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Expand Fletcher-16 NMAX

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The limit when calculating the value of NMAX is 2^16-1 only when sum1
and sum2 are represented as 16-bit. We're representing them as 32-bit.

This will be helpful for SIMD implementations of Fletcher-16, because
the old limit of 22 bytes would have meant that we could only process
16 bytes at a time before having to compute modulo.
This commit is contained in:
JosJuice
2023-09-23 23:01:17 +02:00
committed by Natalia Portillo
parent 449c25012d
commit a9b6c4868b
3 changed files with 10 additions and 30 deletions
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34
fletcher16.c
View File

@@ -84,7 +84,7 @@ AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx *ctx, const uint8_t *
}
/* in case short lengths are provided, keep it somewhat fast */
if(len < 11)
if(len < 6)
{
while(len--)
{
@@ -102,7 +102,7 @@ AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx *ctx, const uint8_t *
while(len >= NMAX)
{
len -= NMAX;
n = NMAX / 11; /* NMAX is divisible by 11 */
n = NMAX / 6; /* NMAX is divisible by 6 */
do
{
sum1 += data[0];
@@ -117,19 +117,9 @@ AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx *ctx, const uint8_t *
sum2 += sum1;
sum1 += data[0 + 4 + 1];
sum2 += sum1;
sum1 += data[0 + 4 + 2];
sum2 += sum1;
sum1 += data[0 + 4 + 2 + 1];
sum2 += sum1;
sum1 += data[8];
sum2 += sum1;
sum1 += data[8 + 1];
sum2 += sum1;
sum1 += data[8 + 2];
sum2 += sum1;
/* 11 sums unrolled */
data += 11;
/* 6 sums unrolled */
data += 6;
}
while(--n);
sum1 %= FLETCHER16_MODULE;
@@ -139,9 +129,9 @@ AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx *ctx, const uint8_t *
/* do remaining bytes (less than NMAX, still just one modulo) */
if(len)
{ /* avoid modulos if none remaining */
while(len >= 11)
while(len >= 6)
{
len -= 11;
len -= 6;
sum1 += data[0];
sum2 += sum1;
sum1 += data[0 + 1];
@@ -154,18 +144,8 @@ AARU_EXPORT int AARU_CALL fletcher16_update(fletcher16_ctx *ctx, const uint8_t *
sum2 += sum1;
sum1 += data[0 + 4 + 1];
sum2 += sum1;
sum1 += data[0 + 4 + 2];
sum2 += sum1;
sum1 += data[0 + 4 + 2 + 1];
sum2 += sum1;
sum1 += data[8];
sum2 += sum1;
sum1 += data[8 + 1];
sum2 += sum1;
sum1 += data[8 + 2];
sum2 += sum1;
data += 11;
data += 6;
}
while(len--)
{

4
fletcher16.h
View File

@@ -20,8 +20,8 @@
#define AARU_CHECKSUMS_NATIVE_FLETCHER16_H
#define FLETCHER16_MODULE 0xFF
/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^16-1 */
#define NMAX 22
/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(FLETCHER16_MODULE-1) <= 2^32-1 */
#define NMAX 5802
typedef struct
{

2
fletcher32.h
View File

@@ -20,7 +20,7 @@
#define AARU_CHECKSUMS_NATIVE_FLETCHER32_H
#define FLETCHER32_MODULE 0xFFFF
/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(ADLER_MODULE-1) <= 2^32-1 */
/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(FLETCHER32_MODULE-1) <= 2^32-1 */
#define NMAX 5552
typedef struct