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Optimize day 25 with baby-step giant-step

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Bert Peters
2020-12-25 08:38:21 +01:00
parent d5d5f97432
commit 261628b120
1 changed files with 25 additions and 7 deletions
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32
2020/src/day25.rs
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@@ -1,3 +1,4 @@
use std::collections::HashMap;
use std::io::Read; use std::io::Read;
use crate::common::from_lines; use crate::common::from_lines;
@@ -7,17 +8,34 @@ const MOD_BASE: u32 = 20201227;
const SUBJECT_NUMBER: u32 = 7; const SUBJECT_NUMBER: u32 = 7;
fn loop_count(public_key: u32) -> u64 { fn loop_count(public_key: u32) -> u64 {
let mut value = 1; discrete_log(SUBJECT_NUMBER, public_key, MOD_BASE).unwrap() as u64
let mut loops = 0; }
while value != public_key { // Implementation of the baby-step giant-step algorithm
value *= SUBJECT_NUMBER; //
value %= MOD_BASE; // Based on:https://en.wikipedia.org/wiki/Baby-step_giant-step#C++_algorithm_(C++17)
fn discrete_log(g: u32, h: u32, mod_base: u32) -> Option<u32> {
let m = (mod_base as f64).sqrt().ceil() as u32;
let mut table = HashMap::new();
let mut e: u32 = 1;
loops += 1; for i in 0..m {
table.insert(e, i);
e = ((e as u64 * g as u64) % mod_base as u64) as u32;
} }
loops as u64 let factor = mod_exp(g as u64, (mod_base - m - 1) as u64, mod_base as u64);
e = h;
for i in 0..m {
if let Some(&val) = table.get(&e) {
return Some(i * m + val);
}
e = ((e as u64 * factor) % mod_base as u64) as u32;
}
None
} }
#[inline] #[inline]