[PATCH v4 2/6] lib/base64: Optimize base64_decode() with reverse lookup tables

[Guan-Chun Wu]

From: Kuan-Wei Chiu <visitorckw at gmail.com>

Replace the use of strchr() in base64_decode() with precomputed reverse
lookup tables for each variant. This avoids repeated string scans and
improves performance. Use -1 in the tables to mark invalid characters.

Decode:
  64B   ~1530ns  ->  ~80ns    (~19.1x)
  1KB  ~27726ns  -> ~1239ns   (~22.4x)

Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
Co-developed-by: Guan-Chun Wu <409411716 at gms.tku.edu.tw>
Signed-off-by: Guan-Chun Wu <409411716 at gms.tku.edu.tw>
---
 lib/base64.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/lib/base64.c b/lib/base64.c
index a7c20a8e8..8a0d28908 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -21,6 +21,21 @@ static const char base64_tables[][65] = {
 	[BASE64_IMAP] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,",
 };
 
+#define BASE64_REV_INIT(ch_62, ch_63) { \
+	[0 ... 255] = -1, \
+	['A'] =  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, \
+		13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, \
+	['a'] = 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, \
+		39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, \
+	['0'] = 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, \
+	[ch_62] = 62, [ch_63] = 63, \
+}
+
+static const s8 base64_rev_maps[][256] = {
+	[BASE64_STD] = BASE64_REV_INIT('+', '/'),
+	[BASE64_URLSAFE] = BASE64_REV_INIT('-', '_'),
+	[BASE64_IMAP] = BASE64_REV_INIT('+', ',')
+};
 /**
  * base64_encode() - Base64-encode some binary data
  * @src: the binary data to encode
@@ -84,10 +99,9 @@ int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base6
 	int bits = 0;
 	int i;
 	u8 *bp = dst;
-	const char *base64_table = base64_tables[variant];
+	s8 ch;
 
 	for (i = 0; i < srclen; i++) {
-		const char *p = strchr(base64_table, src[i]);
 		if (padding) {
 			if (src[i] == '=') {
 				ac = (ac << 6);
@@ -97,9 +111,10 @@ int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base6
 				continue;
 			}
 		}
-		if (p == NULL || src[i] == 0)
+		ch = base64_rev_maps[variant][(u8)src[i]];
+		if (ch == -1)
 			return -1;
-		ac = (ac << 6) | (p - base64_table);
+		ac = (ac << 6) | ch;
 		bits += 6;
 		if (bits >= 8) {
 			bits -= 8;
-- 
2.34.1