echo/examples/algos/power.echo at main · csfh/echo · GitHub

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; Integer power — exponentiation by squaring.
; Run: xo run examples/algos/power.echo

/ std/io
/ std/str

; base^exp for non-negative exp. 0^0 is treated as 1.
$ pow = (base, exp) {
    ? exp < 0 {
        ! "negative exponent"
    }
    ~ b = base
    ~ e = exp
    ~ acc = 1
    * e > 0 {
        ? e % 2 == 1 {
            ~ acc = acc * b
        }
        ~ b = b * b
        ~ e = e / 2
    }
    ^ acc
}

; Modular power: (base^exp) mod m  (m > 0).
$ pow_mod = (base, exp, m) {
    ? exp < 0 {
        ! "negative exponent"
    }
    ? m <= 0 {
        ! "modulus must be positive"
    }
    ~ b = base % m
    ? b < 0 {
        ~ b = b + m
    }
    ~ e = exp
    ~ acc = 1
    * e > 0 {
        ? e % 2 == 1 {
            ~ acc = acc * b % m
        }
        ~ b = b * b % m
        ~ e = e / 2
    }
    ^ acc
}

| pow(2, 0) {
    $ v {
        io.print(str.from_int(v))
    }
    ! err {
        io.print(str.from_int(err))
    }
}
| pow(2, 10) {
    $ v {
        io.print(str.from_int(v))
    }
    ! err {
        io.print(str.from_int(err))
    }
}
| pow(3, 5) {
    $ v {
        io.print(str.from_int(v))
    }
    ! err {
        io.print(str.from_int(err))
    }
}
| pow_mod(2, 10, 1000) {
    $ v {
        io.print(str.from_int(v))
    }
    ! err {
        io.print(str.from_int(err))
    }
}
| pow_mod(7, 3, 5) {
    $ v {
        io.print(str.from_int(v))
    }
    ! err {
        io.print(str.from_int(err))
    }
}

\ pow, pow_mod