Improve PEM exception text on incorrect usage

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## 3.0
 
+### NEXT
+
+
 ### 3.16.1 (Supreme 0.8.1) Hotfix
 * Generalized, proper COSE to MAC mapping, preventing unexpected behaviour for `HS265_24`
 

gradle.properties

@@ -2,8 +2,8 @@ kotlin.code.style=official
 kotlin.js.compiler=ir
 org.gradle.jvmargs=-Xmx4g -Dfile.encoding=UTF-8
 
-artifactVersion=3.16.1
-supremeVersion=0.8.1
+artifactVersion=3.17.0-SNAPSHOT
+supremeVersion=0.9.0-SNAPSHOT
 
 # This is not a well-defined property, the ASP convention plugin respects it, though
 jdk.version=17

indispensable-asn1/src/commonMain/kotlin/at/asitplus/signum/indispensable/asn1/PemEncodable.kt

@@ -45,6 +45,31 @@ abstract class PemDecodable<A : Asn1Element, out T : PemEncodable<A>>
     constructor(vararg ebStrings: String) : this(ebStrings.associateWith { PemDecoder.Default })
     constructor(vararg decoders: Pair<String, ((ByteArray)->T)?>) : this(decoders.associate { it.first to PemDecoder(it.second) })
 
+    companion object {
+        private val KNOWN_DECODERS = mutableMapOf<String, MutableSet<String>>()
+    }
+
+    init {
+        val myName = this::class.simpleName
+        if (myName != null) {
+            decoders.keys.forEach { ebString ->
+                KNOWN_DECODERS.getOrPut(ebString) { mutableSetOf() }.add(myName)
+            }
+        }
+    }
+
+    class UnknownEncapsulationBoundaryException(val ebString: String)
+        : IllegalArgumentException(
+            "Unknown encapsulation boundary string $ebString" +
+            KNOWN_DECODERS[ebString].let { decoderClasses ->
+                when {
+                    (decoderClasses == null) -> ""
+                    (decoderClasses.size == 1) -> "\nDid you mean to use ${decoderClasses.first()}.decodeFromPem(...)?"
+                    else -> "\nDid you mean to use one of the following?\n${
+                        decoderClasses.joinToString("\n") { "- $it.decodeFromPem(...)" }}"
+                }
+            })
+
     /** Decodes a PEM-encoded string into [T] */
     fun decodeFromPem(src: String): KmmResult<T> = catching {
         src.lineSequence()
@@ -56,7 +81,7 @@ abstract class PemDecodable<A : Asn1Element, out T : PemEncodable<A>>
                 val firstLine = it.next()
                 val ebString = firstLine.substring(FENCE_PREFIX_BEGIN.length, firstLine.length - FENCE_SUFFIX.length)
                 val decoder: PemDecoder<T> = decoders.getOrElse(ebString)
-                    { throw IllegalArgumentException("Unknown encapsulation boundary string $ebString") }
+                    { throw UnknownEncapsulationBoundaryException(ebString) }
                 val b64data = StringBuilder()
                 while (it.hasNext()) {
                     val line = it.next()

indispensable/src/commonTest/kotlin/Asn1AddonsTest.kt

@@ -1,12 +1,14 @@
 import at.asitplus.signum.indispensable.asn1.*
 import at.asitplus.signum.indispensable.asn1.encoding.parse
+import at.asitplus.signum.indispensable.pki.X509Certificate
 import com.ionspin.kotlin.bignum.integer.BigInteger
 import com.ionspin.kotlin.bignum.integer.Sign
 import io.kotest.common.Platform
 import io.kotest.common.platform
 import io.kotest.core.spec.style.FreeSpec
 import io.kotest.datatest.withData
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldContain
 import io.kotest.matchers.types.shouldBeTypeOf
 import io.kotest.property.Arb
 import io.kotest.property.arbitrary.*
@@ -86,4 +88,15 @@ class Asn1AddonsTest: FreeSpec({
             }
         }
     }
+
+    "PemDecodable error messages" {
+        val x = X509Certificate.decodeFromPem("-----BEGIN PUBLIC KEY-----\n" +
+                "MFswDQYJKoZIhvcNAQEBBQADSgAwRwJAXWRPQyGlEY+SXz8Uslhe+MLjTgWd8lf/\n" +
+                "nA0hgCm9JFKC1tq1S73cQ9naClNXsMqY7pwPt1bSY8jYRqHHbdoUvwIDAQAB\n" +
+                "-----END PUBLIC KEY-----")
+            .exceptionOrNull()
+            .shouldBeTypeOf<PemDecodable.UnknownEncapsulationBoundaryException>()
+        x.ebString shouldBe "PUBLIC KEY"
+        x.message shouldContain "CryptoPublicKey.decodeFromPem"
+    }
 })