gistfile1.c

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <math.h>
#include <stdbool.h>

#define MAX(x,m) ((x)>(m) ? (x) : (m))

int bin2dec(char *str, int len) {
    int ret = 0, i;
    for(i=0; i < len; ++i)
        ret += (str[i] - '0') * pow(2, len-i-1);
    return ret;
}

int str2idx(char *str, int len) {
    int ret = bin2dec(str, len), i;
    char ones[len-1];
    memset(ones, '1', len-1);
    for(i = 1; i < len; ++i)
        ret += bin2dec(ones, i);
    return ret;
}

void decodeTransmission(char *transmission, char *output, size_t outLen)
{
    char *msg = NULL;
    for(msg = transmission; *msg != '\0'; ++msg) {
        if(*msg >= '0' && *msg <= '1')
            break;
    }
    assert(msg != NULL);
    int headerLen = msg - transmission;
    assert(headerLen > 0);
    char header[headerLen];
    memcpy(header, transmission, headerLen);
    
    int msgLen = strlen(msg);
    char *end = msg+msgLen-4;
    assert(strncmp(end, "000\n", 4) == 0);
        
    int keyCount = (-1 + sqrt(1 + 8*msgLen))/2.0;
    char result[keyCount];
    memset(result, '\0', keyCount);
    int maxKeyLen = MAX(3, keyCount);
    
    char keyBuf[maxKeyLen];
    memset(keyBuf, 0, maxKeyLen);
    int keyLen = 3;
    char terminator[maxKeyLen];
    memset(terminator, '1', maxKeyLen);
    int keyCursor = 0, resultCursor = 0;
    bool counting = true;

    for(; msg < end; ++msg) {
        if(*msg == '\r') continue;
        keyBuf[keyCursor] = *msg; 
        if(++keyCursor != keyLen) continue;

        keyCursor = 0;
        int dec = bin2dec(keyBuf, keyLen);
        if(counting) {
            keyLen = dec;
            counting = false;
        } else if(strncmp(keyBuf, terminator, keyLen) == 0) {
            counting = true;
            keyLen = 3;
        } else
            result[resultCursor++] = header[str2idx(keyBuf, keyLen)];
    }
    if(output)
        strncpy(output, result, outLen);
}

int main(int argc, char *argv[]) {
    FILE *input = fopen(argv[1], "r");
    char transmission[1024];
    char result[1024];
    while(fgets(transmission, 1024, input)) {
        if(transmission[0] == '\n') continue;
        decodeTransmission(transmission, result, 1024);
        puts(result);
    }
    return 0;
}