zig/lib/std/math/powi.zig

// Based on Rust, which is licensed under the MIT license.
// https://github.com/rust-lang/rust/blob/360432f1e8794de58cd94f34c9c17ad65871e5b5/LICENSE-MIT
//
// https://github.com/rust-lang/rust/blob/360432f1e8794de58cd94f34c9c17ad65871e5b5/src/libcore/num/mod.rs#L3423

const std = @import("../std.zig");
const math = std.math;
const assert = std.debug.assert;
const testing = std.testing;

/// Returns the power of x raised by the integer y (x^y).
///
/// Errors:
///  - Overflow: Integer overflow or Infinity
///  - Underflow: Absolute value of result smaller than 1
/// Edge case rules ordered by precedence:
///  - powi(T, x, 0)   = 1 unless T is i1, i0, u0
///  - powi(T, 0, x)   = 0 when x > 0
///  - powi(T, 0, x)   = Overflow
///  - powi(T, 1, y)   = 1
///  - powi(T, -1, y)  = -1 for y an odd integer
///  - powi(T, -1, y)  = 1 unless T is i1, i0, u0
///  - powi(T, -1, y)  = Overflow
///  - powi(T, x, y)   = Overflow when y >= @bitSizeOf(x)
///  - powi(T, x, y)   = Underflow when y < 0
pub fn powi(comptime T: type, x: T, y: T) (error{
    Overflow,
    Underflow,
}!T) {
    const bit_size = @typeInfo(T).int.bits;

    // `y & 1 == 0` won't compile when `does_one_overflow`.
    const does_one_overflow = math.maxInt(T) < 1;
    const is_y_even = !does_one_overflow and y & 1 == 0;

    if (x == 1 or y == 0 or (x == -1 and is_y_even)) {
        if (does_one_overflow) {
            return error.Overflow;
        } else {
            return 1;
        }
    }

    if (x == -1) {
        return -1;
    }

    if (x == 0) {
        if (y > 0) {
            return 0;
        } else {
            // Infinity/NaN, not overflow in strict sense
            return error.Overflow;
        }
    }
    // x >= 2 or x <= -2 from this point
    if (y >= bit_size) {
        return error.Overflow;
    }
    if (y < 0) {
        return error.Underflow;
    }

    // invariant :
    // return value = powi(T, base, exp) * acc;

    var base = x;
    var exp = y;
    var acc: T = if (does_one_overflow) unreachable else 1;

    while (exp > 1) {
        if (exp & 1 == 1) {
            const ov = @mulWithOverflow(acc, base);
            if (ov[1] != 0) return error.Overflow;
            acc = ov[0];
        }

        exp >>= 1;

        const ov = @mulWithOverflow(base, base);
        if (ov[1] != 0) return error.Overflow;
        base = ov[0];
    }

    if (exp == 1) {
        const ov = @mulWithOverflow(acc, base);
        if (ov[1] != 0) return error.Overflow;
        acc = ov[0];
    }

    return acc;
}

test powi {
    try testing.expectError(error.Overflow, powi(i8, -66, 6));
    try testing.expectError(error.Overflow, powi(i16, -13, 13));
    try testing.expectError(error.Overflow, powi(i32, -32, 21));
    try testing.expectError(error.Overflow, powi(i64, -24, 61));
    try testing.expectError(error.Overflow, powi(i17, -15, 15));
    try testing.expectError(error.Overflow, powi(i42, -6, 40));

    try testing.expect((try powi(i8, -5, 3)) == -125);
    try testing.expect((try powi(i16, -16, 3)) == -4096);
    try testing.expect((try powi(i32, -91, 3)) == -753571);
    try testing.expect((try powi(i64, -36, 6)) == 2176782336);
    try testing.expect((try powi(i17, -2, 15)) == -32768);
    try testing.expect((try powi(i42, -5, 7)) == -78125);

    try testing.expect((try powi(u8, 6, 2)) == 36);
    try testing.expect((try powi(u16, 5, 4)) == 625);
    try testing.expect((try powi(u32, 12, 6)) == 2985984);
    try testing.expect((try powi(u64, 34, 2)) == 1156);
    try testing.expect((try powi(u17, 16, 3)) == 4096);
    try testing.expect((try powi(u42, 34, 6)) == 1544804416);

    try testing.expectError(error.Overflow, powi(i8, 120, 7));
    try testing.expectError(error.Overflow, powi(i16, 73, 15));
    try testing.expectError(error.Overflow, powi(i32, 23, 31));
    try testing.expectError(error.Overflow, powi(i64, 68, 61));
    try testing.expectError(error.Overflow, powi(i17, 15, 15));
    try testing.expectError(error.Overflow, powi(i42, 121312, 41));

    try testing.expectError(error.Overflow, powi(u8, 123, 7));
    try testing.expectError(error.Overflow, powi(u16, 2313, 15));
    try testing.expectError(error.Overflow, powi(u32, 8968, 31));
    try testing.expectError(error.Overflow, powi(u64, 2342, 63));
    try testing.expectError(error.Overflow, powi(u17, 2723, 16));
    try testing.expectError(error.Overflow, powi(u42, 8234, 41));

    const minInt = std.math.minInt;
    try testing.expect((try powi(i8, -2, 7)) == minInt(i8));
    try testing.expect((try powi(i16, -2, 15)) == minInt(i16));
    try testing.expect((try powi(i32, -2, 31)) == minInt(i32));
    try testing.expect((try powi(i64, -2, 63)) == minInt(i64));

    try testing.expectError(error.Underflow, powi(i8, 6, -2));
    try testing.expectError(error.Underflow, powi(i16, 5, -4));
    try testing.expectError(error.Underflow, powi(i32, 12, -6));
    try testing.expectError(error.Underflow, powi(i64, 34, -2));
    try testing.expectError(error.Underflow, powi(i17, 16, -3));
    try testing.expectError(error.Underflow, powi(i42, 34, -6));
}

test "powi.special" {
    try testing.expectError(error.Overflow, powi(i8, -2, 8));
    try testing.expectError(error.Overflow, powi(i16, -2, 16));
    try testing.expectError(error.Overflow, powi(i32, -2, 32));
    try testing.expectError(error.Overflow, powi(i64, -2, 64));
    try testing.expectError(error.Overflow, powi(i17, -2, 17));
    try testing.expectError(error.Overflow, powi(i17, -2, 16));
    try testing.expectError(error.Overflow, powi(i42, -2, 42));

    try testing.expect((try powi(i8, -1, 3)) == -1);
    try testing.expect((try powi(i16, -1, 2)) == 1);
    try testing.expect((try powi(i32, -1, 16)) == 1);
    try testing.expect((try powi(i64, -1, 6)) == 1);
    try testing.expect((try powi(i17, -1, 15)) == -1);
    try testing.expect((try powi(i42, -1, 7)) == -1);

    try testing.expect((try powi(u8, 1, 2)) == 1);
    try testing.expect((try powi(u16, 1, 4)) == 1);
    try testing.expect((try powi(u32, 1, 6)) == 1);
    try testing.expect((try powi(u64, 1, 2)) == 1);
    try testing.expect((try powi(u17, 1, 3)) == 1);
    try testing.expect((try powi(u42, 1, 6)) == 1);

    try testing.expectError(error.Overflow, powi(i8, 2, 7));
    try testing.expectError(error.Overflow, powi(i16, 2, 15));
    try testing.expectError(error.Overflow, powi(i32, 2, 31));
    try testing.expectError(error.Overflow, powi(i64, 2, 63));
    try testing.expectError(error.Overflow, powi(i17, 2, 16));
    try testing.expectError(error.Overflow, powi(i42, 2, 41));

    try testing.expectError(error.Overflow, powi(u8, 2, 8));
    try testing.expectError(error.Overflow, powi(u16, 2, 16));
    try testing.expectError(error.Overflow, powi(u32, 2, 32));
    try testing.expectError(error.Overflow, powi(u64, 2, 64));
    try testing.expectError(error.Overflow, powi(u17, 2, 17));
    try testing.expectError(error.Overflow, powi(u42, 2, 42));

    try testing.expect((try powi(u8, 6, 0)) == 1);
    try testing.expect((try powi(u16, 5, 0)) == 1);
    try testing.expect((try powi(u32, 12, 0)) == 1);
    try testing.expect((try powi(u64, 34, 0)) == 1);
    try testing.expect((try powi(u17, 16, 0)) == 1);
    try testing.expect((try powi(u42, 34, 0)) == 1);
}

test "powi.narrow" {
    try testing.expectError(error.Overflow, powi(u0, 0, 0));
    try testing.expectError(error.Overflow, powi(i0, 0, 0));
    try testing.expectError(error.Overflow, powi(i1, 0, 0));
    try testing.expectError(error.Overflow, powi(i1, -1, 0));
    try testing.expectError(error.Overflow, powi(i1, 0, -1));
    try testing.expect((try powi(i1, -1, -1)) == -1);
}