IntersectUsing() new function

[AlessandroLorenzi]

---

This change is 

[AlessandroLorenzi]

refs: #72

[codecov-io]

Codecov Report

Merging #161 into master will increase coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
+ Coverage   98.66%   98.69%   +0.03%     
==========================================
  Files           8        8              
  Lines        1792     1842      +50     
==========================================
+ Hits         1768     1818      +50     
  Misses         12       12              
  Partials       12       12

Impacted Files	Coverage Δ
pie/float64s_pie.go	98.82% <100%> (+0.04%)	:arrow_up:
pie/carpointers_pie.go	98.95% <100%> (+0.01%)	:arrow_up:
pie/cars_pie.go	98.95% <100%> (+0.01%)	:arrow_up:
pie/ints_pie.go	98.82% <100%> (+0.04%)	:arrow_up:
pie/strings_pie.go	97.89% <100%> (+0.08%)	:arrow_up:

---

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[elliotchance]

Please fix the merge conflict before I review.

[AlessandroLorenzi]

hi @elliotchance , what you think about a solution like this?

too overcomplicated?

package functions

// IntersectUsing returns items that exist in all lists, using equals function
//
// It returns slice without any duplicates.
// If zero slice arguments are provided, then nil is returned.
func (ss SliceType) IntersectUsing(equals func(ElementType, ElementType) (bool, interface{}), slices ...SliceType) []interface{} {
    if slices == nil {
        return nil
    }

    found := map[interface{}]int{}

    for _, e1 := range ss {
        for _, s2 := range slices {
            foundInSlice := false
            for _, e2 := range s2 {
                chekFound, checkValue := equals(e1, e2)
                if chekFound {
                    found[checkValue]++
                    foundInSlice = true
                    break // if found the element don't check other elements in this slice
                }
            }
            if !foundInSlice {
                break // if not found in this slice don't check other slices
            }
        }
    }
    ss2 := []interface{}{}

    for value, count := range found {
        if count == len(slices) {
            ss2 = append(ss2, value)
        }
    }
    return ss2
}

[AlessandroLorenzi]

I'll restart this function from zero