Detekt: use code snippet as fingerprint instead of position.

The signature for some formatting warnings is just file name and line
number. In this case use the surrounding code to build issue fingerprint.

detekt-gitlab-output-plugin/src/main/kotlin/outputreport/GitlabQualityOutputReport.kt

@@ -20,9 +20,13 @@
 package outputreport
 
 import io.gitlab.arturbosch.detekt.api.Detektion
+import io.gitlab.arturbosch.detekt.api.Location
 import io.gitlab.arturbosch.detekt.api.OutputReport
 import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
+import java.io.File
+
+private const val CONTEXT_SIZE = 10
 
 class GitlabQualityOutputReport : OutputReport() {
 
@@ -33,17 +37,36 @@ class GitlabQualityOutputReport : OutputReport() {
     override fun render(detektion: Detektion): String {
         val findings = detektion.findings.values.flatten()
         val reports = findings.map {
-            val location = with(it.entity.location) {
+            val location = with(it.location) {
                 GitlabQualityReport.Location(
                     GitlabQualityReport.Location.Lines(source.line, source.line),
                     (filePath.relativePath ?: filePath.absolutePath).toString()
                 )
             }
-            val fingerprint = it.issue.id + " - " + it.signature
+            val fingerprintSignature = if (isFilePosition(it.signature, it.location))
+                with(it.location) { getTextContext(filePath.absolutePath.toString(), text.start, text.end) }
+            else
+                it.signature
+
+            val fingerprint = it.issue.id + " - " + fingerprintSignature
             // Detekt severity doesn't map well to GitLab severity.
             val severity = "info"
             GitlabQualityReport(it.messageOrDescription(), fingerprint, location, severity)
         }
         return json.encodeToString(reports)
     }
+
+    private fun isFilePosition(signature: String, location: Location): Boolean = with(location) {
+        signature.endsWith("${filePath.absolutePath.fileName}:${source.line}")
+    }
+
+    private fun getTextContext(filePath: String, startOffset: Int, endOffset: Int): String {
+        val contents = File(filePath).readText().replace("\r\n", "\n")
+        val selection = contents.slice(startOffset until endOffset)
+        return selection.ifBlank {
+            val preceding = contents.slice((startOffset - CONTEXT_SIZE).coerceAtLeast(0) until startOffset)
+            val following = contents.slice(endOffset until (endOffset + CONTEXT_SIZE).coerceAtMost(contents.length))
+            preceding + following
+        }
+    }
 }