Commit ae442e2c2906f768098de307f3388ef0e8d47e5b | zig

author	David Rubin <david@vortan.dev> 2025-03-20 22:16:57-07:00
committer	David Rubin <david@vortan.dev> 2025-03-24 06:58:01-07:00
log	ae442e2c2906f768098de307f3388ef0e8d47e5b
tree	177ff5c8be5d31f88bcf0cb5142f43e6f31ce630
parent	adee3ee9a2c679069a15bd5476860d45f05b356b
signature	Commit is signed but in an unrecognized format.

big.int: return normalized results from `{add,sub}Carry`

3 files changed, 44 insertions(+), 5 deletions(-)

lib
- std
  - math
    big
    int.zig
    int_test.zig
src
- Value.zig

lib/std/math/big/int.zig+15-4

...	...	@@ -76,9 +76,18 @@ pub fn calcSqrtLimbsBufferLen(a_bit_count: usize) usize {
76	76	return a_limb_count + 3 * u_s_rem_limb_count + calcDivLimbsBufferLen(a_limb_count, u_s_rem_limb_count);
77	77	}
78	78
79		// Compute the number of limbs required to store a 2s-complement number of `bit_count` bits.
	79	/// Compute the number of limbs required to store a 2s-complement number of `bit_count` bits.
	80	pub fn calcNonZeroTwosCompLimbCount(bit_count: usize) usize {
	81	assert(bit_count != 0);
	82	return calcTwosCompLimbCount(bit_count);
	83	}
	84
	85	/// Compute the number of limbs required to store a 2s-complement number of `bit_count` bits.
	86	///
	87	/// Special cases `bit_count == 0` to return 1. Zero-bit integers can only store the value zero
	88	/// and this big integer implementation stores zero using one limb.
80	89	pub fn calcTwosCompLimbCount(bit_count: usize) usize {
81		return std.math.divCeil(usize, bit_count, @bitSizeOf(Limb)) catch unreachable;
	90	return @max(std.math.divCeil(usize, bit_count, @bitSizeOf(Limb)) catch unreachable, 1);
82	91	}
83	92
84	93	/// a + b * c + *carry, sets carry to the overflow bits
...	...	@@ -188,8 +197,10 @@ pub const Mutable = struct {
188	197	if (self.limbs.ptr != other.limbs.ptr) {
189	198	@memcpy(self.limbs[0..other.limbs.len], other.limbs[0..other.limbs.len]);
190	199	}
191		self.positive = other.positive;
192		self.len = other.limbs.len;
	200	// Normalize before setting `positive` so the `eqlZero` doesn't need to iterate
	201	// over the extra zero limbs.
	202	self.normalize(other.limbs.len);
	203	self.positive = other.positive or other.eqlZero();
193	204	}
194	205
195	206	/// Efficiently swap an Mutable with another. This swaps the limb pointers and a full copy is not

lib/std/math/big/int_test.zig+28

...	...	@@ -726,6 +726,34 @@ test "subWrap single-multi, signed, limb aligned" {
726	726	try testing.expect((try a.toInt(SignedDoubleLimb)) == maxInt(SignedDoubleLimb));
727	727	}
728	728
	729	test "addWrap returns normalized result" {
	730	var x = try Managed.initSet(testing.allocator, 0);
	731	defer x.deinit();
	732	var y = try Managed.initSet(testing.allocator, 0);
	733	defer y.deinit();
	734
	735	// make them both non normalized "-0"
	736	x.setMetadata(false, 1);
	737	y.setMetadata(false, 1);
	738
	739	var r = try Managed.init(testing.allocator);
	740	defer r.deinit();
	741	try testing.expect(!(try r.addWrap(&x, &y, .unsigned, 64)));
	742	try testing.expect(r.isPositive() and r.len() == 1 and r.limbs[0] == 0);
	743	}
	744
	745	test "subWrap returns normalized result" {
	746	var x = try Managed.initSet(testing.allocator, 0);
	747	defer x.deinit();
	748	var y = try Managed.initSet(testing.allocator, 0);
	749	defer y.deinit();
	750
	751	var r = try Managed.init(testing.allocator);
	752	defer r.deinit();
	753	try testing.expect(!(try r.subWrap(&x, &y, .unsigned, 64)));
	754	try testing.expect(r.isPositive() and r.len() == 1 and r.limbs[0] == 0);
	755	}
	756
729	757	test "addSat single-single, unsigned" {
730	758	var a = try Managed.initSet(testing.allocator, maxInt(u17) - 5);
731	759	defer a.deinit();

src/Value.zig+1-1

...	...	@@ -2223,7 +2223,7 @@ pub fn shlSatScalar(
2223	2223	const shift: usize = @intCast(rhs.toUnsignedInt(zcu));
2224	2224	const limbs = try arena.alloc(
2225	2225	std.math.big.Limb,
2226		std.math.big.int.calcTwosCompLimbCount(info.bits) + 1,
	2226	std.math.big.int.calcTwosCompLimbCount(info.bits),
2227	2227	);
2228	2228	var result_bigint = BigIntMutable{
2229	2229	.limbs = limbs,