Criterion-like formatting of numbers

main.c

#define _POSIX_C_SOURCE 200809L // For strdup.

#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#include <math.h> // Remember to link with -lm.

#define ARRAY_LEN(array) (sizeof(array) / sizeof((array)[0]))

typedef struct {
    int exponent;
    const char *symbol;
    const char *bin_symbol;
} Prefix;

Prefix PREFIXES[] = {
    {8, "Y", "Yi"},
    {7, "Z", "Zi"},
    {6, "E", "Ei"},
    {5, "P", "Pi"},
    {4, "T", "Ti"},
    {3, "G", "Gi"},
    {2, "M", "Mi"},
    {1, "k", "Ki"},
    {0, "", ""},
    {-1, "m", NULL},
    {-2, "μ", NULL},
    {-3, "n", NULL},
    {-4, "p", NULL},
    {-5, "f", NULL},
    {-6, "a", NULL},
    {-7, "z", NULL},
    {-8, "y", NULL}
};

char *format_num_with_prefix(double num, const char *prefix) {
    // Format to 4 digits, even if they are trailing zeros.

    double num_abs = fabs(num);

    char buffer[128];
    if (num_abs >= 1e9) {
        snprintf(buffer, ARRAY_LEN(buffer), "%.4g %s", num, prefix);
        return strdup(buffer);
    }

    for (int exponent = 3; exponent >= 1; exponent--) {
        if (num_abs >= pow(10, exponent)) {
            snprintf(buffer, ARRAY_LEN(buffer), "%.*f %s", 3 - exponent, num, prefix);
            return strdup(buffer);
        }
    }

    snprintf(buffer, ARRAY_LEN(buffer), "%.3f %s", num, prefix);
    return strdup(buffer);
}

char *format_num(double num, bool binary) {
    // Format a number with a SI prefix.
    // Based on the Criterion library for Haskell.
    // Alternative: <http://jkorpela.fi/c/eng.html>.

    assert(isfinite(num));

    double num_abs = fabs(num);
    double base = binary ? 1024 : 1000;

    for (size_t i_prefix = 0; i_prefix < ARRAY_LEN(PREFIXES); i_prefix++) {
        Prefix prefix = PREFIXES[i_prefix];

        const char *symbol = binary ? prefix.bin_symbol : prefix.symbol;
        if (symbol == NULL) {
            continue;
        }

        if (num_abs >= pow(base, prefix.exponent)) {
            return format_num_with_prefix(num * pow(base, -prefix.exponent), symbol);
        }
    }

    char buffer[128];
    snprintf(buffer, ARRAY_LEN(buffer), "%g ", num);
    return strdup(buffer);
}

int main(int argc, char **argv) {
    double example_time = 0.000005235724;
    char *example_time_str = format_num(example_time, false);
    printf("Example time: %ss\n", example_time_str);
    free(example_time_str);

    int example_size = 52 * 1024 * 1024;
    char *example_size_str = format_num(example_size, true);
    printf("Example file size: %sB\n", example_size_str);
    free(example_size_str);
}