[6.0.0] Conversion from Long-encoded nBits representation to BigInt and back

core/shared/src/main/scala/sigma/SigmaDsl.scala

@@ -695,6 +695,20 @@ trait SigmaDslBuilder {
     */
   def groupGenerator: GroupElement
 
+  /**
+    * @return big integer provided as input approximately encoded using NBits,
+    *         see (https://bitcoin.stackexchange.com/questions/57184/what-does-the-nbits-value-represent)
+    *         for format details
+    */
+  def encodeNbits(bi: BigInt): Long
+
+  /**
+    * @return big integer decoded from NBits value provided,
+    *         see (https://bitcoin.stackexchange.com/questions/57184/what-does-the-nbits-value-represent)
+    *         for format details
+    */
+  def decodeNbits(l: Long): BigInt
+
   /**
     * Transforms serialized bytes of ErgoTree with segregated constants by replacing constants
     * at given positions with new values. This operation allow to use serialized scripts as

core/shared/src/main/scala/sigma/ast/SType.scala

@@ -452,7 +452,7 @@ case object SLong extends SPrimType with SEmbeddable with SNumericType with SMon
   }
 }
 
-/** Type of 256 bit integet values. Implemented using [[java.math.BigInteger]]. */
+/** Type of 256 bit integer values. Implemented using [[java.math.BigInteger]]. */
 case object SBigInt extends SPrimType with SEmbeddable with SNumericType with SMonoType {
   override type WrappedType = BigInt
   override val typeCode: TypeCode = 6: Byte

core/shared/src/main/scala/sigma/reflection/ReflectionData.scala

@@ -443,6 +443,12 @@ object ReflectionData {
         },
         mkMethod(clazz, "decodePoint", Array[Class[_]](cColl)) { (obj, args) =>
           obj.asInstanceOf[SigmaDslBuilder].decodePoint(args(0).asInstanceOf[Coll[Byte]])
+        },
+        mkMethod(clazz, "encodeNbits", Array[Class[_]](classOf[BigInt])) { (obj, args) =>
+          obj.asInstanceOf[SigmaDslBuilder].encodeNbits(args(0).asInstanceOf[BigInt])
+        },
+        mkMethod(clazz, "decodeNbits", Array[Class[_]](classOf[Long])) { (obj, args) =>
+          obj.asInstanceOf[SigmaDslBuilder].decodeNbits(args(0).asInstanceOf[Long])
         }
       )
     )

core/shared/src/main/scala/sigma/util/NBitsUtils.scala

@@ -0,0 +1,84 @@
+package sigma.util
+
+import java.math.BigInteger
+
+object NBitsUtils {
+
+  /**
+    * <p>The "compact" format is a representation of a whole number N using an unsigned 32 bit number similar to a
+    * floating point format. The most significant 8 bits are the unsigned exponent of base 256. This exponent can
+    * be thought of as "number of bytes of N". The lower 23 bits are the mantissa. Bit number 24 (0x800000) represents
+    * the sign of N. Therefore, N = (-1^sign) * mantissa * 256^(exponent-3).</p>
+    *
+    * <p>Satoshi's original implementation used BN_bn2mpi() and BN_mpi2bn(). MPI uses the most significant bit of the
+    * first byte as sign. Thus 0x1234560000 is compact 0x05123456 and 0xc0de000000 is compact 0x0600c0de. Compact
+    * 0x05c0de00 would be -0x40de000000.</p>
+    *
+    * <p>Bitcoin only uses this "compact" format for encoding difficulty targets, which are unsigned 256bit quantities.
+    * Thus, all the complexities of the sign bit and using base 256 are probably an implementation accident.</p>
+    */
+  def decodeCompactBits(compact: Long): BigInt = {
+    val size: Int = (compact >> 24).toInt & 0xFF
+    val bytes: Array[Byte] = new Array[Byte](4 + size)
+    bytes(3) = size.toByte
+    if (size >= 1) bytes(4) = ((compact >> 16) & 0xFF).toByte
+    if (size >= 2) bytes(5) = ((compact >> 8) & 0xFF).toByte
+    if (size >= 3) bytes(6) = (compact & 0xFF).toByte
+    decodeMPI(bytes)
+  }
+
+  /**
+    * @see Utils#decodeCompactBits(long)
+    */
+  def encodeCompactBits(requiredDifficulty: BigInt): Long = {
+    val value = requiredDifficulty.bigInteger
+    var result: Long = 0L
+    var size: Int = value.toByteArray.length
+    if (size <= 3) {
+      result = value.longValue << 8 * (3 - size)
+    } else {
+      result = value.shiftRight(8 * (size - 3)).longValue
+    }
+    // The 0x00800000 bit denotes the sign.
+    // Thus, if it is already set, divide the mantissa by 256 and increase the exponent.
+    if ((result & 0x00800000L) != 0) {
+      result >>= 8
+      size += 1
+    }
+    result |= size << 24
+    val a: Int = if (value.signum == -1) 0x00800000 else 0
+    result |= a
+    result
+  }
+
+
+  /** Parse 4 bytes from the byte array (starting at the offset) as unsigned 32-bit integer in big endian format. */
+  def readUint32BE(bytes: Array[Byte]): Long = ((bytes(0) & 0xffL) << 24) | ((bytes(1) & 0xffL) << 16) | ((bytes(2) & 0xffL) << 8) | (bytes(3) & 0xffL)
+
+  /**
+    * MPI encoded numbers are produced by the OpenSSL BN_bn2mpi function. They consist of
+    * a 4 byte big endian length field, followed by the stated number of bytes representing
+    * the number in big endian format (with a sign bit).
+    *
+    */
+  private def decodeMPI(mpi: Array[Byte]): BigInteger = {
+
+    val length: Int = readUint32BE(mpi).toInt
+    val buf = new Array[Byte](length)
+    System.arraycopy(mpi, 4, buf, 0, length)
+
+    if (buf.length == 0) {
+      BigInteger.ZERO
+    } else {
+      val isNegative: Boolean = (buf(0) & 0x80) == 0x80
+      if (isNegative) buf(0) = (buf(0) & 0x7f).toByte
+      val result: BigInteger = new BigInteger(buf)
+      if (isNegative) {
+        result.negate
+      } else {
+        result
+      }
+    }
+  }
+
+}

data/shared/src/main/scala/sigma/ast/methods.scala

@@ -2,16 +2,17 @@ package sigma.ast
 
 import org.ergoplatform._
 import org.ergoplatform.validation._
-import sigma._
+import sigma.{VersionContext, _}
 import sigma.ast.SCollection.{SBooleanArray, SBoxArray, SByteArray, SByteArray2, SHeaderArray}
 import sigma.ast.SMethod.{MethodCallIrBuilder, MethodCostFunc, javaMethodOf}
 import sigma.ast.SType.TypeCode
 import sigma.ast.syntax.{SValue, ValueOps}
 import sigma.data.OverloadHack.Overloaded1
-import sigma.data.{DataValueComparer, KeyValueColl, Nullable, RType, SigmaConstants}
+import sigma.data.{CBigInt, DataValueComparer, KeyValueColl, Nullable, RType, SigmaConstants}
 import sigma.eval.{CostDetails, ErgoTreeEvaluator, TracedCost}
 import sigma.reflection.RClass
 import sigma.serialization.CoreByteWriter.ArgInfo
+import sigma.util.NBitsUtils
 import sigma.utils.SparseArrayContainer
 
 import scala.annotation.unused
@@ -309,13 +310,6 @@ case object SBigIntMethods extends SNumericTypeMethods {
   /** Type for which this container defines methods. */
   override def ownerType: SMonoType = SBigInt
 
-  final val ToNBitsCostInfo = OperationCostInfo(
-    FixedCost(JitCost(5)), NamedDesc("NBitsMethodCall"))
-
-  //id = 8 to make it after toBits
-  val ToNBits = SMethod(this, "nbits", SFunc(this.ownerType, SLong), 8, ToNBitsCostInfo.costKind)
-                  .withInfo(ModQ, "Encode this big integer value as NBits")
-
   /** The following `modQ` methods are not fully implemented in v4.x and this descriptors.
     * This descritors are remain here in the code and are waiting for full implementation
     * is upcoming soft-forks at which point the cost parameters should be calculated and
@@ -333,7 +327,7 @@ case object SBigIntMethods extends SNumericTypeMethods {
 
   protected override def getMethods(): Seq[SMethod]  = {
     if (VersionContext.current.isV6SoftForkActivated) {
-      super.getMethods() ++ Seq(ToNBits)
+      super.getMethods()
       //    ModQMethod,
       //    PlusModQMethod,
       //    MinusModQMethod,
@@ -343,14 +337,6 @@ case object SBigIntMethods extends SNumericTypeMethods {
       super.getMethods()
     }
   }
-
-  /**
-    *
-    */
-  def nbits_eval(mc: MethodCall, bi: sigma.BigInt)(implicit E: ErgoTreeEvaluator): Long = {
-    ???
-  }
-
 }
 
 /** Methods of type `String`. */
@@ -1519,9 +1505,50 @@ case object SGlobalMethods extends MonoTypeMethods {
     Xor.xorWithCosting(ls, rs)
   }
 
-  protected override def getMethods() = super.getMethods() ++ Seq(
-    groupGeneratorMethod,
-    xorMethod
-  )
+  private lazy val EnDecodeNBitsCost = FixedCost(JitCost(5)) // the same cost for nbits encoding and decoding
+
+  lazy val encodeNBitsMethod: SMethod = SMethod(
+    this, "encodeNbits", SFunc(Array(SGlobal, SBigInt), SLong), 3, EnDecodeNBitsCost)
+    .withIRInfo(MethodCallIrBuilder)
+    .withInfo(MethodCall, "Encode big integer number as nbits", ArgInfo("bigInt", "Big integer"))
+
+  lazy val decodeNBitsMethod: SMethod = SMethod(
+    this, "decodeNbits", SFunc(Array(SGlobal, SLong), SBigInt), 4, EnDecodeNBitsCost)
+    .withIRInfo(MethodCallIrBuilder)
+    .withInfo(MethodCall, "Decode nbits-encoded big integer number", ArgInfo("nbits", "NBits-encoded argument"))
+
+  /**
+    * encodeNBits evaluation with costing
+    */
+  def encodeNbits_eval(mc: MethodCall, G: SigmaDslBuilder, bigInt: BigInt)(implicit E: ErgoTreeEvaluator): Long = {
+    E.addFixedCost(EnDecodeNBitsCost, encodeNBitsMethod.opDesc) {
+      NBitsUtils.encodeCompactBits(bigInt.asInstanceOf[CBigInt].wrappedValue)
+    }
+  }
+
+  /**
+    * decodeNBits evaluation with costing
+    */
+  def decodeNbits_eval(mc: MethodCall, G: SigmaDslBuilder, l: Long)(implicit E: ErgoTreeEvaluator): BigInt = {
+    E.addFixedCost(EnDecodeNBitsCost, decodeNBitsMethod.opDesc) {
+      CBigInt(NBitsUtils.decodeCompactBits(l).bigInteger)
+    }
+  }
+
+  protected override def getMethods() = {
+    if (VersionContext.current.isV6SoftForkActivated) {
+      super.getMethods() ++ Seq(
+        groupGeneratorMethod,
+        xorMethod,
+        encodeNBitsMethod,
+        decodeNBitsMethod
+      )
+    } else {
+      super.getMethods() ++ Seq(
+        groupGeneratorMethod,
+        xorMethod
+      )
+    }
+  }
 }
 

data/shared/src/main/scala/sigma/data/CSigmaDslBuilder.scala

@@ -10,6 +10,7 @@ import sigma.crypto.{CryptoConstants, EcPointType, Ecp}
 import sigma.eval.Extensions.EvalCollOps
 import sigma.serialization.{GroupElementSerializer, SigmaSerializer}
 import sigma.util.Extensions.BigIntegerOps
+import sigma.util.NBitsUtils
 import sigma.validation.SigmaValidationSettings
 import sigma.{AvlTree, BigInt, Box, Coll, CollBuilder, GroupElement, SigmaDslBuilder, SigmaProp, VersionContext}
 
@@ -175,6 +176,14 @@ class CSigmaDslBuilder extends SigmaDslBuilder { dsl =>
 
   override def groupGenerator: GroupElement = _generatorElement
 
+  def encodeNbits(bi: BigInt): Long = {
+    NBitsUtils.encodeCompactBits(bi.asInstanceOf[CBigInt].wrappedValue)
+  }
+
+  def decodeNbits(l: Long): BigInt = {
+    CBigInt(NBitsUtils.decodeCompactBits(l).bigInteger)
+  }
+
   /**
     * @return the identity of the Dlog group used in ErgoTree
     */

data/shared/src/main/scala/sigma/eval/ErgoTreeEvaluator.scala

@@ -59,9 +59,9 @@ abstract class ErgoTreeEvaluator {
     * @param opDesc   the operation descriptor to associate the cost with (when costTracingEnabled)
     * @param block    operation executed under the given cost
     */
-  def addFixedCost(costKind: FixedCost, opDesc: OperationDesc)(block: => Unit): Unit
+  def addFixedCost[R](costKind: FixedCost, opDesc: OperationDesc)(block: => R): R
 
-  def addFixedCost(costInfo: OperationCostInfo[FixedCost])(block: => Unit): Unit
+  def addFixedCost[R](costInfo: OperationCostInfo[FixedCost])(block: => R): R
 
   /** Adds the given cost to the `coster`. If tracing is enabled, creates a new cost item
     * with the given operation.

interpreter/shared/src/main/scala/sigmastate/interpreter/CErgoTreeEvaluator.scala

@@ -293,7 +293,7 @@ class CErgoTreeEvaluator(
   }
 
   /** @hotspot don't beautify the code */
-  override def addFixedCost(costKind: FixedCost, opDesc: OperationDesc)(block: => Unit): Unit = {
+  override def addFixedCost[R](costKind: FixedCost, opDesc: OperationDesc)(block: => R): R = {
     var costItem: FixedCostItem = null
     if (settings.costTracingEnabled) {
       costItem = FixedCostItem(opDesc, costKind)
@@ -305,16 +305,17 @@ class CErgoTreeEvaluator(
       }
       val start = System.nanoTime()
       coster.add(costKind.cost)
-      val _ = block
+      val res = block
       val end = System.nanoTime()
       profiler.addCostItem(costItem, end - start)
+      res
     } else {
       coster.add(costKind.cost)
       block
     }
   }
 
-  override def addFixedCost(costInfo: OperationCostInfo[FixedCost])(block: => Unit): Unit = {
+  override def addFixedCost[R](costInfo: OperationCostInfo[FixedCost])(block: => R): R = {
     addFixedCost(costInfo.costKind, costInfo.opDesc)(block)
   }
 

interpreter/shared/src/test/scala/sigma/serialization/MethodCallSerializerSpecification.scala

@@ -24,12 +24,34 @@ class MethodCallSerializerSpecification extends SerializationSpecification {
     roundTripTest(expr)
   }
 
-  property("MethodCall deserialization round trip for BigInt.nbits") {
+  property("MethodCall deserialization round trip for Global.encodeNBits") {
     def code = {
       val bi = BigIntConstant(5)
-      val expr = MethodCall(bi,
-        SBigIntMethods.ToNBits,
-        Vector(),
+      val expr = MethodCall(Global,
+        SGlobalMethods.encodeNBitsMethod,
+        Vector(bi),
+        Map()
+      )
+      roundTripTest(expr)
+    }
+
+    // should be ok
+    VersionContext.withVersions(VersionContext.V6SoftForkVersion, 1) {
+      code
+    }
+
+    an[ValidationException] should be thrownBy (
+      VersionContext.withVersions((VersionContext.V6SoftForkVersion - 1).toByte, 1) {
+        code
+      })
+  }
+
+  property("MethodCall deserialization round trip for Global.decodeNBits") {
+    def code = {
+      val l = LongConstant(5)
+      val expr = MethodCall(Global,
+        SGlobalMethods.decodeNBitsMethod,
+        Vector(l),
         Map()
       )
       roundTripTest(expr)

interpreter/shared/src/test/scala/sigmastate/eval/BasicOpsTests.scala

@@ -2,12 +2,17 @@ package sigmastate.eval
 
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers
+import sigma.ast.{BigIntConstant, ErgoTree, Global, JitCost, MethodCall, SBigIntMethods, SGlobalMethods}
 import sigma.crypto.SecP256K1Group
-import sigma.data.{CSigmaDslBuilder => SigmaDsl, TrivialProp}
+import sigma.data.{CBigInt, TrivialProp}
+import sigma.eval.SigmaDsl
 import sigma.util.Extensions.SigmaBooleanOps
+import sigma.util.NBitsUtils
 
 import java.math.BigInteger
 import sigma.{ContractsTestkit, SigmaProp}
+import sigmastate.interpreter.{CErgoTreeEvaluator, CostAccumulator}
+import sigmastate.interpreter.CErgoTreeEvaluator.DefaultProfiler
 
 import scala.language.implicitConversions
 
@@ -63,4 +68,27 @@ class BasicOpsTests extends AnyFunSuite with ContractsTestkit with Matchers {
     box.creationInfo._1 shouldBe a [Integer]
   }
 
+  /**
+    * Checks BigInt.nbits evaluation for SigmaDSL as well as AST interpreter (MethodCall) layers
+    */
+  test("nbits evaluation") {
+    SigmaDsl.encodeNbits(CBigInt(BigInteger.valueOf(0))) should be
+      (NBitsUtils.encodeCompactBits(0))
+
+    val es = CErgoTreeEvaluator.DefaultEvalSettings
+    val accumulator = new CostAccumulator(
+      initialCost = JitCost(0),
+      costLimit = Some(JitCost.fromBlockCost(es.scriptCostLimitInEvaluator)))
+    val evaluator = new CErgoTreeEvaluator(
+      context = null,
+      constants = ErgoTree.EmptyConstants,
+      coster = accumulator, DefaultProfiler, es)
+
+    val res = MethodCall(Global, SGlobalMethods.encodeNBitsMethod, IndexedSeq(BigIntConstant(BigInteger.valueOf(0))), Map.empty)
+        .evalTo[Long](Map.empty)(evaluator)
+
+    res should be (NBitsUtils.encodeCompactBits(0))
+
+  }
+
 }