zig/lib/compiler_rt/subosi4_test.zig

const subo = @import("subo.zig");
const testing = @import("std").testing;

fn test__subosi4(a: i32, b: i32) !void {
    var result_ov: c_int = undefined;
    var expected_ov: c_int = undefined;
    const result = subo.__subosi4(a, b, &result_ov);
    const expected: i32 = simple_subosi4(a, b, &expected_ov);
    try testing.expectEqual(expected, result);
    try testing.expectEqual(expected_ov, result_ov);
}

// 2 cases on evaluating `a-b`:
// 1. `a-b` may underflow, iff b>0 && a<0 and a-b < min <=> a<min+b
// 2. `a-b` may overflow,  iff b<0 && a>0 and a-b > max <=> a>max+b
// `-b` evaluation may overflow, iff b==min, but this is handled by the hardware
pub fn simple_subosi4(a: i32, b: i32, overflow: *c_int) i32 {
    overflow.* = 0;
    const min: i32 = -2147483648;
    const max: i32 = 2147483647;
    if (((b > 0) and (a < min + b)) or
        ((b < 0) and (a > max + b)))
        overflow.* = 1;
    return a -% b;
}

test "subosi3" {
    // -2^31 <= i32 <= 2^31-1
    // 2^31 = 2147483648
    // 2^31-1 = 2147483647
    const min: i32 = -2147483648;
    const max: i32 = 2147483647;
    var i: i32 = 1;
    while (i < max) : (i *|= 2) {
        try test__subosi4(i, i);
        try test__subosi4(-i, -i);
        try test__subosi4(i, -i);
        try test__subosi4(-i, i);
    }

    // edge cases
    // 0 - 0       = 0
    // MIN - MIN   = 0
    // MAX - MAX   = 0
    // 0   - MIN     overflow
    // 0   - MAX   = MIN+1
    // MIN - 0     = MIN
    // MAX - 0     = MAX
    // MIN - MAX     overflow
    // MAX - MIN     overflow
    try test__subosi4(0, 0);
    try test__subosi4(min, min);
    try test__subosi4(max, max);
    try test__subosi4(0, min);
    try test__subosi4(0, max);
    try test__subosi4(min, 0);
    try test__subosi4(max, 0);
    try test__subosi4(min, max);
    try test__subosi4(max, min);

    // derived edge cases
    // MIN+1 - MIN = 1
    // MAX-1 - MAX = -1
    // 1     - MIN   overflow
    // -1    - MIN = MAX
    // -1    - MAX = MIN
    // +1    - MAX = MIN+2
    // MIN   - 1     overflow
    // MIN   - -1  = MIN+1
    // MAX   - 1   = MAX-1
    // MAX   - -1    overflow
    try test__subosi4(min + 1, min);
    try test__subosi4(max - 1, max);
    try test__subosi4(1, min);
    try test__subosi4(-1, min);
    try test__subosi4(-1, max);
    try test__subosi4(1, max);
    try test__subosi4(min, 1);
    try test__subosi4(min, -1);
    try test__subosi4(max, -1);
    try test__subosi4(max, 1);
}