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std.math.big.int: address log2/log10 reviews

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There were good reviews made after #31365 was merged, so this commit
addresses them separately.

1. Assert that the number is greater than zero
2. Use `constants` instead of calculating constants manually
3. Use `Const.bitCountAbs` for log2
This commit is contained in:
hemisputnik 2026-03-01 14:05:22 +02:00 • committed by Andrew Kelley
parent da6d4e28ef
commit 74f361a5ce
1 changed files with 7 additions and 9 deletions
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16
lib/std/math/big/int.zig
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@ -2678,7 +2678,9 @@ pub const Const = struct {
/// Calculate the base 2 logarithm, rounded down.
pub fn log2(a: Const) Limb {
return a.limbs.len * @bitSizeOf(Limb) - 1 - @clz(a.limbs[a.limbs.len - 1]);
assert(a.positive);
assert(!a.eqlZero());
return a.bitCountAbs() - 1;
}
/// Calculate the base 10 logarithm, rounded down.
@ -2695,13 +2697,9 @@ pub const Const = struct {
///
/// `limbs_buffer` is used for temporary storage. The amount required is given by `calcLog10LimbsBufferLen`.
pub fn log10(a: Const, limbs_buffer: []Limb) Limb {
const max_digits_per_limb = std.math.log10(std.math.maxInt(Limb));
const limb_base = comptime calc: {
var limb_base: comptime_int = 1;
for (0..max_digits_per_limb) |_| limb_base *= 10;
break :calc limb_base;
};
const limb_base_as_bigint: Const = .{ .limbs = &.{limb_base}, .positive = true };
assert(a.positive);
assert(!a.eqlZero());
const limb_base_as_bigint: Const = .{ .limbs = &.{constants.big_bases[10]}, .positive = true };
var q: Mutable = .{
.limbs = limbs_buffer[0 .. a.limbs.len + 2],
@ -2721,7 +2719,7 @@ pub const Const = struct {
var num_digits: Limb = 0;
while (q.len >= 2) {
q.divTrunc(&remainder, q.toConst(), limb_base_as_bigint, division_buf);
num_digits += max_digits_per_limb;
num_digits += constants.digits_per_limb[10];
}
var remaining_limb = q.limbs[0];
while (remaining_limb != 0) {