# Sizehint! a Dict with a BigInt size not possible?
**URL:** https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151
**Category:** General Usage
**Tags:** question
**Created:** [October 27, 2020, 9:54pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151 "2020-10-27T21:54:03Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@malacroi](https://discourse.julialang.org/u/malacroi)
#### Post date: [October 27, 2020, 9:54pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/1 "2020-10-27T21:54:03Z")
I’m using a dictionary to store some values indexed by integer partitions of 40. Since I know that there are 37338 of them, I attempted to sizehint! the dictionary, but instead of manually entering 37338, I used a function that computed that number as a BigInt, with the result that the sizehint! failed. As a minimal example, consider:
```julia
julia> d = Dict{Int64, Int64}()
Dict{Int64,Int64}()
julia> sizehint!(d, BigInt(37338))
ERROR: MethodError: no method matching leading_zeros(::BigInt)
Closest candidates are:
leading_zeros(::Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}) at int.jl:384
Stacktrace:
[1] _tablesz(::BigInt) at ./abstractdict.jl:526
[2] rehash!(::Dict{Int64,Int64}, ::BigInt) at ./dict.jl:179
[3] sizehint!(::Dict{Int64,Int64}, ::BigInt) at ./dict.jl:242
[4] top-level scope at REPL[39]:1
```
why can’t the sizehint! be specified as a BigInt?
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [October 27, 2020, 10:05pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/2 "2020-10-27T22:05:13Z")
is it possible to have memory larger than ~`typemax(Int64) * 2` bytes?
in terms of implementation, `sizehint!(array)` is able to take a `BigInt` so maybe dictionary should take it too.
As a work around you can probably convert it back to `Int` after calculation? Since you don’t \*actually need to hint a size that is beyond Int64’s capability
---
### Author:  [@malacroi](https://discourse.julialang.org/u/malacroi)
#### Post date: [October 27, 2020, 10:13pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/3 "2020-10-27T22:13:20Z")
By way of comparison, `sizehint!(d,Int128(40))` works just fine, and I certainly can’t have memory approaching `typemax(Int128)` bytes, but we don’t require an explicit conversion in that case.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [October 27, 2020, 10:17pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/4 "2020-10-27T22:17:27Z")
the issue is simply `leading_zeros` is only meaningful for `BitInteger`, for which `BigInt` is not one
---
### Author:  [@malacroi](https://discourse.julialang.org/u/malacroi)
#### Post date: [October 27, 2020, 10:49pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/5 "2020-10-27T22:49:11Z")
So it’s just a matter of not having an efficient way to compute `ceil(Int, log2(x))` when `x::BigInt`?
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [October 27, 2020, 10:56pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/6 "2020-10-27T22:56:07Z")
no, `leading_zeros` is describing the binary representation of a number, this is not well-defined for `BigInt`. Thus:
```julia
julia> bitstring(BigInt(1239))
ERROR: MethodError: no method matching bitstring(::BigInt)
```
---
### Author:  [@malacroi](https://discourse.julialang.org/u/malacroi)
#### Post date: [October 27, 2020, 11:07pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/7 "2020-10-27T23:07:16Z")
But there’s no need to calculate the number of leading zeros, what the table needs is the number of digits trailing the leading zeros. It’s only used via its negation to determine the number of significant bits to the representation of `x`, in the calculation
`_tablesz(x::Integer) = x < 16 ? 16 : one(x)<<((sizeof(x)<<3)-leading_zeros(x-1))`
i.e. the table that can hold `x` items needs `(sizeof(x)<<3)-leading_zeros(x-1)` bits, so it can actually hold 2^(number of bits to represent x) items. But this agrees with the slower calculation
`_tablesz(x::Integer) = x < 16 ? 16 : one(x)<
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [October 27, 2020, 11:09pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/8 "2020-10-27T23:09:15Z")
not very familiar with this code but sounds like a possible route, you can open a PR maybe
---
### Author:  [@Palli](https://discourse.julialang.org/u/Palli)
#### Post date: [October 27, 2020, 11:25pm UTC](https://discourse.julialang.org/t/sizehint-a-dict-with-a-bigint-size-not-possible/49151/9 "2020-10-27T23:25:54Z")
You can do something like, then your sizehint! would have worked:
```julia
julia> import Base.sizehint!
julia> function sizehint!(d::Dict{T}, newsz::BigInt) where T
newsz > 1000000 ? Error("sizehint! has suspiciously high value") : sizehint!(d, Int(newsz))
end
```
Feel free to make a PR on Dict with (or without?) some cut-off. I’m not going to make a PR, just note, you do not need to do anything about `leading_zeros` or `_tablesz`.
I tried on my machine with 128 GB of RAM, just note, allocating 119 on yours might be slow or likely crash the machine:
```julia
julia> @time sizehint!(d, 373380000)
0.766696 seconds (9.99 k allocations: 17.001 GiB, 3.20% gc time, 1.62% compilation time)
Dict{Int64, Int64}()
julia> @time sizehint!(d, 3733800000)
21.486587 seconds (3 allocations: 119.000 GiB, 0.62% gc time)
Dict{Int64, Int64}()
```