From: Andre Noll <maan@tuebingen.mpg.de>
Date: Sat, 9 Apr 2016 18:44:28 +0000 (+0200)
Subject: base64: Speed up decoder by using a table.
X-Git-Tag: v0.5.7~20^2
X-Git-Url: http://git.tue.mpg.de/?a=commitdiff_plain;h=d865486282660af20194463e8e36a6d84ab579c8;p=paraslash.git

base64: Speed up decoder by using a table.

The current implementation calls strchr() for each character in the
decoded data to find the offset in the Base64[] array that corresponds
to six decoded bits. This makes the algorithm scale poorly.

This commit introduces a lookup table of size 256 which simplifies
the code and improves the performance of the decoder.
---

diff --git a/base64.c b/base64.c
index cee51571..7b8fe292 100644
--- a/base64.c
+++ b/base64.c
@@ -15,6 +15,40 @@
 
 static const char Base64[] =
 	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+static const unsigned char base64_tab[256] = {
+	255, 255, 255, 255, 255, 255, 255, 255, /* 00-07 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 08-0f */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 10-17 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 18-1f */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 20-2f */
+	255, 255, 255,  62, 255, 255, 255,  63, /* 28-2f */
+	52 ,  53,  54,  55,  56,  57,  58,  59, /* 30-37 */
+	60 ,  61, 255, 255, 255, 255, 255, 255, /* 38-3f */
+	255,   0,   1,   2,   3,   4,   5,   6, /* 40-47 */
+	7  ,   8,   9,  10,  11,  12,  13,  14, /* 48-4f */
+	15 ,  16,  17,  18,  19,  20,  21,  22, /* 50-57 */
+	23 ,  24,  25, 255, 255, 255, 255, 255, /* 58-5f */
+	255,  26,  27,  28,  29,  30,  31,  32, /* 60-6f */
+	33 ,  34,  35,  36,  37,  38,  39,  40, /* 68-6f */
+	41 ,  42,  43,  44,  45,  46,  47,  48, /* 70-77 */
+	49 ,  50,  51, 255, 255, 255, 255, 255, /* 78-7f */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 80-87 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 88-8f */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 90-97 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* 98-9f */
+	255, 255, 255, 255, 255, 255, 255, 255, /* a0-a7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* a8-af */
+	255, 255, 255, 255, 255, 255, 255, 255, /* b0-b7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* b8-bf */
+	255, 255, 255, 255, 255, 255, 255, 255, /* c0-c7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* c8-cf */
+	255, 255, 255, 255, 255, 255, 255, 255, /* d0-d7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* d8-df */
+	255, 255, 255, 255, 255, 255, 255, 255, /* e0-e7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* e8-ef */
+	255, 255, 255, 255, 255, 255, 255, 255, /* f0-f7 */
+	255, 255, 255, 255, 255, 255, 255, 255, /* f8-ff */
+};
 
 /** Maximal possible size of the decoded data. */
 #define BASE64_MAX_DECODED_SIZE(_encoded_size) ((_encoded_size) / 4 * 3)
@@ -41,61 +75,54 @@ static const char Base64[] =
 int base64_decode(char const *src, size_t encoded_size, char **result,
 		size_t *decoded_size)
 {
-	unsigned int tarindex, state;
-	int ch;
-	char *pos;
-	const char *end = src + encoded_size;
-	unsigned char *target;
+	size_t i, j, state; /* source/target indices */
+	const char *end = src + encoded_size, *p;
+	unsigned char *target, uch;
 
 	if (encoded_size == (size_t)-1)
 		encoded_size = strlen(src);
 	target = para_malloc(BASE64_MAX_DECODED_SIZE(encoded_size) + 1);
 
-	state = 0;
-	tarindex = 0;
-
-	while (src < end) {
-		ch = *src++;
-		if (para_isspace(ch)) /* Skip whitespace anywhere. */
+	for (
+		i = 0, j = 0, state = 0;
+		i < encoded_size && (uch = src[i]) != '\0';
+		i++
+	) {
+		if (para_isspace(uch)) /* Skip whitespace anywhere. */
 			continue;
-
-		if (ch == PAD64)
+		if (uch == PAD64)
 			break;
-
-		pos = strchr(Base64, ch);
-		if (pos == NULL) /* A non-base64 character. */
+		if (base64_tab[uch] == 255) /* A non-base64 character. */
 			goto fail;
-
+		uch = base64_tab[uch];
 		switch (state) {
 		case 0:
-			target[tarindex] = (pos - Base64) << 2;
-			state = 1;
+			target[j] = uch << 2;
 			break;
 		case 1:
-			target[tarindex] |= (pos - Base64) >> 4;
-			target[tarindex + 1] = ((pos - Base64) & 0x0f) << 4;
-			tarindex++;
-			state = 2;
+			target[j] |= uch >> 4;
+			j++;
+			target[j] = (uch & 0x0f) << 4;
 			break;
 		case 2:
-			target[tarindex] |= (pos - Base64) >> 2;
-			target[tarindex + 1] = ((pos - Base64) & 0x03) << 6;
-			tarindex++;
-			state = 3;
+			target[j] |= uch >> 2;
+			j++;
+			target[j] = (uch & 0x03) << 6;
 			break;
 		case 3:
-			target[tarindex] |= pos - Base64;
-			tarindex++;
-			state = 0;
+			target[j] |= uch;
+			j++;
 			break;
 		}
+		state = (state + 1) % 4;
 	}
+	p = (i < encoded_size)? src + i : NULL;
 	/*
 	 * We are done decoding Base-64 chars.  Let's see if we ended
 	 * on a byte boundary, and/or with erroneous trailing characters.
 	 */
-	if (*src == PAD64) { /* We got a pad char. */
-		ch = *src++; /* Skip it, get next. */
+	if (p && *p == PAD64) { /* We got a pad char. Skip it, get next. */
+		p++;
 		switch (state) {
 		case 0: /* Invalid = in first position */
 		case 1: /* Invalid = in second position */
@@ -103,22 +130,22 @@ int base64_decode(char const *src, size_t encoded_size, char **result,
 
 		case 2: /* Valid, means one byte of info */
 			/* Skip any number of spaces. */
-			for (; ch != '\0'; ch = *src++)
-				if (!para_isspace(ch))
+			for (; p < end && *p != '\0'; p++)
+				if (!para_isspace(*p))
 					break;
 			/* Make sure there is another trailing = sign. */
-			if (ch != PAD64)
+			if (*p != PAD64)
 				goto fail;
-			ch = *src++;		/* Skip the = */
 			/* Fall through to "single trailing =" case. */
+			p++;
 
 		case 3: /* Valid, means two bytes of info */
 			/*
 			 * We know this char is an =.  Is there anything but
 			 * whitespace after it?
 			 */
-			for (; ch != '\0'; ch = *src++)
-				if (!para_isspace(ch))
+			for (; p < end && *p != '\0'; p++)
+				if (!para_isspace(*p))
 					goto fail;
 			/*
 			 * Now make sure for cases 2 and 3 that the "extra"
@@ -126,7 +153,7 @@ int base64_decode(char const *src, size_t encoded_size, char **result,
 			 * zeros.  If we don't check them, they become a
 			 * subliminal channel.
 			 */
-			if (target[tarindex] != 0)
+			if (target[j] != 0)
 				goto fail;
 		}
 	} else {
@@ -138,10 +165,10 @@ int base64_decode(char const *src, size_t encoded_size, char **result,
 			goto fail;
 	}
 	/* success */
-	target[tarindex] = '\0'; /* just to be sure */
-	*result = (char *)target;
+	target[j] = '\0'; /* just to be sure */
 	if (decoded_size)
-		*decoded_size = tarindex;
+		*decoded_size = j;
+	*result = (char *)target;
 	return 1;
 fail:
 	free(target);