53 lines
1.4 KiB
Rust
53 lines
1.4 KiB
Rust
extern crate separator;
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use separator::Separatable;
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use std::io;
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///! Calculate a number in the fibonnacci sequence,
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///! using a lookup table for better worst-case performance.
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///
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/// Can calculate up to 186 using native unsigned 128 bit integers.
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fn fibonacci(n: usize, table: &mut Vec<u128>) -> u128 {
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match table.get(n) {
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Some(x) => *x, // Vec<T>.get returns a Option with a reference to the value, so deref
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None => {
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let a = n - 1;
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let b = n - 2;
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let current = fibonacci(a, table) + fibonacci(b, table);
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table.insert(n, current);
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current
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}
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}
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}
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fn main() {
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// The lookup table for previously calculated values
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let mut table: Vec<u128> = vec![0, 1, 1, 2, 3, 5];
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println!("Fibonacci calculator.");
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println!("Any non-number will exit.");
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loop {
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println!("\nWhich index in the sequence to calculate?");
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let mut index = String::new();
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io::stdin().read_line(&mut index)
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.expect("Failed to read line");
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let index = match index.trim().parse() {
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Ok(num) => num,
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Err(_) => break, // Exit on non-number
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};
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let calculated = fibonacci(index, &mut table);
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println!("Calculating number for index: {}", index);
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println!("The Fibonnacci number at index: {} is {}", index, calculated.separated_string());
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}
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}
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