Add merging node and prettified toString

usvm-core/src/main/kotlin/org/usvm/PathNode.kt

@@ -3,46 +3,66 @@ package org.usvm
 import org.usvm.algorithms.findLcaLinear
 import org.usvm.merging.UMergeable
 
+sealed interface PathSegment<Statement> {
+    val statement: Statement
+
+    data class Single<Statement>(
+        override val statement: Statement,
+    ) : PathSegment<Statement> {
+        override fun toString(): String = "$statement"
+    }
+
+    data class Merged<Statement>(
+        override val statement: Statement,
+        val left: List<PathSegment<Statement>>,
+        val right: List<PathSegment<Statement>>,
+    ) : PathSegment<Statement> {
+        override fun toString(): String = buildString {
+            appendLine(statement)
+            val targetSize = kotlin.math.max(left.size, right.size)
+            val leftStrs = left.map { it.toString() } + List(targetSize - left.size) { "" }
+            val rightStrs = right.map { it.toString() } + List(targetSize - right.size) { "" }
+            val leftColumnnSize = (leftStrs.maxOfOrNull { it.length } ?: 0) + 8
+            leftStrs.zip(rightStrs).forEach { (left, right) ->
+                append(left.padEnd(leftColumnnSize))
+                appendLine(right)
+            }
+        }
+    }
+}
+
 class PathNode<Statement> private constructor(
     val parent: PathNode<Statement>?,
-    private val _statement: Statement?,
+    private val _segment: PathSegment<Statement>?,
     depth: Int,
 ) : UMergeable<PathNode<Statement>, Unit> {
     var depth: Int = depth
         private set
 
-    val statement: Statement get() = requireNotNull(_statement)
+    val statement: Statement get() = requireNotNull(_segment).statement
 
     operator fun plus(statement: Statement): PathNode<Statement> {
-        return PathNode(this, statement, depth + 1)
+        return PathNode(this, PathSegment.Single(statement), depth + 1)
     }
 
     val allStatements
         get(): Iterable<Statement> = Iterable {
             object : Iterator<Statement> {
                 var cur = this@PathNode
-                override fun hasNext(): Boolean = cur._statement != null
+                override fun hasNext(): Boolean = cur._segment != null
 
                 override fun next(): Statement {
                     if (!hasNext()) {
                         throw NoSuchElementException()
                     }
-                    val element = requireNotNull(cur._statement)
+                    val element = requireNotNull(cur._segment)
                     cur = requireNotNull(cur.parent)
-                    return element
+                    return element.statement
                 }
 
             }
         }
 
-    companion object {
-        private val EMPTY = PathNode(parent = null, _statement = null, depth = 0)
-
-        @Suppress("UNCHECKED_CAST")
-        fun <Statement> root(): PathNode<Statement> = EMPTY as PathNode<Statement>
-    }
-
-
     /**
      * Check if this [PathNode] can be merged with [other] path node.
      *
@@ -54,10 +74,32 @@ class PathNode<Statement> private constructor(
      * @return the merged path node.
      */
     override fun mergeWith(other: PathNode<Statement>, by: Unit): PathNode<Statement>? {
-        if (_statement != other._statement) {
+        if (_segment != other._segment) {
             return null
         }
-        val (lca, _, _) = findLcaLinear(this, other, { it.parent!! }, { it.depth }, { it.statement })
-        return lca + statement
+        val (lca, suffixLeft, suffixRight) = findLcaLinear(
+            this,
+            other,
+            { it.parent!! },
+            { it.depth },
+            { it._segment!! }
+        )
+
+        val segment = PathSegment.Merged(statement, suffixLeft, suffixRight)
+
+        return PathNode(lca, segment, lca.depth + 1)
+    }
+
+    companion object {
+        private val EMPTY = PathNode<Nothing?>(parent = null, _segment = null, depth = 0)
+
+        @Suppress("UNCHECKED_CAST")
+        fun <Statement> root(): PathNode<Statement> = EMPTY as PathNode<Statement>
     }
+
+    override fun toString(): String =
+        buildString {
+            appendLine(_segment)
+            appendLine(parent)
+        }
 }