Add and test initRandomBigInt for issue #101

bigints.nimble

@@ -16,7 +16,7 @@ task test, "Test bigints":
     echo "testing " & backend & " backend"
     for gc in ["refc", "arc", "orc"]:
       echo "  using " & gc & " GC"
-      for file in ["tbigints.nim", "tbugs.nim"]:
+      for file in ["trandom.nim", "tbigints.nim", "tbugs.nim"]:
         exec "nim r --hints:off --experimental:strictFuncs --backend:" & backend & " --gc:" & gc & " tests/" & file
       exec "nim doc --hints:off --backend:" & backend & " --gc:" & gc & " src/bigints.nim"
 

src/bigints.nim

@@ -1,6 +1,6 @@
 ## Arbitrary precision integers.
 
-import std/[algorithm, bitops, math, options]
+import std/[algorithm, bitops, math, options, random]
 
 type
   BigInt* = object
@@ -66,6 +66,27 @@ else:
 func initBigInt*(val: BigInt): BigInt =
   result = val
 
+proc initRandomBigInt*(nbits: Natural): BigInt =
+  ## Initializes a standalone BigInt with exactly `nbits` bits.
+  let
+    remainder = nbits mod 32
+    n_limbs = (if remainder == 0: nbits shr 5 else: nbits shr 5 + 1)
+    remainingBits  = (if remainder == 0: 32 else: remainder)
+  result.limbs.setLen(n_limbs)
+
+  # mask ensures only remainingBits bits can be set to 1
+  # Ensures the first bit is set to 1
+  var
+    mask: uint32 = 0xFFFF_FFFF'u32
+    mask2: uint32 = 0x8000_0000'u32
+  if remainingBits != 32:
+    mask = 1'u32 shl remainingBits - 1
+    mask2 = 1'u32 shl (remainingBits-1)
+  for i in 0 ..< result.limbs.len-1:
+    result.limbs[i] = rand(uint32)
+  let word = rand(uint32)
+  result.limbs[result.limbs.len-1] = word and mask or mask2
+
 const
   zero = initBigInt(0)
   one = initBigInt(1)
@@ -1198,3 +1219,4 @@ func powmod*(base, exponent, modulus: BigInt): BigInt =
         result = (result * basePow) mod modulus
       basePow = (basePow * basePow) mod modulus
       exponent = exponent shr 1
+

tests/trandom.nim

@@ -0,0 +1,18 @@
+import bigints
+import random
+
+const
+  zero = initBigInt(0)
+  one = initBigInt(1)
+
+proc main() =
+  block:
+    randomize()
+    # Repeat probabilistic tests
+    for nBits in [29, 32, 1037]:
+      for _ in 1 .. 5:
+        let a: BigInt = initRandomBigInt(nBits)
+        assert (toString(a, 2)).len == nBits
+        doAssert fastLog2(a) == (nBits - 1)
+
+main()