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//! Manipulates the probabilities of working on different solutions. //! //! A portion of the bees in an artificial bee colony are tasked with observing //! the dedicated workers, and doing extra work on promising solutions. To //! enable this, the solutions' fitnesses are gathered as a `Vec<f64>`. A //! [`ScalingFunction`](type.ScalingFunction.html) is then run on the fitnesses to //! get weighting factors, and a solution is chosen with likelihood //! proportionate to its scaled fitness. This is expressed as: //! //! <center>P(*i*) = *scaled*<sub>*i*</sub> //! / ∑<sub>*j* = 1 … N</sub> *scaled*<sub>*j*</sub></center> //! //! By default, [proportionate](fn.proportionate.html) scaling is used. //! //! # Examples //! //! Several constructors for scaling functions are available in this module. //! However, users may also implement custom scaling functions. These can, //! for example, exaggerate differences in fitness: //! //! ``` //! # extern crate abc; fn main() { //! Box::new(move |fitnesses: Vec<f64>| { //! // Square the fitnesses. //! fitnesses.iter().map(|fitness| fitness.powf(2_f64)).collect::<Vec<_>>() //! }); //! # } //! ``` //! //! If you have a large number of active solutions, and don't want to replicate //! the fitnesses vector, you can mutate and return the same vector. Since the //! actual storage portion of a `Vec` is is heap-allocated, the scaling function //! should be reasonably well-behaved with respect to memory. /// Transform a set of fitnesses into weights for observers' random choices. pub type ScalingFunction = Fn(Vec<f64>) -> Vec<f64> + Send + Sync + 'static; /// Chooses solutions in direct proportion to their fitness. /// /// scaled<sub>*i*</sub> = fitness<sub>*i*</sub> pub fn proportionate() -> Box<ScalingFunction> { Box::new(move |fitnesses: Vec<f64>| fitnesses) } /// Chooses more fit solutions exponentially more often. /// /// scaled<sub>*i*</sub> = fitness<sub>*i*</sub><sup>*k*</sup> pub fn power(k: f64) -> Box<ScalingFunction> { Box::new(move |mut fitnesses: Vec<f64>| { for f in &mut fitnesses { *f = f.powf(k); } fitnesses }) } /// Chooses solutions according to their rank. /// /// Rather than use the fitness directly, this formula ranks the N solutions /// 1 to N, in ascending order of fitness, then chooses in proportion to the /// value of the rank. So, the solution with rank 6 will be chosen twice as /// often as the solution with rank 3, and three times as often as the solution /// with rank 2. /// /// scaled<sub>*i*</sub> = rank<sub>*i*</sub> pub fn rank() -> Box<ScalingFunction> { // power_rank is be implemented on its own because composing scaling // functions involves allocating extra vectors, which we'd like to avoid. power_rank(1_f64) } /// Chooses solutions according to their rank, raised to a certain power. /// /// This scaling formula was proposed by Yudong Zhang et al for the /// Fitness-Scaling Chaotic ABC in the 2013 volume of *Mathematical Problems /// in Engineering*. Conceptually, it composes the [power](fn.power.html) and /// [rank](fn.rank.html) scaling techniques. /// /// scaled<sub>*i*</sub> = rank<sub>*i*</sub><sup>*k*</sup> /// /// As with rank scaling, rank<sub>*i*</sub> starts with 1 for the least fit, /// and continues up to N for the most fit. pub fn power_rank(k: f64) -> Box<ScalingFunction> { Box::new(move |fitnesses: Vec<f64>| { // Pair each fitness with its index, so that we can remember which goes // where after sorting. let mut with_indices = fitnesses.iter().enumerate().collect::<Vec<_>>(); // Sort by fitness, ascending. After this, we can ignore fitness. with_indices.sort_by(|&(_, f1), &(_, f2)| f1.partial_cmp(f2).unwrap()); // The rank of solution i now corresponds to the index in with_indices // of (i, fitness_i). But we want the original index to be the index, // and the rank to be used to generate the value. // Create a blank (not empty) vector, so that we can use random access // to sort by original index. let mut ranks = vec![0_f64;with_indices.len()]; for (rank_minus_one, &(index, _)) in with_indices.iter().enumerate() { ranks[index] = ((rank_minus_one + 1) as f64).powf(k); } ranks }) }