signed int exp

stint/intops.nim

@@ -184,12 +184,12 @@ func setBit*(a: var StInt, k: Natural) =
 
 func clearBit*(a: var StInt, k: Natural) =
   ## set bit at position `k`
-  ## k = 0..a.bits-1  
+  ## k = 0..a.bits-1
   a.imp.clearBit(k)
 
 func getBit*(a: StInt, k: Natural): bool =
   ## set bit at position `k`
-  ## k = 0..a.bits-1  
+  ## k = 0..a.bits-1
   a.imp.getBit(k)
 
 {.pop.}
@@ -234,10 +234,29 @@ func `+=`*(a: var StInt, b: SomeUnsignedInt) =
 
 {.push raises: [], noinit, gcsafe.}
 
-func pow*(a: StUint, e: Natural): StUint =
+func isOdd(x: Natural): bool =
+  bool(x and 1)
+
+func pow*(a: StInt, e: Natural): StInt =
   ## Compute ``a`` to the power of ``e``,
   ## ``e`` must be non-negative
-  
-func pow*[aBits, eBits](a: StUint[aBits], e: StUint[eBits]): StUint[aBits] =
+  if a.isNegative:
+    let base = a.neg
+    result.imp = base.imp.pow(e)
+    if e.isOdd:
+      result.negate
+  else:
+    result.imp = a.imp.pow(e)
+
+func pow*[aBits, eBits](a: StInt[aBits], e: StInt[eBits]): StInt[aBits] =
   ## Compute ``x`` to the power of ``y``,
-  ## ``x`` must be non-negative
+  ## ``x`` must be non-negative
+  doAssert e.isNegative.not, "exponent must be non-negative"
+
+  if a.isNegative:
+    let base = a.neg
+    result.imp = base.imp.pow(e.imp)
+    if e.isOdd:
+      result.negate
+  else:
+    result.imp = a.imp.pow(e.imp)

tests/test_int_addsub.nim

@@ -92,10 +92,10 @@ template testAddSub(chk, tst: untyped) =
     chkNegation(chk, 32767, -32767, 128)
     # With Nim 1.6, it seems like https://github.com/status-im/nim-stint/issues/92
     # can now happen on 32-bit platforms.
-    when (NimMajor,NimMinor,NimPatch) < (1,6,0):
-      chkNegation(chk, 2147483648, -2147483648, 128)
+    #when (NimMajor,NimMinor,NimPatch) < (1,6,0):
+    chkNegation(chk, 2147483648, -2147483648, 128)
     chkNegation(chk, 2147483647, -2147483647, 128)
-    #chkNegation(chk, 9223372036854775808, -9223372036854775808, 128) # TODO: bug #92
+    # chkNegation(chk, 9223372036854775808, -9223372036854775808, 128) # TODO: bug #92
 
   tst "absolute integer":
     chkAbs(chk, 0, 0, 128)

tests/test_int_exp.nim

@@ -0,0 +1,64 @@
+# Stint
+# Copyright 2018-2023 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+import ../stint, unittest, math, test_helpers
+
+template chkPow(chk: untyped, a, b, c: string, bits: int) =
+  chk pow(fromHex(StInt[bits], a), fromHex(StInt[bits], b)) == fromHex(StInt[bits], c)
+
+template chkPow(chk: untyped, a: string, b: SomeInteger, c: string, bits: int) =
+  chk pow(fromHex(StInt[bits], a), b) == fromHex(StInt[bits], c)
+
+template testExp(chk, tst: untyped) =
+  tst "signed BigInt BigInt Pow":
+    chkPow(chk, "F", "2", "E1", 128)
+    chkPow(chk, "FF", "2", "FE01", 128)
+    chkPow(chk, "FF", "3", "FD02FF", 128)
+    chkPow(chk, "FFF", "3", "FFD002FFF", 128)
+    chkPow(chk, "FFFFF", "3", "ffffd00002fffff", 128)
+
+    chk pow(-10.i128, 2.i128) == 100.i128
+    chk pow(-10.i128, 3.i128) == -1000.i128
+
+  tst "signed BigInt Natural Pow":
+    chkPow(chk, "F", 2, "E1", 128)
+    chkPow(chk, "FF", 2, "FE01", 128)
+    chkPow(chk, "FF", 3, "FD02FF", 128)
+    chkPow(chk, "FFF", 3, "FFD002FFF", 128)
+    chkPow(chk, "FFFFF", 3, "ffffd00002fffff", 128)
+
+    chk pow(-10.i128, 2) == 100.i128
+    chk pow(-10.i128, 3) == -1000.i128
+
+static:
+  testExp(ctCheck, ctTest)
+
+suite "Wider signed int exp coverage":
+  testExp(check, test)
+
+suite "Testing signed exponentiation":
+  test "Simple exponentiation 5^3":
+
+    let
+      a = 5'u64
+      b = 3
+      u = a.stint(64)
+
+    check:
+      cast[uint64](u.pow(b)) == a ^ b
+      cast[uint64](u.pow(b.stint(64))) == a ^ b
+
+  test "12 ^ 34 == 4922235242952026704037113243122008064":
+    # https://www.wolframalpha.com/input/?i=12+%5E+34
+    let
+      a = 12.stint(256)
+      b = 34
+
+    check: a.pow(b) == "4922235242952026704037113243122008064".i256
+    check: a.pow(b.stint(256)) == "4922235242952026704037113243122008064".i256