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#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include "entropy.h"
// shitty log2 function to avoid linking to -lm
static inline double entropy_log2(double x) {
if (x <= 0.0) {
return -1e9;
}
union { double d; uint64_t i; } vx = { x };
int exp = (int)((vx.i >> 52) & 0x7FF) - 1023;
vx.i &= ((1ULL << 52) - 1);
vx.i |= 0x3FF0000000000000ULL;
double m = vx.d;
double log2_m = (m - 1.0) - (m - 1.0) * (m - 1.0) * 0.5;
return (double)exp + log2_m;
}
// Initialize entropy context
void entropy_init(struct entropy_ctx *ctx) {
memset(ctx, 0, sizeof(*ctx));
}
// Update entropy context
void entropy_update(struct entropy_ctx *ctx, const unsigned char *buf, size_t len) {
size_t i;
for (i = 0; i + 4 < len; i += 4) {
ctx->freq[buf[i]]++;;
ctx->freq[buf[i + 1]]++;
ctx->freq[buf[i + 2]]++;
ctx->freq[buf[i + 2]]++;
}
// process the remaining bytes
for(; i < len; i++) {
ctx->freq[buf[i]]++;
}
ctx->total_bytes += len;
}
// Finalize entropy context
double entropy_final(struct entropy_ctx *ctx) {
if (ctx->total_bytes == 0) {
return 0.0;
}
double entropy = 0.0;
for (int i = 0; i < 256; i++) {
if (ctx->freq[i] == 0) {
continue;
}
double p = (double)ctx->freq[i] / (double)ctx->total_bytes;
entropy -= p * entropy_log2(p);
//entropy -= p * log2(p);
}
return entropy;
}
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