Fix the grammar sampling method

[THinnerichs]

HerbSearch.jl/src/sampling_grammar.jl

Line 75 in 004f8c6

function _sample(node::RuleNode, x::RuleNodeAndCount, maxdepth::Int)

The sampling function is buggy and doesn't really do what we want to do here. (See discussion in #36 (comment))

[THinnerichs]

I ran a few examples and the sampling seems to quite skewed. It either generates very large or very small trees. Intermediate sizes barely happen.

julia> arithmetic_grammar = @cfgrammar begin
                   X = X * X
                   X = X + X
                   X = X - X
                   X = |(1:4)
               end
...               
             
julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
4,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
6,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
5,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
2{1{3{3{2{1{5,2{4,3{4,6}}}6}1{6,7}}2{1{1{4,3{6,7}}3{7,3{6,3{5,6}}}}3{3{7,2{6,1{5,6}}}1{4,2{5,2{1{6,6}4}}}}}}4}4}

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
4,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
6,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
6,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
2{3{5,1{7,4}}1{4,3{1{4,1{4,4}}7}}}

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
5,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
1{7,5}

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
6,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
5,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
3{6,6}

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
3{2{7,3{5,6}}3{3{6,3{1{1{2{2{2{1{1{4,1{6,1{4,4}}}6}7}5}1{2{3{6,6}3{4,4}}4}}7}6}5}}2{2{1{1{3{7,1{1{2{7,1{6,2{4,3{7,2{6,5}}}}}4}4}}2{2{3{6,5}2{2{3{1{5,3{4,2{6,4}}}6}2{4,1{1{2{5,5}7}6}}}1{4,1{4,4}}}}6}}5}6}6}}}

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
6,

julia> expression_generated = rand(RuleNode, arithmetic_grammar, :X, 15)
3{1{2{7,4}7}6}