Jacky Qwerty/29A Compression Algorithm

jqwerty.c

//
// Jacky Qwerty/29A compression algorithm, by Matt Mahoney
// modified by odzhan
// 2019-12-07
//
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>

FILE *in, *out;

void jqwerty_compress(const void *inbuf, uint32_t inlen, void *outbuf) {
    int flags=1;  // 8 packed bits: 1 = mispredicted byte. bit 8 = sentinel
    int cxt=0;    // 2 byte context
    int n=0;      // number of pending literals to write
    int c;        // last byte
    int i;
    unsigned char buf[8]={0};        // pending literals to write
    unsigned char model[1<<16]={0};  // order 2 prediction of next byte
  
    while(1) {
      c = getc(in);
      if(c != EOF) {
        flags += flags;
        if(c != model[cxt]) {
          ++flags; 
          buf[n++] = model[cxt] = c;
        }
        cxt=((cxt << 8) | c) & 0xffff;
      } else {
        while (flags < 256) {
          flags += flags + 1;  // pad
        }
      }
      if(flags >= 256) {
        putc(flags & 255, out);
        for(i=0; i<n; ++i) {
          putc(buf[i], out);
        }
        flags = 1;
        n = 0;
      }
      if (c == EOF) break;
    }
}

void jqwerty_decompress(const void *inbuf, uint32_t inlen, void *outbuf) {
    int flags=1;  // 8 packed bits: 1 = mispredicted byte. bit 8 = sentinel
    int cxt=0;    // 2 byte context
    int c;        // last byte
    int i;
    unsigned char model[1<<16]={0};  // order 2 prediction of next byte
    
    while((flags = getc(in)) != EOF) {
      for(i=7; i>=0; --i) {
        if ((flags>>i)&1) {
          c = getc(in);
          if(c == EOF) break;
          else putc(model[cxt] = c, out);
        } else putc(c = model[cxt], out);
        cxt=(cxt << 8 | c) & 0xffff;
      }
    }
}

// User interface.  Args are input and output file.
int main(int argc, char **argv) {
    // Check arguments
    if ((argc!=4)||((argv[1][0]!='c')&&(argv[1][0]!='d'))) {
        printf("Usage: jqwerty c/d input output\n");
        return 0;
    }

    in = fopen(argv[2], "rb");
    if (!in) {perror(argv[2]); return 1;}
    out = fopen(argv[3], "wb");
    if (!out) {perror(argv[3]); return 1;}
    
    if (argv[1][0]=='c') {
        printf("Compressing %s to %s ...\n", argv[2], argv[3]);
        jqwerty_compress(0, 0, 0); // Compress
    } else {
        printf("Decompressing %s from %s ...\n", argv[3], argv[2]);
        jqwerty_decompress(0, 0, 0); // Decompress
    }
    fclose(in);
    fclose(out);
    return 0;
}