From: Ralf Jung <post@ralfj.de>
Date: Sat, 13 Jun 2015 12:01:34 +0000 (+0200)
Subject: start on a BigInt class
X-Git-Url: https://git.ralfj.de/rust-101.git/commitdiff_plain/6a83fbe44cc324f35f99da3ad290f0c0ef71260c?ds=sidebyside

start on a BigInt class
---

diff --git a/src/part05.rs b/src/part05.rs
new file mode 100644
index 0000000..274e3b5
--- /dev/null
+++ b/src/part05.rs
@@ -0,0 +1,2 @@
+// Rust-101, Part 05: Copy, Clone
+// ==============================
diff --git a/src/part06.rs b/src/part06.rs
new file mode 100644
index 0000000..3b867aa
--- /dev/null
+++ b/src/part06.rs
@@ -0,0 +1,67 @@
+// Rust-101, Part 06: Abstract Datastructure, Testing
+// ==================================================
+
+use std::cmp;
+use std::ops;
+
+pub struct BigInt {
+    data: Vec<u64>, // least significant digits first. The last block will *not* be 0.
+}
+
+impl BigInt {
+    pub fn new(x: u64) -> Self {
+        if x == 0 {
+            BigInt { data: vec![] }
+        } else {
+            BigInt { data: vec![x] }
+        }
+    }
+}
+
+/// 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)
+        }
+    }
+}
+
+#[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));
+}
+
+impl ops::Add<BigInt> for BigInt {
+    type Output = BigInt;
+    fn add(self, rhs: BigInt) -> Self::Output {
+        let mut result_vec:Vec<u64> = 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 { data: result_vec }
+    }
+}
+
+
+