Add a simple TransactionGraphSearch class that can be used to query transaction graphs recursively loaded from storage. Right now it only supports a single type of query.

core/src/main/kotlin/core/TransactionGraphSearch.kt

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+/*
+ * Copyright 2015 Distributed Ledger Group LLC.  Distributed as Licensed Company IP to DLG Group Members
+ * pursuant to the August 7, 2015 Advisory Services Agreement and subject to the Company IP License terms
+ * set forth therein.
+ *
+ * All other rights reserved.
+ */
+
+package core
+
+import core.crypto.SecureHash
+import java.util.*
+import java.util.concurrent.Callable
+
+/**
+ * Given a map of transaction id to [SignedTransaction], performs a breadth first search of the dependency graph from
+ * the starting point down in order to find transactions that match the given query criteria.
+ *
+ * Currently, only one kind of query is supported: find any transaction that contains a command of the given type.
+ *
+ * In future, this should support restricting the search by time, and other types of useful query.
+ *
+ * TODO: Write unit tests for this.
+ */
+class TransactionGraphSearch(val transactions: Map<SecureHash, SignedTransaction>,
+                             val startPoints: List<WireTransaction>) : Callable<List<WireTransaction>> {
+    class Query(
+        val withCommandOfType: Class<out CommandData>? = null
+    )
+
+    var query: Query = Query()
+
+    override fun call(): List<WireTransaction> {
+        val q = query
+
+        val next = ArrayList<SecureHash>()
+        next += startPoints.flatMap { it.inputs.map { it.txhash } }
+
+        val results = ArrayList<WireTransaction>()
+
+        while (next.isNotEmpty()) {
+            val hash = next.removeAt(next.lastIndex)
+            val tx = transactions[hash]?.tx ?: continue
+
+            if (q.matches(tx))
+                results += tx
+
+            next += tx.inputs.map { it.txhash }
+        }
+
+        return results
+    }
+
+    private fun Query.matches(tx: WireTransaction): Boolean {
+        if (withCommandOfType != null) {
+            if (tx.commands.any { it.data.javaClass.isAssignableFrom(withCommandOfType) })
+                return true
+        }
+        return false
+    }
+}