Test fixed-point number formatting

test.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <limits.h>

void print_number_f(double x) {
    double y = x;
    int c = 0;
    if (y < 0.0) {
        y = -y;
    }
    while (y > 0 && y < 100.0) {
        y *= 10.0;
        c++;
    }
    printf("%.*f", c, x);
}

#define FP_EXP (6)
#define FP_MULT (1000000LL)

/* Format fixed point number. */
void print_number(const int64_t x) {
    int64_t x_abs, y;
    int c, i, rounding;
    size_t ptr;
    char buffer[30];

    if (x == INT64_MIN) {
        /* Prevent UB. */
        printf("ERR");
        return;
    }
    x_abs = x < 0 ? -x : x;

    /* Determine how many decimals we want to show (more than FP_EXP makes no
     * sense). */
    y = x_abs;
    c = 0;
    while (y > 0LL && y < 100LL * FP_MULT && c < FP_EXP) {
        y *= 10LL;
        c++;
    }

    /* Round to 'c' decimals. */
    y = x_abs;
    rounding = 0;
    for (i = c; i < FP_EXP; ++i) {
        rounding = (y % 10) >= 5;
        y /= 10;
    }
    y += rounding;

    /* Format and print the number. */
    ptr = sizeof(buffer) - 1;
    buffer[ptr] = 0;
    if (c != 0) {
        for (i = 0; i < c; ++i) {
            buffer[--ptr] = '0' + (y % 10);
            y /= 10;
        }
        buffer[--ptr] = '.';
    }
    do {
        buffer[--ptr] = '0' + (y % 10);
        y /= 10;
    } while (y != 0);
    if (x < 0) {
        buffer[--ptr] = '-';
    }
    printf("%s", &buffer[ptr]);
}

int main()
{
    printf("[comparison]\n");
    print_number_f(0.0);
    printf(" ");
    print_number(0LL);
    printf("\n");

    print_number_f(0.01);
    printf(" ");
    print_number(10000LL);
    printf("\n");

    print_number_f(0.0999);
    printf(" ");
    print_number(99900LL);
    printf("\n");

    print_number_f(0.09999);
    printf(" ");
    print_number(99990LL);
    printf("\n");

    print_number_f(0.1);
    printf(" ");
    print_number(100000LL);
    printf("\n");

    print_number_f(1.0);
    printf(" ");
    print_number(1000000LL);
    printf("\n");

    print_number_f(15.0);
    printf(" ");
    print_number(15000000LL);
    printf("\n");

    print_number_f(150.0);
    printf(" ");
    print_number(150000000LL);
    printf("\n");

    print_number_f(-0.01);
    printf(" ");
    print_number(-10000LL);
    printf("\n");

    print_number_f(-150.0);
    printf(" ");
    print_number(-150000000LL);
    printf("\n");

    print_number_f(-150.51);
    printf(" ");
    print_number(-150510000LL);
    printf("\n");

    printf("[special values]\n");
    print_number(1);
    printf(" ");
    print_number(-1);
    printf(" ");
    print_number(INT64_MAX);
    printf(" ");
    print_number(INT64_MIN);
    printf(" ");
    print_number((int64_t)-INT_MAX * FP_MULT);
    printf(" ");
    print_number((int64_t)INT_MAX * FP_MULT);
    printf("\n");
    return 0;
}