From a2b628c9cbb85d2b6fbffe91b1f66e9b915e29f0 Mon Sep 17 00:00:00 2001
From: "Avi Halachmi (:avih)" <avihpit@yahoo.com>
Date: Wed, 16 Feb 2022 11:35:09 +0200
Subject: [PATCH] array join: avoid strcat, speedup from O(N^2) to O(N)

Previously, each iteration (except the 1st) did this amount of calls:
  2x strlen(result-so-far) + 2x strlen(element) + strlen(sep)
Where except one strlen(element) they're implicit inside strcat, and
of sizes which we already know.

Now each iteration does one strlen(element), and no strcat.

The big speedup is avoiding strlen of the result so far (twice) on
each iteration - O(N^2), but the other extra 2x strlen can add up too.

Join of an array of 2000 strings of 80 chars each:
  Windows: before: 80ms, after: 2ms
  Linux:   before: 20ms, after: 2ms
Measured using Date.now()
---
 jsarray.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/jsarray.c b/jsarray.c
index ca1bb70..e655f93 100644
--- a/jsarray.c
+++ b/jsarray.c
@@ -91,7 +91,7 @@ static void Ap_join(js_State *J)
 	const char *sep;
 	const char *r;
 	int seplen;
-	int k, n, len;
+	int k, n, len, outlen, rlen;
 
 	len = js_getlength(J, 0);
 
@@ -120,20 +120,22 @@ static void Ap_join(js_State *J)
 			r = "";
 		else
 			r = js_tostring(J, -1);
-		n += strlen(r);
+		outlen = n - 1;
+		rlen = strlen(r);
+		n += rlen;
 
 		if (k == 0) {
 			if (n > JS_STRLIMIT)
 				js_rangeerror(J, "invalid string length");
 			out = js_malloc(J, (int)n);
-			strcpy(out, r);
+			memcpy(out, r, rlen + 1);
 		} else {
 			n += seplen;
 			if (n > JS_STRLIMIT)
 				js_rangeerror(J, "invalid string length");
 			out = js_realloc(J, out, (int)n);
-			strcat(out, sep);
-			strcat(out, r);
+			memcpy(out + outlen, sep, seplen);
+			memcpy(out + outlen + seplen, r, rlen + 1);
 		}
 
 		js_pop(J, 1);