C function for comparing float values that are close enough (<= 10 ULP)

almost-equal32.c

// generated from https://chatgpt.com/canvas/shared/67afab561d4c8191bbb497e5cef5147b

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

#define ULP_TOLERANCE 10

int float_almost_equal(float a, float b) {
    // Handle NaN cases
    if (isnan(a) || isnan(b)) return 0;
    
    // Handle exact equality
    if (a == b) return 1;
    
    // Handle large numbers as almost equal to infinity using bit manipulation
    int32_t ai = *(int32_t*)&a;
    int32_t bi = *(int32_t*)&b;
    int32_t abs_ai = ai & 0x7FFFFFFF;
    int32_t abs_bi = bi & 0x7FFFFFFF;
    int32_t float_max_bits = 0x7F7FFFFF; // Largest finite float
    if (abs_ai >= float_max_bits || abs_bi >= float_max_bits) {
        return ((ai == 0x7F800000 || ai == 0xFF800000) && (bi == 0x7F800000 || bi == 0xFF800000)) || (abs(abs_ai - abs_bi) < 0x100000);
    }
    
    // Handle negative and positive zero as equal
    if (ai == 0 && bi == 0) return 1;
    
    // Make the integer representation ordered as a two's complement integer
    if (ai < 0) ai = 0x80000000 - ai;
    if (bi < 0) bi = 0x80000000 - bi;
    
    // Compute the difference in ULPs
    int32_t ulp_diff = abs(ai - bi);
    
    return ulp_diff <= ULP_TOLERANCE;
}

int main() {
    float x = 0.1f;
    float y = nextafterf(x, 1.0f); // Get next representable float
    
    if (float_almost_equal(x, y)) {
        printf("Numbers are almost equal.\n");
    } else {
        printf("Numbers are not equal.\n");
    }
    
    return 0;
}