$NetBSD: patch-af,v 1.1 2006/03/30 13:51:29 salo Exp $

--- ars.c.orig	2003-07-28 11:00:54.000000000 +0200
+++ ars.c	2005-11-05 21:35:00.000000000 +0100
@@ -372,7 +372,7 @@
 	}
 	if (nbytes == 1) {
 		oddbyte = 0;
-		*((u_int16_t *) &oddbyte) = *(u_int8_t *) buf;
+		*((u_int8_t *) &oddbyte) = *((u_int8_t *) buf);
 		sum += oddbyte;
 	}
 	sum = (sum >> 16) + (sum & 0xffff);
@@ -381,6 +381,13 @@
 }
 
 /* Multiple buffers checksum facility */
+/*
+ * Important facts about the checksum (sum = ~cksum, Ai are bytes):
+ * 1. sum(A0...A2i-1,A2i...A2n-1) = sum(A0...A2i-1) + sum(A2i...B2n-1),
+ * 2. sum(A0...A2i,A2i+1...A2n-1) = sum(A0...A2i,0) + sum(0,A2i+1...A2n-1)
+ * 3. sum(A0...A2n-1,A2n) = sum(A0...A2n-1,A2n,0)  (ex definitiones)
+ * 4. sum(0,A0...Ak) = swap_bytes(sum(A0...Ak))
+ */
 u_int16_t ars_multi_cksum(struct mc_context *c, int op, void *vbuf,
 							size_t nbytes)
 {
@@ -394,38 +401,27 @@
 		c->old = 0;
 		return -ARS_OK;
 	} else if (op == ARS_MC_UPDATE) {
-		if (c->oddbyte_flag) {
-			u_int8_t *x = (u_int8_t*)&oddbyte;
-			oddbyte = 0;
-			*((u_int16_t *) &oddbyte) = c->oddbyte << 8;
-			*((u_int16_t *) &oddbyte) |= *(u_int8_t *) buf;
-			oddbyte = (x[0] << 8) | x[1]; /* fix endianess */
-			c->old += oddbyte;
-			nbytes--;
-			c->oddbyte_flag = 0;
-			/* We need to stay aligned -- bad slowdown, fix? */
-			tmp = alloca(nbytes);
-			memcpy(tmp, vbuf+1, nbytes);
-			buf = tmp;
-		}
-		sum = c->old;
+		sum = 0;
 		while (nbytes > 1) {
 			sum += *buf++;
 			nbytes -= 2;
 		}
-		c->old = sum;
 		if (nbytes == 1) {
-			c->oddbyte = *(u_int8_t*) buf;
-			c->oddbyte_flag++;
+			oddbyte = 0;
+			*((u_int8_t *) &oddbyte) = *((u_int8_t *) buf);
+			sum += oddbyte;
 		}
+		if (c->oddbyte_flag) {
+			/* reverse checksum endianness */
+			sum = (sum >> 16) + (sum & 0xffff);
+			sum += (sum >> 16);
+			sum = ((sum & 0xff00) >> 8) | ((sum & 0xff) << 8);
+		}
+		c->old += sum;
+		c->oddbyte_flag ^= (nbytes == 1 ? 1 : 0);
 		return -ARS_OK;
 	} else if (op == ARS_MC_FINAL) {
 		sum = c->old;
-		if (c->oddbyte_flag == 1) {
-			oddbyte = 0;
-			*((u_int16_t *) &oddbyte) = c->oddbyte;
-			sum += oddbyte;
-		}
 		sum = (sum >> 16) + (sum & 0xffff);
 		sum += (sum >> 16);
 		return (u_int16_t) ~sum;