sellerator/sellerator.c

#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <math.h>

#define max(a,b) ({                 \
        __typeof__ (a) _a = (a);    \
        __typeof__ (b) _b = (b);    \
        _a > _b ? _a : _b;          \
    })

void usage(FILE *out, const char *name) {
    fprintf(out, "Usage: %s n p\n", name);
    fprintf(out, "\n  n: Number of reviews\n  p: Percent positive\n");
}

double wilson_lower(long n, double p) {
    // https://en.wikipedia.org/wiki/Binomial_proportion_confidence_interval#Wilson_score_interval_with_continuity_correction
    double z = 1.96; // Let's use a good ol' 95% CI
    double w = (2.0*n*p + pow(z,2) - (1.0 + z*sqrt(pow(z,2)-1.0/n+4*n*p*(1-p)+(4*p-2))))
        /(2.0*((double)n+pow(z,2)));
    return(max(0.0, w));
}

int score(int argc, char **argv) {
    if(argc != 3) {
        usage(stderr, argv[0]);
        exit(1);
    }
    long n = 0;
    char *endptr = NULL;
    errno = 0;
    n = strtol(argv[1], &endptr, 10);
    if(errno != 0) {
        fprintf(stderr, "While parsing '%s', got error %d: %s\n", argv[1], errno, strerror(errno));
        usage(stderr, argv[0]);
        exit(1);
    }
    if(*endptr != '\0') {
        fprintf(stderr, "Error: '%s' is not numeric.\n", argv[1]);
        usage(stderr, argv[0]);
        exit(1);
    }
    if(n <= 0) {
        fprintf(stderr, "Error: got a non-positive number of reviews: %ld.\n", n);
        usage(stderr, argv[0]);
        exit(1);
    }
    double p = 0;
    endptr = NULL;
    errno = 0;
    p = strtod(argv[2], &endptr);
    if(errno != 0) {
        fprintf(stderr, "While parsing '%s', got error %d: %s\n", argv[1], errno, strerror(errno));
        usage(stderr, argv[0]);
        exit(1);
    }
    if(*endptr != '\0') {
        fprintf(stderr, "Error: '%s' is not numeric.\n", argv[2]);
        usage(stderr, argv[0]);
        exit(1);
    }
    if(p < 0 || p > 100) {
        fprintf(stderr, "Got a percentage of %0.1f, which is not between 0.0%% and 100.0%%\n", p);
        usage(stderr, argv[0]);
        exit(1);
    }
    double lower = wilson_lower(n, p/100.0);
    printf("%f\n", 1.0/(1.0-lower));
    return 0;
}

int main(int argc, char **argv) {
    if(argc < 2) {
        usage(stderr, argv[0]);
        exit(1);
    }
    score(argc, argv);
    // TODO: add another subcommand for testing hypothesis of difference between sellers
    return 0;
}