gistfile1.txt

const std = @import("std");
const big = std.math.big;
const Allocator = std.mem.Allocator;
const crypto = std.crypto;

const Managed = big.int.Managed;
const Const = big.int.Const;
const Order = std.math.Order;

fn powMod(res: *Managed, b: Const, e: Const, m: Const) !void {
    if (m.eqZero()) {
        return error.DivideByZero;
    }
    const e_abs = e.abs();
    const m_abs = m.abs();
    if (m_abs.to(u1)) |m_abs_int| {
        if (m_abs_int == 1) {
            return res.set(0);
        }
    } else |_| {}
    try res.set(1);
    var bm = try Managed.init(res.allocator);
    var _q = try Managed.init(res.allocator);
    try _q.divTrunc(&bm, b, m);
    var em = try Managed.init(res.allocator);
    try em.copy(e);
    while (!em.eqZero()) {
        try em.shiftRight(em, 1);
        try bm.sqr(bm.toConst());
        try _q.divTrunc(&bm, bm.toConst(), m);
    }
}