From: Ralf Jung Date: Mon, 15 Jun 2015 15:40:22 +0000 (+0200) Subject: add a "solutions" crate with a BigInt class X-Git-Url: https://git.ralfj.de/rust-101.git/commitdiff_plain/931a4309e60e7a4915cfbf88dee7f0c3e875a63f?ds=inline;hp=177a1f127af6601f373be9742c31351f556633ab add a "solutions" crate with a BigInt class --- diff --git a/solutions/Cargo.lock b/solutions/Cargo.lock new file mode 100644 index 0000000..ffb21e6 --- /dev/null +++ b/solutions/Cargo.lock @@ -0,0 +1,4 @@ +[root] +name = "solutions" +version = "0.1.0" + diff --git a/solutions/Cargo.toml b/solutions/Cargo.toml new file mode 100644 index 0000000..8aebfa9 --- /dev/null +++ b/solutions/Cargo.toml @@ -0,0 +1,4 @@ +[package] +name = "solutions" +version = "0.1.0" +authors = ["Ralf Jung "] diff --git a/solutions/src/bigint.rs b/solutions/src/bigint.rs new file mode 100644 index 0000000..e9dfdf0 --- /dev/null +++ b/solutions/src/bigint.rs @@ -0,0 +1,211 @@ +use std::ops; +use std::cmp; +use std::fmt; + +pub struct BigInt { + data: Vec, // least significant digits first. The last block will *not* be 0. +} + +// Add with carry, returning the sum and the carry +fn overflowing_add(a: u64, b: u64, carry: bool) -> (u64, bool) { + match u64::checked_add(a, b) { + Some(sum) if !carry => (sum, false), + Some(sum) => { // we have to increment the sum by 1, where it may overflow again + match u64::checked_add(sum, 1) { + Some(total_sum) => (total_sum, false), + None => (0, true) // we overflowed incrementing by 1, so we are just "at the edge" + } + }, + None => { + // Get the remainder, i.e., the wrapping sum. This cannot overflow again by adding just 1, so it is safe + // to add the carry here. + let rem = u64::wrapping_add(a, b) + if carry { 1 } else { 0 }; + (rem, true) + } + } +} + + +impl BigInt { + /// Construct a BigInt from a "small" one. + pub fn new(x: u64) -> Self { + if x == 0 { // take care of our invariant! + BigInt { data: vec![] } + } else { + BigInt { data: vec![x] } + } + } + + fn test_invariant(&self) -> bool { + if self.data.len() == 0 { + true + } else { + self.data[self.data.len() - 1] != 0 + } + } + + /// Construct a BigInt from a vector of 64-bit "digits", with the last significant digit being first + pub fn from_vec(mut v: Vec) -> Self { + // remove trailing zeroes + while v.len() > 0 && v[v.len()-1] == 0 { + v.pop(); + } + BigInt { data: v } + } + + /// Return the smaller of the two numbers + pub fn min(self, other: Self) -> Self { + debug_assert!(self.test_invariant() && other.test_invariant()); + if self.data.len() < other.data.len() { + self + } else if self.data.len() > other.data.len() { + other + } else { + // compare back-to-front, i.e., most significant digit first + let mut idx = self.data.len()-1; + while idx > 0 { + if self.data[idx] < other.data[idx] { + return self; + } else if self.data[idx] > other.data[idx] { + return other; + } + else { + idx = idx-1; + } + } + // the two are equal + return self; + } + } + + /// Returns a view on the raw digits representing the number. + /// + /// ``` + /// use solutions::bigint::BigInt; + /// let b = BigInt::new(13); + /// let d = b.data(); + /// assert_eq!(d, [13]); + /// ``` + pub fn data(&self) -> &[u64] { + &self.data[..] + } + + /// Increments the number by "by". + pub fn inc(&mut self, mut by: u64) { + let mut idx = 0; + // This loop adds "by * (1 << idx)". Think of "by" as the carry from incrementing the last digit. + while idx < self.data.len() { + let cur = self.data[idx]; + let sum = u64::wrapping_add(cur, by); + self.data[idx] = sum; + if sum >= cur { + // No overflow, we are done. + return; + } else { + // We need to add a carry. + by = 1; + idx += 1; + } + } + // If we came here, there is a last carry to add + self.data.push(by); + } + + /// Return the nth power-of-2 as BigInt + pub fn power_of_2(mut power: u64) -> BigInt { + let mut v = Vec::new(); + while power >= 64 { + v.push(0); + power -= 64; + } + v.push(1 << power); + BigInt::from_vec(v) + } +} + +impl Clone for BigInt { + fn clone(&self) -> Self { + BigInt { data: self.data.clone() } + } +} + + +impl PartialEq for BigInt { + fn eq(&self, other: &BigInt) -> bool { + debug_assert!(self.test_invariant() && other.test_invariant()); + self.data() == other.data() + } +} + +impl fmt::Debug for BigInt { + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { + self.data().fmt(f) + } +} + +impl<'a, 'b> ops::Add<&'a BigInt> for &'b BigInt { + type Output = BigInt; + fn add(self, rhs: &'a BigInt) -> Self::Output { + let mut result_vec:Vec = Vec::with_capacity(cmp::max(self.data().len(), rhs.data().len())); + let mut carry:bool = false; // the carry bit + for (i, val) in self.data().into_iter().enumerate() { + // compute next digit and carry + let rhs_val = if i < rhs.data().len() { rhs.data()[i] } else { 0 }; + let (sum, new_carry) = overflowing_add(*val, rhs_val, carry); + // store them + result_vec.push(sum); + carry = new_carry; + } + BigInt::from_vec(result_vec) + } +} + +impl<'a> ops::Add for &'a BigInt { + type Output = BigInt; + #[inline] + fn add(self, rhs: BigInt) -> Self::Output { + self + &rhs + } +} + +impl<'a> ops::Add<&'a BigInt> for BigInt { + type Output = BigInt; + #[inline] + fn add(self, rhs: &'a BigInt) -> Self::Output { + &self + rhs + } +} + +impl ops::Add for BigInt { + type Output = BigInt; + #[inline] + fn add(self, rhs: BigInt) -> Self::Output { + &self + &rhs + } +} + +#[cfg(test)] +mod tests { + use super::overflowing_add; + use super::BigInt; + + #[test] + fn test_overflowing_add() { + assert_eq!(overflowing_add(10, 100, false), (110, false)); + assert_eq!(overflowing_add(10, 100, true), (111, false)); + assert_eq!(overflowing_add(1 << 63, 1 << 63, false), (0, true)); + assert_eq!(overflowing_add(1 << 63, 1 << 63, true), (1, true)); + assert_eq!(overflowing_add(1 << 63, (1 << 63) -1 , true), (0, true)); + } + + #[test] + fn test_power_of_2() { + assert_eq!(BigInt::power_of_2(0), BigInt::new(1)); + assert_eq!(BigInt::power_of_2(13), BigInt::new(1 << 13)); + assert_eq!(BigInt::power_of_2(64), BigInt::from_vec(vec![0, 1])); + assert_eq!(BigInt::power_of_2(96), BigInt::from_vec(vec![0, 1 << 32])); + assert_eq!(BigInt::power_of_2(128), BigInt::from_vec(vec![0, 0, 1])); + } +} + + diff --git a/solutions/src/lib.rs b/solutions/src/lib.rs new file mode 100644 index 0000000..8f8d148 --- /dev/null +++ b/solutions/src/lib.rs @@ -0,0 +1 @@ +pub mod bigint;