Add revdep calculation

Add CompilationSpecs type to handle slices of CompilationSpec, to
perform operation such as Unique() and Remove().

Add also dependencies and the spec associated to the Artifact, to track
how the artifact was generated.

Add revdeps compilation wrapper, and unit tests

pkg/compiler/spec.go

@@ -23,6 +23,68 @@ import (
 	yaml "gopkg.in/yaml.v2"
 )
 
+type LuetCompilationspecs []LuetCompilationSpec
+
+func NewLuetCompilationspecs(s ...CompilationSpec) CompilationSpecs {
+	all := LuetCompilationspecs{}
+
+	for _, spec := range s {
+		all.Add(spec)
+	}
+	return &all
+}
+
+func (specs LuetCompilationspecs) Len() int {
+	return len(specs)
+}
+
+func (specs *LuetCompilationspecs) Remove(s CompilationSpecs) CompilationSpecs {
+	newSpecs := LuetCompilationspecs{}
+SPECS:
+	for _, spec := range specs.All() {
+		for _, target := range s.All() {
+			if target.GetPackage().Matches(spec.GetPackage()) {
+				continue SPECS
+			}
+		}
+		newSpecs.Add(spec)
+	}
+	return &newSpecs
+}
+
+func (specs *LuetCompilationspecs) Add(s CompilationSpec) {
+	c, ok := s.(*LuetCompilationSpec)
+	if !ok {
+		panic("LuetCompilationspecs supports only []LuetCompilationSpec")
+	}
+	*specs = append(*specs, *c)
+}
+
+func (specs *LuetCompilationspecs) All() []CompilationSpec {
+	var cspecs []CompilationSpec
+	for i, _ := range *specs {
+		f := (*specs)[i]
+		cspecs = append(cspecs, &f)
+	}
+
+	return cspecs
+}
+
+func (specs *LuetCompilationspecs) Unique() CompilationSpecs {
+	newSpecs := LuetCompilationspecs{}
+	seen := map[string]bool{}
+
+	for i, _ := range *specs {
+		j := (*specs)[i]
+		_, ok := seen[j.GetPackage().GetFingerPrint()]
+		if !ok {
+			seen[j.GetPackage().GetFingerPrint()] = true
+			newSpecs = append(newSpecs, j)
+		}
+	}
+	return &newSpecs
+}
+
 type LuetCompilationSpec struct {
 	Steps      []string    `json:"steps"`   // Are run inside a container and the result layer diff is saved
 	Prelude    []string    `json:"prelude"` // Are run inside the image which will be our builder