X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/931a4309e60e7a4915cfbf88dee7f0c3e875a63f..1a691352b57b7338388ff568403495ecb44272eb:/solutions/src/bigint.rs diff --git a/solutions/src/bigint.rs b/solutions/src/bigint.rs index e9dfdf0..8d67a78 100644 --- a/solutions/src/bigint.rs +++ b/solutions/src/bigint.rs @@ -2,29 +2,54 @@ use std::ops; use std::cmp; use std::fmt; +pub trait Minimum { + /// Return the smaller of the two + fn min<'a>(&'a self, other: &'a Self) -> &'a Self; +} + +/// Return a pointer to the minimal value of `v`. +pub fn vec_min(v: &Vec) -> Option<&T> { + let mut min = None; + for e in v { + min = Some(match min { + None => e, + Some(n) => e.min(n) + }); + } + min +} + 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) - } + let sum = u64::wrapping_add(a, b); + let carry_n = if carry { 1 } else { 0 }; + if sum >= a { // the first sum did not overflow + let sum_total = u64::wrapping_add(sum, carry_n); + let had_overflow = sum_total < sum; + (sum_total, had_overflow) + } else { // the first sum did overflow + // it is impossible for this to overflow again, as we are just adding 0 or 1 + (sum + carry_n, true) } } +// Subtract with carry, returning the difference and the carry +fn overflowing_sub(a: u64, b: u64, carry: bool) -> (u64, bool) { + let diff = u64::wrapping_sub(a, b); + let carry_n = if carry { 1 } else { 0 }; + if diff <= a { // the first diff did not wrap + let diff_total = u64::wrapping_sub(diff, carry_n); + let had_wrap = diff_total > diff; + (diff_total, had_wrap) + } else { // the first diff did wrap + // it is impossible for this to wrap again, as we are just substracting 0 or 1 + (diff - carry_n, true) + } +} impl BigInt { /// Construct a BigInt from a "small" one. @@ -44,50 +69,33 @@ impl BigInt { } } - /// Construct a BigInt from a vector of 64-bit "digits", with the last significant digit being first + /// Construct a BigInt from a vector of 64-bit "digits", with the last significant digit being first. Solution to 05.1. pub fn from_vec(mut v: Vec) -> Self { - // remove trailing zeroes + // remove trailing zeros 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; - } + /// Increments the number by 1. + pub fn inc1(&mut self) { + let mut idx = 0; + // This loop adds "(1 << idx)". If there is no more carry, we leave. + while idx < self.data.len() { + let cur = self.data[idx]; + let sum = u64::wrapping_add(cur, 1); + self.data[idx] = sum; + if sum >= cur { + // No overflow, we are done. + return; + } else { + // We need to go on. + 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[..] + // If we came here, there is a last carry to add + self.data.push(1); } /// Increments the number by "by". @@ -129,34 +137,68 @@ impl Clone for BigInt { } } - impl PartialEq for BigInt { fn eq(&self, other: &BigInt) -> bool { debug_assert!(self.test_invariant() && other.test_invariant()); - self.data() == other.data() + self.data == other.data + } +} + +impl Minimum for BigInt { + // This is essentially the solution to 06.1. + fn min<'a>(&'a self, other: &'a Self) -> &'a 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; + } } } impl fmt::Debug for BigInt { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { - self.data().fmt(f) + 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 max_len = cmp::max(self.data.len(), rhs.data.len()); + let mut result_vec:Vec = Vec::with_capacity(max_len); let mut carry:bool = false; // the carry bit - for (i, val) in self.data().into_iter().enumerate() { + for i in 0..max_len { // 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); + let lhs_val = if i < self.data.len() { self.data[i] } else { 0 }; + let rhs_val = if i < rhs.data.len() { rhs.data[i] } else { 0 }; + let (sum, new_carry) = overflowing_add(lhs_val, rhs_val, carry); // store them result_vec.push(sum); carry = new_carry; } - BigInt::from_vec(result_vec) + if carry { + result_vec.push(1); + } + // We know that the invariant holds: overflowing_add would only return (0, false) if + // the arguments are (0, 0, false), but we know that in the last iteration, one od the two digits + // is the last of its number and hence not 0. + BigInt { data: result_vec } } } @@ -184,10 +226,57 @@ impl ops::Add for BigInt { } } +impl<'a, 'b> ops::Sub<&'a BigInt> for &'b BigInt { + type Output = BigInt; + fn sub(self, rhs: &'a BigInt) -> Self::Output { + let max_len = cmp::max(self.data.len(), rhs.data.len()); + let mut result_vec:Vec = Vec::with_capacity(max_len); + let mut carry:bool = false; // the carry bit + for i in 0..max_len { + // compute next digit and carry + let lhs_val = if i < self.data.len() { self.data[i] } else { 0 }; + let rhs_val = if i < rhs.data.len() { rhs.data[i] } else { 0 }; + let (sum, new_carry) = overflowing_sub(lhs_val, rhs_val, carry); + // store them + result_vec.push(sum); + carry = new_carry; + } + if carry { + panic!("Wrapping subtraction of BigInt"); + } + // We may have trailing zeroes, so get rid of them + BigInt::from_vec(result_vec) + } +} + +impl<'a> ops::Sub for &'a BigInt { + type Output = BigInt; + #[inline] + fn sub(self, rhs: BigInt) -> Self::Output { + self - &rhs + } +} + +impl<'a> ops::Sub<&'a BigInt> for BigInt { + type Output = BigInt; + #[inline] + fn sub(self, rhs: &'a BigInt) -> Self::Output { + &self - rhs + } +} + +impl ops::Sub for BigInt { + type Output = BigInt; + #[inline] + fn sub(self, rhs: BigInt) -> Self::Output { + &self - &rhs + } +} + #[cfg(test)] mod tests { - use super::overflowing_add; - use super::BigInt; + use std::u64; + use super::{overflowing_add,overflowing_sub,BigInt}; #[test] fn test_overflowing_add() { @@ -198,6 +287,72 @@ mod tests { assert_eq!(overflowing_add(1 << 63, (1 << 63) -1 , true), (0, true)); } + #[test] + fn test_overflowing_sub() { + assert_eq!(overflowing_sub(100, 10, false), (90, false)); + assert_eq!(overflowing_sub(100, 10, true), (89, false)); + assert_eq!(overflowing_sub(10, 1 << 63, false), ((1 << 63) + 10, true)); + assert_eq!(overflowing_sub(10, 1 << 63, true), ((1 << 63) + 9, true)); + assert_eq!(overflowing_sub(42, 42 , true), (u64::max_value(), true)); + } + + #[test] + fn test_add() { + let b1 = BigInt::new(1 << 32); + let b2 = BigInt::from_vec(vec![0, 1]); + let b3 = BigInt::from_vec(vec![0, 0, 1]); + let b4 = BigInt::new(1 << 63); + + assert_eq!(&b1 + &b2, BigInt::from_vec(vec![1 << 32, 1])); + assert_eq!(&b2 + &b1, BigInt::from_vec(vec![1 << 32, 1])); + assert_eq!(&b2 + &b3, BigInt::from_vec(vec![0, 1, 1])); + assert_eq!(&b2 + &b3 + &b4 + &b4, BigInt::from_vec(vec![0, 2, 1])); + assert_eq!(&b2 + &b4 + &b3 + &b4, BigInt::from_vec(vec![0, 2, 1])); + assert_eq!(&b4 + &b2 + &b3 + &b4, BigInt::from_vec(vec![0, 2, 1])); + } + + #[test] + fn test_sub() { + let b1 = BigInt::new(1 << 32); + let b2 = BigInt::from_vec(vec![0, 1]); + let b3 = BigInt::from_vec(vec![0, 0, 1]); + let b4 = BigInt::new(1 << 63); + + assert_eq!(&b2 - &b1, BigInt::from_vec(vec![u64::max_value() - (1 << 32) + 1])); + assert_eq!(&b3 - &b2, BigInt::from_vec(vec![0, u64::max_value(), 0])); + assert_eq!(&b2 - &b4 - &b4, BigInt::new(0)); + assert_eq!(&b3 - &b2 - &b4 - &b4, BigInt::from_vec(vec![0, u64::max_value() - 1])); + assert_eq!(&b3 - &b4 - &b2 - &b4, BigInt::from_vec(vec![0, u64::max_value() - 1])); + assert_eq!(&b3 - &b4 - &b4 - &b2, BigInt::from_vec(vec![0, u64::max_value() - 1])); + } + + #[test] + #[should_panic(expected = "Wrapping subtraction of BigInt")] + fn test_sub_panic1() { + let _ = BigInt::new(1) - BigInt::new(5); + } + + #[test] + #[should_panic(expected = "Wrapping subtraction of BigInt")] + fn test_sub_panic2() { + let _ = BigInt::from_vec(vec![5,8,3,33,1<<13,46,1<<49, 1, 583,1<<60,2533]) - BigInt::from_vec(vec![5,8,3,33,1<<13,46,1<<49, 5, 583,1<<60,2533]); + } + + #[test] + fn test_inc1() { + let mut b = BigInt::new(0); + b.inc1(); + assert_eq!(b, BigInt::new(1)); + b.inc1(); + assert_eq!(b, BigInt::new(2)); + + b = BigInt::new(u64::MAX); + b.inc1(); + assert_eq!(b, BigInt::from_vec(vec![0, 1])); + b.inc1(); + assert_eq!(b, BigInt::from_vec(vec![1, 1])); + } + #[test] fn test_power_of_2() { assert_eq!(BigInt::power_of_2(0), BigInt::new(1));