Adds Candidates function to generic emitter

- Adds functionality that deduplicates printing entries with the same
version source and version to help decluter logging

scribe/emitter.go

@@ -2,6 +2,7 @@ package scribe
 
 import (
 	"io"
+	"strconv"
 	"time"
 
 	"github.com/paketo-buildpacks/packit"
@@ -41,3 +42,44 @@ func (e Emitter) SelectedDependency(entry packit.BuildpackPlanEntry, dependency
 	}
 	e.Break()
 }
+
+func (e Emitter) Candidates(entries []packit.BuildpackPlanEntry) {
+	e.Subprocess("Candidate version sources (in priority order):")
+
+	var (
+		sources [][2]string
+		maxLen  int
+	)
+
+Entries:
+	for _, entry := range entries {
+		versionSource, ok := entry.Metadata["version-source"].(string)
+		if !ok {
+			versionSource = "<unknown>"
+		}
+
+		if len(versionSource) > maxLen {
+			maxLen = len(versionSource)
+		}
+
+		version, ok := entry.Metadata["version"].(string)
+		if !ok {
+			version = ""
+		}
+
+		source := [2]string{versionSource, version}
+		for _, s := range sources {
+			if s == source {
+				continue Entries
+			}
+		}
+
+		sources = append(sources, source)
+	}
+
+	for _, source := range sources {
+		e.Action(("%-" + strconv.Itoa(maxLen) + "s -> %q"), source[0], source[1])
+	}
+
+	e.Break()
+}

scribe/emitter_test.go

@@ -118,4 +118,75 @@ func testEmitter(t *testing.T, context spec.G, it spec.S) {
 			})
 		})
 	})
+
+	context("Candidates", func() {
+		it("logs the candidate entries", func() {
+			emitter.Candidates([]packit.BuildpackPlanEntry{
+				{
+					Metadata: map[string]interface{}{
+						"version-source": "some-source",
+						"version":        "some-version",
+					},
+				},
+				{
+					Metadata: map[string]interface{}{
+						"version": "other-version",
+					},
+				},
+			})
+			Expect(buffer.String()).To(Equal(`    Candidate version sources (in priority order):
+      some-source -> "some-version"
+      <unknown>   -> "other-version"
+
+`))
+		})
+
+		context("when there are deuplicate version sources with the same version", func() {
+			it("logs the candidate entries and removes duplicates", func() {
+				emitter.Candidates([]packit.BuildpackPlanEntry{
+					{
+						Metadata: map[string]interface{}{
+							"version-source": "some-source",
+							"version":        "some-version",
+						},
+					},
+					{
+						Metadata: map[string]interface{}{
+							"version-source": "some-source",
+							"version":        "other-version",
+						},
+					},
+					{
+						Metadata: map[string]interface{}{
+							"version-source": "other-source",
+							"version":        "some-version",
+						},
+					},
+					{
+						Metadata: map[string]interface{}{
+							"version": "other-version",
+						},
+					},
+					{
+						Metadata: map[string]interface{}{
+							"version": "other-version",
+						},
+					},
+					{
+						Metadata: map[string]interface{}{
+							"version-source": "some-source",
+							"version":        "some-version",
+						},
+					},
+				})
+				Expect(buffer.String()).To(Equal(`    Candidate version sources (in priority order):
+      some-source  -> "some-version"
+      some-source  -> "other-version"
+      other-source -> "some-version"
+      <unknown>    -> "other-version"
+
+`), buffer.String())
+			})
+		})
+	})
 }