linux-kernel - [PATCH 2/2 v2] lib/vsprintf.c: Further simplify uuid

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Date:	12 May 2015 05:10:36 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	joe@...ches.com, linux@...izon.com
Cc:	linux-kernel@...r.kernel.org, linux@...musvillemoes.dk
Subject: [PATCH 2/2 v2] lib/vsprintf.c: Further simplify uuid_string()

Make the endianness permutation table do double duty by having it
list not source offsets, but destination offsets.  Thus, it both puts
the bytes in the right order and skips the hyphens.

This further shrinks the code from 256 to 214 bytes.  Eliminating
erratic branches probably helps speed, too.

Signed-off-by: George Spelvin <linux@...izon.com>
---
> These might be better with a little comment/explanation
> of the values as output offsets for each index.

Like this?  I had thought about it, and had decied not to change the
existing lacomic code style, as it didn't seem any harder to understand
than the original.  But I'm happy to add comments.

 lib/vsprintf.c | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index c3fb18bb..e5db83a9 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1265,14 +1265,14 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 		  struct printf_spec spec, const char *fmt)
 {
 	char uuid[sizeof("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx")];
-	char *p = uuid;
-	int i;
-	static const u8 be[16] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
-	static const u8 le[16] = {3,2,1,0,5,4,7,6,8,9,10,11,12,13,14,15};
+	/* Offset in uuid[] where each byte is printed, two cases. */
+	static const u8 be[16] = {0,2,4,6,9,11,14,16,19,21,24,26,28,30,32,34};
+	static const u8 le[16] = {6,4,2,0,11,9,16,14,19,21,24,26,28,30,32,34};
 	const u8 *index = be;
 	const char *hex = hex_asc;
+	int i;
 
-	switch (*(++fmt)) {
+	switch (fmt[1]) {
 	case 'L':
 		hex = hex_asc_upper;	/* fall-through */
 	case 'l':
@@ -1283,21 +1283,17 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 		break;
 	}
 
+	/* Format each byte of the raw uuid into the buffer */
 	for (i = 0; i < 16; i++) {
-		u8 byte = addr[index[i]];
-		*p++ = hex[byte >> 4];
-		*p++ = hex[byte & 0x0f];
-		switch (i) {
-		case 3:
-		case 5:
-		case 7:
-		case 9:
-			*p++ = '-';
-			break;
-		}
-	}
+		u8 byte = addr[i];
+		char *p = uuid + index[i];
 
-	*p = 0;
+		p[0] = hex[byte >> 4];
+		p[1] = hex[byte & 0x0f];
+	}
+	/* Insert the fixed punctuation */
+	uuid[23] = uuid[18] = uuid[13] = uuid[8] = '-';
+	uuid[36] = 0;
 
 	return string(buf, end, uuid, spec);
 }
-- 
2.1.4

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