From a03f9548d3dd32876f99f5b7bdf1d678c5a5b98e Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Sun, 31 Jan 2021 20:58:11 +0100
Subject: [PATCH] std/math/big/int: normalize after a right shift

After a right shift, top limbs may be all zero. However, without
normalization, the number of limbs is not going to change.

In order to check if a big number is zero, we used to assume that the
number of limbs is 1. Which may not be the case after right shifts,
even if the actual value is zero.

- Normalize after a right shift
- Add a test for that issue
- Check all the limbs in `eqlZero()`. It may not be necessary if
callers always remember to normalize before calling the function.
But checking all the limbs is very cheap and makes the function less
bug-prone.
---
 lib/std/math/big/int.zig      | 8 +++++---
 lib/std/math/big/int_test.zig | 6 ++++++
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/lib/std/math/big/int.zig b/lib/std/math/big/int.zig
index 3cd72dd8e4..81982eac51 100644
--- a/lib/std/math/big/int.zig
+++ b/lib/std/math/big/int.zig
@@ -549,8 +549,8 @@ pub const Mutable = struct {
             return;
         }
 
-        const r_len = llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
-        r.len = a.limbs.len - (shift / limb_bits);
+        llshr(r.limbs[0..], a.limbs[0..a.limbs.len], shift);
+        r.normalize(a.limbs.len - (shift / limb_bits));
         r.positive = a.positive;
     }
 
@@ -1348,7 +1348,9 @@ pub const Const = struct {
 
     /// Returns true if `a == 0`.
     pub fn eqZero(a: Const) bool {
-        return a.limbs.len == 1 and a.limbs[0] == 0;
+        var d: Limb = 0;
+        for (a.limbs) |limb| d |= limb;
+        return d == 0;
     }
 
     /// Returns true if `|a| == |b|`.
diff --git a/lib/std/math/big/int_test.zig b/lib/std/math/big/int_test.zig
index b73e9f90d6..179e55ff69 100644
--- a/lib/std/math/big/int_test.zig
+++ b/lib/std/math/big/int_test.zig
@@ -1287,6 +1287,12 @@ test "big.int shift-right multi" {
     try a.shiftRight(a, 67);
 
     testing.expect((try a.to(u64)) == 0x1fffe0001dddc222);
+
+    try a.set(0xffff0000eeee1111dddd2222cccc3333);
+    try a.shiftRight(a, 63);
+    try a.shiftRight(a, 63);
+    try a.shiftRight(a, 2);
+    testing.expect(a.eqZero());
 }
 
 test "big.int shift-left single" {