen = function (lset) {return lset.charAt(0);};
+
+	var padNumString = function (str) {
+		switch (str.length) {
+		case 1: return '00' + str;
+		case 2: return '0' + str;
+		case 3: return '' + str;
+		}
+	};
+	var inLearnset = function (lset, learnset) {
+		var secondChar = lset.charAt(1);
+		if (secondChar !== 'L') return learnset.indexOf(lset) >= 0;
+
+		var firstFragment = lset.substr(0, 2);
+		var levelFragment = lset.substr(2);
+		var paddedLevel = padNumString(levelFragment);
+		for (var i = 0, len = learnset.length; i < len; i++) {
+			if (learnset[i].substring(0, 2) !== firstFragment) continue;
+			// ignore PSdex starter moves sorting data
+			if (learnset[i].substring(2, 5) === paddedLevel) return true;
+		}
+		return false;
+	};
+	var formatLset = function (lset) {
+		var secondChar = lset.charAt(1);
+		if (secondChar !== 'L') return lset;
+		var firstFragment = lset.substr(0, 2);
+		var levelFragment = lset.substr(2).match(numberRegExp)[0];
+		var sortFragment = lset.substr(2 + levelFragment.length);
+		return firstFragment + padNumString(levelFragment) + sortFragment;
+	};
+
+	var Pokedex, Learnsets, oldLearnsetsG6, newLearnsetsG6;
+	newLearnsetsG6 = {};
+	try {
+		Pokedex = require(path.join(rootDir, 'data', 'pokedex.js')).BattlePokedex;
+		Learnsets = require(path.join(rootDir, 'data', 'learnsets.js')).BattleLearnsets;
+	} catch (err) {
+		return callback(err);
+	}
+	try {
+		oldLearnsetsG6 = require(path.join(rootDir, 'data', 'learnsets-g6.js')).BattleLearnsets;
+	} catch (err) {
+		if (err.code !== 'MODULE_NOT_FOUND') return callback(err);
+		oldLearnsetsG6 = {};
+	}
+
+	for (var speciesid in Learnsets) {
+		if (!oldLearnsetsG6[speciesid] || !oldLearnsetsG6[speciesid].learnset) {
+			console.log("NEW ENTRY at learnsets-g6.js: " + Pokedex[speciesid].species + ".");
+			oldLearnsetsG6[speciesid] = {learnset: {}};
+		}
+	}
+	for (var speciesid in oldLearnsetsG6) {
+		if (!oldLearnsetsG6[speciesid] || !oldLearnsetsG6[speciesid].learnset) return callback(new TypeError("Invalid `learnsets-g6.js` entry for " + speciesid + "."));
+		if (!Learnsets[speciesid]) {
+			console.log("REMOVED ENTRY at learnsets-g6.js: " + Pokedex[speciesid].species + ".");
+			continue;
+		}
+		var newLearnset = {};
+		var oldLearnset = oldLearnsetsG6[speciesid].learnset;
+		var fullLearnset = Learnsets[speciesid].learnset;
+		if (!fullLearnset) return callback(new TypeError("Invalid data at `learnsets.js` for " + speciesid + "."));
+
+		// clone
+		for (var moveid in oldLearnset) {
+			if (!Array.isArray(oldLearnset[moveid])) return callback(new TypeError("Invalid data at `learnsets-g6.js` for " + speciesid + ":" + moveid + "."));
+			newLearnset[moveid] = oldLearnset[moveid].slice();
+		}
+
+		for (var moveid in fullLearnset) {
+			if (!Array.isArray(fullLearnset[moveid])) return callback(new TypeError("Invalid data at `learnsets.js` for " + speciesid + ":" + moveid + "."));
+
+			if (!newLearnset[moveid]) newLearnset[moveid] = [];
+			newLearnset[moveid] = newLearnset[moveid].map(formatLset);
+			for (var i = 0, len = fullLearnset[moveid].length; i < len; i++) {
+				if (getLsetGen(fullLearnset[moveid][i]) !== '6') continue;
+				if (inLearnset(fullLearnset[moveid][i], newLearnset[moveid])) continue;
+				newLearnset[moveid].push(formatLset(fullLearnset[moveid][i]));
+			}
+			newLearnset[moveid].sort(alphabetize);
+		}
+
+		newLearnsetsG6[speciesid] = {learnset: newLearnset};
+	}
+
+	var buf = [];
+	var pokemonList = Object.values(Pokedex).sort(function (a, b) {
+		// Missingno. goes first (zeroth); afterwards, CAP in descending dex order (increasingly negative)
+		// Finally, standard Pokémon in ascending dex order
+		if (a.num <= 0 && b.num > 0) return -1;
+		if (b.num <= 0 && a.num > 0) return 1;
+		if (a.num <= 0 && b.num <= 0) return b.num - a.num;
+		return a.num - b.num;
+	}).map('species').map(toId);
+
+	for (var i = 0, len = pokemonList.length; i < len; i++) {
+		var entry = newLearnsetsG6[pokemonList[i]];
+		if (!entry || !entry.learnset) continue;
+		var lsetSerialized = '{' + Object.keys(entry.learnset).sort(alphabetize).map(function (moveid) {
+			return moveid.replace(reservedRegExp, '"$1"') + ':' + JSON.stringify(entry.learnset[moveid]);
+		}).join(',') + '}';
+		buf.push(pokemonList[i] + ':{learnset:' + lsetSerialized + '}');
+	}
+
+	var writeStream = fs.createWriteStream(path.join(rootDir, 'data', 'learnsets-g6.js')).on('error', callback);
+	writeStream.write('exports.BattleLearnsets = {\n\t' + buf.join(',\n\t') + '\n};\n');
+	writeStream.end(callback);
+}
+
 var pendingFiles = 0;
+var indexStats, updateStats, indexMTime, updateMTime;
+try {
+	indexStats = fs.statSync(path.join(rootDir, 'index.html'));
+	indexMTime = indexStats.mtime.getTime();
+} catch (err) {
+	if (err.code !== 'ENOENT') throw err;
+	// It doesn't exist currently, but it will by the end of the script execution.
+	// Any other error is unacceptable and will throw.
+}
+try {
+	updateStats = fs.statSync(thisFile);
+	updateMTime = indexStats.mtime.getTime();
+} catch (err) {
+	throw err; // !!
+}
 
 // update learnsets-g6
+console.log("Updating file `data/learnsets-g6`...");
+
 pendingFiles++;
-console.log('Updating file `data/learnsets-g6`...');
-var reservedKeywords = ['return']; // `return` is the only ES3+ reserved keyword that (currently) raises conflicts
-var numberRegExp = new RegExp('^[0-9]*');
-var reservedRegExp = new RegExp('^(' + reservedKeywords.join('|') + ')$', 'g');
-var alphabetize = function (a, b) {return a.localeCompare(b)};
-var getLsetGen = function (lset) {return lset.charAt(0)};
+var learnsetsStats;
+var learnsetsG6Stats;
+var learnsetsG6ToUpdate = true;
 
-var padNumString = function (str) {
-	switch (str.length) {
-	case 1: return '00' + str;
-	case 2: return '0' + str;
-	case 3: return '' + str;
-	}
-};
-var inLearnset = function (lset, learnset) {
-	var secondChar = lset.charAt(1);
-	if (secondChar !== 'L') return learnset.indexOf(lset) >= 0;
-
-	var firstFragment = lset.substr(0, 2);
-	var levelFragment = lset.substr(2);
-	var paddedLevel = padNumString(levelFragment);
-	for (var i = 0, len = learnset.length; i < len; i++) {
-		if (learnset[i].substring(0, 2) !== firstFragment) continue;
-		// ignore PSdex starter moves sorting data
-		if (learnset[i].substring(2, 5) === paddedLevel) return true;
-	}
-	return false;
-};
-var formatLset = function (lset) {
-	var secondChar = lset.charAt(1);
-	if (secondChar !== 'L') return lset;
-	var firstFragment = lset.substr(0, 2);
-	var levelFragment = lset.substr(2).match(numberRegExp)[0];
-	var sortFragment = lset.substr(2 + levelFragment.length);
-	return firstFragment + padNumString(levelFragment) + sortFragment;
-};
-
-var Learnsets = require(path.join(rootDir, 'data', 'learnsets.js')).BattleLearnsets;
-var oldLearnsetsG6 = require(path.join(rootDir, 'data', 'learnsets-g6.js')).BattleLearnsets;
-var Pokedex = require(path.join(rootDir, 'data', 'pokedex.js')).BattlePokedex;
-var newLearnsetsG6 = {};
-
-for (var speciesid in Learnsets) {
-	if (!oldLearnsetsG6[speciesid] || !oldLearnsetsG6[speciesid].learnset) {
-		console.log("NEW: " + Pokedex[speciesid].species + ".");
-		oldLearnsetsG6[speciesid] = {learnset: {}};
-	}
+try {
+	learnsetsStats = fs.statSync(path.join(rootDir, 'data', 'learnsets.js'));
+} catch (err) {
+	// Couldn't find learnsets.js, but that's not the end of the world: skip to next task.
+	console.error("Couldn't find `data/learnsets.js`. Task aborted.");
+	learnsetsG6ToUpdate = false;
+	if (!--pendingFiles) updateIndex();
 }
-for (var speciesid in oldLearnsetsG6) {
-	if (!Learnsets[speciesid] || !Learnsets[speciesid].learnset) throw new Error("Missing learnsets.js entry for " + speciesid + ".");
-	if (!oldLearnsetsG6[speciesid] || !oldLearnsetsG6[speciesid].learnset) throw new Error("Invalid learnsets-g6.js entry for " + speciesid + ".");
-
-	var fullLearnset = Learnsets[speciesid].learnset;
-	var oldLearnset = oldLearnsetsG6[speciesid].learnset;
-	var newLearnset = {};
-
-	// clone
-	for (var moveid in oldLearnset) {
-		if (!Array.isArray(oldLearnset[moveid])) throw new TypeError("Invalid data at learnsets-g6.js for " + speciesid + ":" + moveid + ".");
-		newLearnset[moveid] = oldLearnset[moveid].slice();
-	}
-
-	for (var moveid in fullLearnset) {
-		if (!Array.isArray(fullLearnset[moveid])) throw new TypeError("Invalid data at learnsets.js for " + speciesid + ":" + moveid + ".");
-
-		if (!newLearnset[moveid]) newLearnset[moveid] = [];
-		newLearnset[moveid] = newLearnset[moveid].map(formatLset);
-		for (var i = 0, len = fullLearnset[moveid].length; i < len; i++) {
-			if (getLsetGen(fullLearnset[moveid][i]) !== '6') continue;
-			if (inLearnset(fullLearnset[moveid][i], newLearnset[moveid])) continue;
-			newLearnset[moveid].push(formatLset(fullLearnset[moveid][i]));
+if (learnsetsG6ToUpdate) {
+	try {
+		learnsetsG6Stats = fs.statSync(path.join(rootDir, 'data', 'learnsets-g6.js'));
+	} catch (err) {
+		if (err.code === 'ENOENT') {
+			// It doesn't exist currently, but it will by the end of the script execution.
+		} else {
+			// Any other error is unacceptable for learnsets-g6.js update: skip to next task.
+			console.error("Failed to read `data/learnsets-g6.js`. Task aborted.");
+			learnsetsG6ToUpdate = false;
+			if (!--pendingFiles) updateIndex();
 		}
-		newLearnset[moveid].sort(alphabetize);
 	}
-
-	newLearnsetsG6[speciesid] = {learnset: newLearnset};
 }
 
-var buf = [];
-var pokemonList = Object.values(Pokedex).sort(function (a, b) {
-	// Missingno. goes first (zeroth); afterwards, CAP in descending dex order (increasingly negative)
-	// Finally, standard Pokémon in ascending dex order
-	if (a.num <= 0 && b.num > 0) return -1;
-	if (b.num <= 0 && a.num > 0) return 1;
-	if (a.num <= 0 && b.num <= 0) return b.num - a.num;
-	return a.num - b.num;
-}).map('species').map(toId);
+if (learnsetsG6ToUpdate && (!indexStats || !learnsetsG6Stats || indexMTime < updateMTime || indexMTime < learnsetsStats.mtime.getTime() || indexMTime < learnsetsG6Stats.mtime.getTime())) {
+	// Only merge learnsets.js with learnsets-g6.js if any of those files, or this one, have been modified recently (or if we don't know what "recently" means)
 
-for (var i = 0, len = pokemonList.length; i < len; i++) {
-	var entry = newLearnsetsG6[pokemonList[i]];
-	if (!entry || !entry.learnset) continue;
-	var lsetSerialized = '{' + Object.keys(entry.learnset).sort(alphabetize).map(function (moveid) {
-		return moveid.replace(reservedRegExp, '"$1"') + ':' + JSON.stringify(entry.learnset[moveid]);
-	}).join(',') + '}';
-	buf.push(pokemonList[i] + ':{learnset:' + lsetSerialized + '}');
+	updateLearnsets(function (err) {
+		if (err) {
+			var stack = err.stack || '';
+			stack = "File `data/learnsets-g6` failed to update.\n" + stack;
+			console.error(stack);
+		} else {
+			console.log("File `data/learnsets-g6` updated.");
+		}
+		if (!--pendingFiles) updateIndex();
+	});
+} else {
+	console.log("Input files not modified. Task aborted.");
+	if (!--pendingFiles) updateIndex();
 }
-
-var writeStream = fs.createWriteStream(path.join(rootDir, 'data', 'learnsets-g6.js')).on('error', function (err) {
-	console.error(err.stack || err);
-});
-writeStream.write('exports.BattleLearnsets = {\n\t' + buf.join(',\n\t') + '\n};\n');
-writeStream.end(function () {
-	console.log('File `data/learnsets-g6` updated.');
-	pendingFiles--;
-	if (!pendingFiles) updateIndex();
-});