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math.big.int: document the purpose of limb_len in scalar methods

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Ideally this duplicated code could be factored out into a function, but
there doesn't seem to be any way in the Zig type system to represent an
argument to a function called at comptime that is only needed if it is
comptime-known.  Instead, we document what is going on in an adjacent
comment in case it gets copy-pasted into new methods in the future.
This commit is contained in:
Jacob Young 2022-10-12 08:56:13 -04:00
parent 38ee512a25
commit e78d7704a4
1 changed files with 12 additions and 0 deletions
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12
lib/std/math/big/int.zig
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@ -394,6 +394,12 @@ pub const Mutable = struct {
/// Asserts the result fits in `r`. An upper bound on the number of limbs needed by
/// r is `math.max(a.limbs.len, calcLimbLen(scalar)) + 1`.
pub fn addScalar(r: *Mutable, a: Const, scalar: anytype) void {
// Normally we could just determine the number of limbs needed with calcLimbLen,
// but that is not comptime-known when scalar is not a comptime_int. Instead, we
// use calcTwosCompLimbCount for a non-comptime_int scalar, which can be pessimistic
// in the case that scalar happens to be small in magnitude within its type, but it
// is well worth being able to use the stack and not needing an allocator passed in.
// Note that Mutable.init still sets operand.len to calcLimbLen(scalar) in any case.
const limb_len = comptime switch (@typeInfo(@TypeOf(scalar))) {
.ComptimeInt => calcLimbLen(scalar),
.Int => |info| calcTwosCompLimbCount(info.bits),
@ -2311,6 +2317,12 @@ pub const Const = struct {
/// Same as `order` but the right-hand operand is a primitive integer.
pub fn orderAgainstScalar(lhs: Const, scalar: anytype) math.Order {
// Normally we could just determine the number of limbs needed with calcLimbLen,
// but that is not comptime-known when scalar is not a comptime_int. Instead, we
// use calcTwosCompLimbCount for a non-comptime_int scalar, which can be pessimistic
// in the case that scalar happens to be small in magnitude within its type, but it
// is well worth being able to use the stack and not needing an allocator passed in.
// Note that Mutable.init still sets rhs.len to calcLimbLen(scalar) in any case.
const limb_len = comptime switch (@typeInfo(@TypeOf(scalar))) {
.ComptimeInt => calcLimbLen(scalar),
.Int => |info| calcTwosCompLimbCount(info.bits),