From da6d7e516c30c5cee9b894c7a73eaf693ddfcbb7 Mon Sep 17 00:00:00 2001
From: make-github-pseudonymous-again
 <5165674+make-github-pseudonymous-again@users.noreply.github.com>
Date: Thu, 3 Jun 2021 14:30:09 +0200
Subject: [PATCH] :bicyclist: perf: Scan middle snake only once.

Fixes #19.
---
 src/backwardExtend.js | 74 +++++++++++++++++++++++++++++++++++++++++++
 src/backwardStep.js   | 27 +++-------------
 src/forwardStep.js    |  9 +++---
 src/index.js          |  1 +
 src/oneWay.js         |  8 +++--
 src/twoWayScan.js     | 12 ++++++-
 6 files changed, 100 insertions(+), 31 deletions(-)
 create mode 100644 src/backwardExtend.js

diff --git a/src/backwardExtend.js b/src/backwardExtend.js
new file mode 100644
index 0000000..7d4cc7b
--- /dev/null
+++ b/src/backwardExtend.js
@@ -0,0 +1,74 @@
+import assert from 'assert';
+
+import bound from './bound.js';
+import longestCommonSuffix from './longestCommonSuffix.js';
+/**
+ * Diagonal backward extension without yield.
+ *
+ * @param {number} center
+ * @param {number} D
+ * @param {Int32Array} V
+ * @param {Function} eq
+ * @param {number} li
+ * @param {number} lj
+ * @param {number} ri
+ * @param {number} rj
+ * @param {number} Delta
+ */
+export default function backwardExtend(
+	center,
+	D,
+	V,
+	eq,
+	li,
+	lj,
+	ri,
+	rj,
+	Delta,
+) {
+	assert(ri > rj && li > lj);
+	// NOTE: We make the bounding box as small as possible.
+	// This should save roughly half of the computation time compared to
+	// letting LB = -D and UB = D.
+	const LB = -bound(D, li - lj);
+	const UB = bound(D, ri - rj);
+	assert(LB <= UB);
+	assert(LB !== D);
+	assert(UB !== -D);
+
+	console.debug('beg backwardExtend', {
+		center,
+		D,
+		V,
+		eq,
+		li,
+		lj,
+		ri,
+		rj,
+		LB,
+		UB,
+		Delta,
+	});
+
+	for (let k = LB; k <= UB; k += 2) {
+		const x = V[center + k];
+		const y = x - (k + Delta);
+		console.debug({k, x, lj, y, rj});
+
+		if (x >= lj && y >= rj && x <= li && y <= ri) {
+			V[center + k] = longestCommonSuffix(eq, x, lj, y, rj);
+		}
+	}
+
+	console.debug('end backwardExtend', {
+		center,
+		D,
+		V,
+		eq,
+		li,
+		lj,
+		ri,
+		rj,
+		Delta,
+	});
+}
diff --git a/src/backwardStep.js b/src/backwardStep.js
index 7ffb404..ba79c94 100644
--- a/src/backwardStep.js
+++ b/src/backwardStep.js
@@ -1,9 +1,8 @@
 import assert from 'assert';
 
 import bound from './bound.js';
-import longestCommonSuffix from './longestCommonSuffix.js';
 /**
- * Diagonal backward step without yielding.
+ * Diagonal backward step without yield.
  *
  * @param {number} center
  * @param {number} D
@@ -46,25 +45,13 @@ export default function backwardStep(center, D, V, eq, li, lj, ri, rj, Delta) {
 	const k = LB;
 	const xp = V[cx + k];
 	const yp = V[cy + k];
-	const x = k !== -D && xp > yp ? yp : xp - 1;
-	const y = x - (k + Delta);
-	console.debug({k, x, lj, y, rj});
-
-	if (x >= lj && y >= rj && x <= li && y <= ri) {
-		V[center + k] = longestCommonSuffix(eq, x, lj, y, rj);
-	}
+	V[center + k] = k !== -D && xp > yp ? yp : xp - 1;
 
 	for (let k = LB + 2; k < UB; k += 2) {
 		assert(k !== -D && k !== D);
 		const xp = V[cx + k];
 		const yp = V[cy + k];
-		const x = xp > yp ? yp : xp - 1;
-		const y = x - (k + Delta);
-		console.debug({k, x, lj, y, rj});
-
-		if (x >= lj && y >= rj && x <= li && y <= ri) {
-			V[center + k] = longestCommonSuffix(eq, x, lj, y, rj);
-		}
+		V[center + k] = xp > yp ? yp : xp - 1;
 	}
 
 	if (UB !== LB) {
@@ -72,13 +59,7 @@ export default function backwardStep(center, D, V, eq, li, lj, ri, rj, Delta) {
 		const xp = V[cx + k];
 		const yp = V[cy + k];
 		assert(k === D || xp > yp);
-		const x = yp;
-		const y = x - (k + Delta);
-		console.debug({k, x, lj, y, rj});
-
-		if (x >= lj && y >= rj && x <= li && y <= ri) {
-			V[center + k] = longestCommonSuffix(eq, x, lj, y, rj);
-		}
+		V[center + k] = yp;
 	}
 
 	console.debug('end backwardStep', {
diff --git a/src/forwardStep.js b/src/forwardStep.js
index 859e86e..8396392 100644
--- a/src/forwardStep.js
+++ b/src/forwardStep.js
@@ -1,9 +1,8 @@
 import assert from 'assert';
 
 import bound from './bound.js';
-import longestCommonPrefix from './longestCommonPrefix.js';
 /**
- * Diagonal forward step.
+ * Diagonal forward step with yield.
  *
  * @param {number} center
  * @param {number} D
@@ -53,7 +52,7 @@ export default function* forwardStep(center, D, V, eq, li, lj, ri, rj, Delta) {
 	console.debug({k, x, lj, y, rj});
 
 	if (x <= lj && y <= rj && x >= li && y >= ri) {
-		V[center + k] = longestCommonPrefix(eq, x, lj, y, rj);
+		V[center + k] = x;
 		yield k;
 	}
 
@@ -66,7 +65,7 @@ export default function* forwardStep(center, D, V, eq, li, lj, ri, rj, Delta) {
 		console.debug({k, x, lj, y, rj});
 
 		if (x <= lj && y <= rj && x >= li && y >= ri) {
-			V[center + k] = longestCommonPrefix(eq, x, lj, y, rj);
+			V[center + k] = x;
 			yield k;
 		}
 	}
@@ -80,7 +79,7 @@ export default function* forwardStep(center, D, V, eq, li, lj, ri, rj, Delta) {
 		console.debug({k, x, lj, y, rj});
 
 		if (x <= lj && y <= rj && x >= li && y >= ri) {
-			V[center + k] = longestCommonPrefix(eq, x, lj, y, rj);
+			V[center + k] = x;
 			yield k;
 		}
 	}
diff --git a/src/index.js b/src/index.js
index bd51327..8fd2b82 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,3 +1,4 @@
+export {default as backwardExtend} from './backwardExtend.js';
 export {default as backwardStep} from './backwardStep.js';
 export {default as bound} from './bound.js';
 export {default as boundAlloc} from './boundAlloc.js';
diff --git a/src/oneWay.js b/src/oneWay.js
index 077c978..52750f0 100644
--- a/src/oneWay.js
+++ b/src/oneWay.js
@@ -2,6 +2,7 @@ import assert from 'assert';
 
 import oneWayAlloc from './oneWayAlloc.js';
 import forwardStep from './forwardStep.js';
+import longestCommonPrefix from './longestCommonPrefix.js';
 
 /**
  * Scan from begin to end.
@@ -27,12 +28,15 @@ const oneWay = (MAX, eq, li, lj, ri, rj) => {
 	const Delta0 = li - ri;
 	for (let D = 1; D <= MAX; ++D) {
 		for (const k of forwardStep(center, D, V, eq, li, lj, ri, rj, Delta0)) {
-			const x = V[center + k];
-			const y = x - (k + Delta0);
+			let x = V[center + k];
+			let y = x - (k + Delta0);
+			x = longestCommonPrefix(eq, x, lj, y, rj);
+			y = x - (k + Delta0);
 			if (x === lj && y === rj)
 				return {
 					distance: D,
 				};
+			V[center + k] = x;
 		}
 	}
 
diff --git a/src/twoWayScan.js b/src/twoWayScan.js
index f6ef5c1..ab08603 100644
--- a/src/twoWayScan.js
+++ b/src/twoWayScan.js
@@ -2,6 +2,9 @@ import assert from 'assert';
 
 import forwardStep from './forwardStep.js';
 import backwardStep from './backwardStep.js';
+import backwardExtend from './backwardExtend.js';
+import longestCommonPrefix from './longestCommonPrefix.js';
+import longestCommonSuffix from './longestCommonSuffix.js';
 
 /**
  * Scan from begin to middle and end to middle.
@@ -42,12 +45,15 @@ const twoWayScan = (MAX, V, centerF, centerB, eq, li, lj, ri, rj) => {
 	for (let D = 1; D <= HALF_MAX; ++D) {
 		if (2 * D > MAX + parityDelta) break;
 		for (const k of forwardStep(centerF, D, V, eq, li, lj, ri, rj, Delta0)) {
+			const x = V[centerF + k];
+			const y = x - (k + Delta0);
+			V[centerF + k] = longestCommonPrefix(eq, x, lj, y, rj);
 			if (k - Delta < -(D - parityDelta)) continue;
 			if (k - Delta > D - parityDelta) continue;
 			if (V[centerF + k] < V[centerB + k - Delta]) continue; // TODO this scans the snake twice
 			return {
 				k: k + Delta0,
-				xBegin: V[centerB + k - Delta],
+				xBegin: longestCommonSuffix(eq, x, li, y, ri),
 				xEnd: V[centerF + k],
 				distance: 2 * D - parityDelta,
 				distanceLeft: D,
@@ -57,6 +63,10 @@ const twoWayScan = (MAX, V, centerF, centerB, eq, li, lj, ri, rj) => {
 
 		if (D === HALF_MAX) break;
 
+		if (D > parityDelta) {
+			backwardExtend(centerB, D - parityDelta, V, eq, lj, li, rj, ri, Delta1);
+		}
+
 		backwardStep(centerB, D + 1 - parityDelta, V, eq, lj, li, rj, ri, Delta1);
 	}