104 lines
2.4 KiB
Rust
104 lines
2.4 KiB
Rust
/// Calculate a number in the fibonacci sequence,
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/// using recursion and a lookup table
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///
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/// Can calculate up to 186 using native unsigned 128 bit integers.
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///
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/// Example:
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/// ```rust
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/// use rusty_fib_facts::mem_fibonacci;
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///
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/// let valid = mem_fibonacci(45); // Some(1134903170)
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/// # assert_eq!(1134903170, mem_fibonacci(45).unwrap());
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/// # assert!(valid.is_some());
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///
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/// let invalid = mem_fibonacci(187); // None
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/// # assert!(invalid.is_none());
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/// ```
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#[inline]
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pub fn mem_fibonacci(n: usize) -> Option<u128> {
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let mut table = [0u128; 187];
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table[0] = 0;
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table[1] = 1;
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table[2] = 1;
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/// Actual calculating function for `fibonacci`
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fn f(n: usize, table: &mut [u128]) -> Option<u128> {
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if n < 2 {
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// The first values are predefined.
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return Some(table[n]);
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}
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if n > 186 {
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return None;
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}
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match table[n] {
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// The lookup array starts out zeroed, so a zero
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// is a not yet calculated value
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0 => {
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let a = f(n - 1, table)?;
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let b = f(n - 2, table)?;
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table[n] = a + b;
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Some(table[n])
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}
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x => Some(x),
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}
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}
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f(n, &mut table)
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}
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/// Calculate a number in the fibonacci sequence,
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/// using naive recursion
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///
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/// REALLY SLOW
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///
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/// Can calculate up to 186 using native unsigned 128 bit integers.
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#[inline]
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pub fn rec_fibonacci(n: usize) -> Option<u128> {
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if matches!(n, 0 | 1) {
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Some(n as u128)
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} else {
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let a = rec_fibonacci(n - 1)?;
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let b = rec_fibonacci(n - 2)?;
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a.checked_add(b)
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}
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}
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/// Calculate a number in the fibonacci sequence,
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///
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/// Can calculate up to 186 using native unsigned 128 bit integers.
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///
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/// Example:
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/// ```rust
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/// use rusty_fib_facts::fibonacci;
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///
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/// let valid = fibonacci(45); // Some(1134903170)
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/// # assert_eq!(1134903170, fibonacci(45).unwrap());
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/// # assert!(valid.is_some());
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///
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/// let invalid = fibonacci(187); // None
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/// # assert!(invalid.is_none());
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/// ```
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#[inline]
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pub fn fibonacci(n: usize) -> Option<u128> {
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let mut a: u128 = 0;
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let mut b: u128 = 1;
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if matches!(n, 0 | 1) {
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Some(n as u128)
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} else {
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for _ in 0..n - 1 {
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let c: u128 = a.checked_add(b)?;
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a = b;
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b = c;
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}
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Some(b)
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}
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}
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