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Add a factorial example, checking for integer overflow

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This commit is contained in:
Timothy Warren 2019-02-22 11:41:57 -05:00
parent b879914688
commit 9c2b4da36e
5 changed files with 167 additions and 86 deletions
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1
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@ -46,6 +46,7 @@
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5
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@ -185,6 +185,10 @@
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8
factorial/Cargo.toml Normal file
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@ -0,0 +1,8 @@
[package]
name = "factorial"
version = "0.1.0"
authors = ["Timothy Warren <twarren@nabancard.com>"]
edition = "2018"
[dependencies]
separator = "0.4.0"

73
factorial/src/main.rs Normal file
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@ -0,0 +1,73 @@
extern crate separator;
use separator::Separatable;
use std::io;
use std::u128;
///! Calculate a number in the fibonnacci sequence,
///! using a lookup table for better worst-case performance.
///
/// Can calculate up to 34! using native unsigned 128 bit integers.
/// If the result overflows, an error message will be displayed.
fn factorial (n: usize, table: &mut Vec<u128>) -> Option<u128> {
match table.get(n) {
Some(x) => {
// Vec<T>.get returns a Option with a reference to the value, so deref
// Wrap in Some() for proper return type
Some(*x)
},
None => {
// If a previous base overflowed, just
// pass on the overflow. The overflow will
// be caught later. Using the max value of u128
// for convenience.
let prev = factorial(n - 1, table)
.unwrap_or(u128::MAX);
// Do an overflow-checked multiply
let attempt = (n as u128).checked_mul(prev);
// If there isn't an overflow, add the result
// to the calculation table
if let Some(current) = attempt {
table.insert(n, current);
}
attempt // Some(x) if no overflow
}
}
}
fn main() {
// The lookup table for previously calculated values
let mut table: Vec<u128> = vec![1, 1, 2];
println!("Factorial calculator.");
println!("Any non-number will exit.");
loop {
println!("\nWhich factorial to calculate?");
let mut index = String::new();
io::stdin().read_line(&mut index)
.expect("Failed to read line");
let index = match index.trim().parse() {
Ok(num) => num,
Err(_) => break, // Exit on non-number
};
println!("Calculating factorial of: {}", index);
match factorial(index, &mut table) {
Some(calculated) => {
println!("{}! is {}", index, calculated.separated_string())
},
None => {
println!("Calculation overflow. The factorial base was too large.");
}
}
}
}