Code shared from the Rust Playground

pi_mantissa_hex.rs

use std::f64;
use std::u8;

#[allow(dead_code)]

fn print_gnostr() {
    let s = "gnostr";
    for byte in s.as_bytes() {
        print!("{:02X} ", byte);
    }
    println!();
    for byte in s.as_bytes() {
        print!("{} ", byte);
    }
    println!();
}

fn main() {
    //print_gnostr();

    let mut pi_mantissa: String = String::from("");
    let index_range: u32 = 197; // 197 // 1021 // 1990; // 1990 consistent on Code Runner
    let index_start: u32 = 3; // 8000 // 3170 // 1018 //6567 ? 
    let index_end: u32 = index_start + index_range;
    if index_end > 8197 {};
    for n in index_start..index_end { // Increased to 6001 to get 6000 digits.
      
        pi_mantissa.push_str(&format!("{:X}", compute_spigot(n)));
        //println!("{:0>30}:\n{}", n, pi_mantissa);
    }
    println!("pi_mantissa[(HEX)]:\n{}", pi_mantissa);

    let pi_hex = pi_mantissa.as_str();
    let gnostr_hex = "AFED"; //"676E6F7374"; // Hex representation of "gnostr"

    println!("Searching: {}", gnostr_hex);
    if let Some(index) = pi_hex.find(gnostr_hex) {
        println!("{} = pi_mantissa[{:}..{:}] ", gnostr_hex, index - 0, index + (gnostr_hex.len() - 1));
        //let decimal_index = (index - 0) / 1; // -2 for "3.", /1 because each hex char is one digit.
        //println!("'{}' found after the {}th decimal place.", gnostr_hex, decimal_index - 0);
    } else {
        println!("'{}' not found in pi hex string.", gnostr_hex);
        // let gnostr_xor = gnostr_hex.clone();
        // println!("{}",gnostr_xor);
    }
}

#[allow(dead_code)]
fn compute_pi_base10(precision: u16) -> f64 {
    let mut pi: f64 = 0.0;
    for ki32 in 0..precision + 1 {
        let k: f64 = f64::from(ki32);
        pi = pi
            + (f64::powf(16.0, k * -1.0)
                * (4.0 / (8.0 * k + 1.0)
                    - 2.0 / (8.0 * k + 4.0)
                    - 1.0 / (8.0 * k + 5.0)
                    - 1.0 / (8.0 * k + 6.0)));
    }
    pi
}

fn mod_pow(mut base: u32, mut exp: u32, modulus: u32) -> u32 {
    if modulus == 1 {
        return 0;
    }
    let mut result = 1;
    base = base % modulus;
    while exp > 0 {
        if exp % 2 == 1 {
            result = result * base % modulus;
        }
        exp = exp >> 1;
        base = base * base % modulus
    }
    result
}

fn sigma(n: u32, j: u32) -> f64 {
    let mut s: f64 = 0.0;
    let mut denom = j;
    for k in 0..n + 1 {
        let r = mod_pow(16, n - k, denom);
        s = fract_mod(s + f64::from(r) / f64::from(denom));
        denom += 8u32;
    }
    let mut num = 1.0 / 16.0;
    while num / f64::from(denom) > f64::EPSILON {
        s += num / f64::from(denom);
        num /= 16.0;
        denom += 8u32;
    }
    s.fract()
}

fn fract_mod(v: f64) -> f64 {
    if v < 0.0 {
        return 1.0 + v.fract() % 1.0;
    }
    v.fract() % 1.0
}

fn f64_to_u8(value: f64) -> u8 {
    let clamped_value = value.round().max(u8::MIN as f64).min(u8::MAX as f64);
    clamped_value as u8
}

fn compute_spigot(digit: u32) -> u8 {
    let r = fract_mod(
        4.0 * sigma(digit - 1, 1)
            - (2.0 * sigma(digit - 1, 4))
            - sigma(digit - 1, 5)
            - sigma(digit - 1, 6),
    );
    f64_to_u8((16.0 * r).floor())
}