overflow.h

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#pragma once
 
#include <stddef.h>
#include <stdint.h>
#include <stdbool.h>
#include <limits.h>
 
#include "../common.h"
 
/* ============================================================================
 * Overflow Prediction Functions
 * ============================================================================
 */
 
static inline bool size_mul_would_overflow(size_t a, size_t b) {
  // Handle zero case (no overflow)
  if (a == 0 || b == 0)
    return false;
 
  // Check if a * b would exceed SIZE_MAX
  // This is safe: if a != 0, we can divide
  return b > SIZE_MAX / a;
}
 
static inline bool size_add_would_overflow(size_t a, size_t b) {
  return b > SIZE_MAX - a;
}
 
/* ============================================================================
 * Checked Arithmetic Operations
 * ============================================================================
 */
 
static inline asciichat_error_t checked_size_mul(size_t a, size_t b, size_t *result) {
  if (size_mul_would_overflow(a, b)) {
    return 1; // ERROR_OVERFLOW - see asciichat_errno.h for exact value
  }
  *result = a * b;
  return 0; // ASCIICHAT_OK
}
 
static inline asciichat_error_t checked_size_add(size_t a, size_t b, size_t *result) {
  if (size_add_would_overflow(a, b)) {
    return 1; // ERROR_OVERFLOW
  }
  *result = a + b;
  return 0; // ASCIICHAT_OK
}
 
static inline asciichat_error_t checked_size_mul3(size_t width, size_t height, size_t depth, size_t *result) {
  // First multiply width * height
  if (size_mul_would_overflow(width, height)) {
    return 1; // ERROR_OVERFLOW
  }
  size_t intermediate = width * height;
 
  // Then multiply result * depth
  if (size_mul_would_overflow(intermediate, depth)) {
    return 1; // ERROR_OVERFLOW
  }
  *result = intermediate * depth;
  return 0; // ASCIICHAT_OK
}
 
/* ============================================================================
 * Unchecked Variants (For Performance-Critical Code)
 * ============================================================================
 */
 
#define SIZE_MUL_SAFE(a, b) (size_mul_would_overflow(a, b) ? SIZE_MAX : (a) * (b))
 
#define SIZE_ADD_SAFE(a, b) (size_add_would_overflow(a, b) ? SIZE_MAX : (a) + (b))