fix float shrinking issue

Author: saem

lib/experimental/property_testing.nim

@@ -977,27 +977,30 @@ proc genFloatScalar[T: SomeFloat](min, max: T,
     if choice > 90 and classes.len > 1:
       cls = classes[int((choice - 91) mod uint64(classes.len - 1)) + 1]
 
+    let
+      fHigh = when T is float64: 1.7976931348623157e+308 else: 3.4028235e+38f
+      actualMin = if classify(min) == fcNaN: (if allowInf: T(-Inf) else: T(-fHigh)) else: min
+      actualMax = if classify(max) == fcNaN: (if allowInf: T(Inf) else: T(fHigh)) else: max
+      minOrd = when T is float64: float64ToOrdinal(actualMin) else: int64(float32ToOrdinal(actualMin))
+      maxOrd = when T is float64: float64ToOrdinal(actualMax) else: int64(float32ToOrdinal(actualMax))
+      simplest = if actualMin > 0: actualMin elif actualMax < 0: actualMax else: T(0.0)
+      simplestOrd = when T is float64: float64ToOrdinal(simplest) else: int64(float32ToOrdinal(simplest))
+
+      numBelow = cast[uint64](simplestOrd) - cast[uint64](minOrd)
+      numAbove = cast[uint64](maxOrd) - cast[uint64](simplestOrd)
+      rangeSize = numBelow + numAbove
+      tgtKind = when T is float64: sk8Bytes else: sk4Bytes
+
+    # always pick a rank, so that the same number of bytes written/read
+    # is the same every time
+    let rank = s.chooseRange(0'u64, rangeSize, tgtKind)
+
     case cls
     of 1: return T(-0.0)
     of 2: return T(Inf)
     of 3: return T(-Inf)
     of 4: return T(NaN)
     else:
-      let
-        fHigh = when T is float64: 1.7976931348623157e+308 else: 3.4028235e+38f
-        actualMin = if classify(min) == fcNaN: (if allowInf: T(-Inf) else: T(-fHigh)) else: min
-        actualMax = if classify(max) == fcNaN: (if allowInf: T(Inf) else: T(fHigh)) else: max
-        minOrd = when T is float64: float64ToOrdinal(actualMin) else: int64(float32ToOrdinal(actualMin))
-        maxOrd = when T is float64: float64ToOrdinal(actualMax) else: int64(float32ToOrdinal(actualMax))
-        simplest = if actualMin > 0: actualMin elif actualMax < 0: actualMax else: T(0.0)
-        simplestOrd = when T is float64: float64ToOrdinal(simplest) else: int64(float32ToOrdinal(simplest))
-
-        numBelow = cast[uint64](simplestOrd) - cast[uint64](minOrd)
-        numAbove = cast[uint64](maxOrd) - cast[uint64](simplestOrd)
-        rangeSize = numBelow + numAbove
-        tgtKind = when T is float64: sk8Bytes else: sk4Bytes
-
-      let rank = s.chooseRange(0'u64, rangeSize, tgtKind)
       let uRes = rankToOrdinal(rank, renumerateInt64ToUint64(minOrd),
                                      renumerateInt64ToUint64(maxOrd),
                                      renumerateInt64ToUint64(simplestOrd))

tests/stdlib/experimental/tproperty_testing.nim

@@ -1398,6 +1398,30 @@ suite "Mixed Data Type Generation & Shrinking":
     check si < 76
 
 
+  test "Float stream alignment: (float64, int) shrinking":
+    let prop = forAll(
+      (f: genFloat64(allowInf = true),
+       i: genInt(100, 100)) # Always 100
+    ):
+      # Fail if float is Infinity. 
+      # This forces the shrinker to explore the 'Inf' branch of genFloatScalar.
+      # If the stream is corrupted, reading 'i' will fail or get a wrong value.
+      if f.classify == fcInf: 
+        if i != 100: 
+          return psError # Stream corruption
+        return psFail
+      return psPass
+
+    let res = runProperty(prop, trials=1000, seed=2)
+    # Depending on seed, we might hit Inf.
+    if res.status == psFail:
+      check res.shrunk
+      let (sf, si) = res.shrunkValue.get()
+      checkpoint "Shrunk float: " & $sf & " int: " & $si
+      if sf.classify == fcInf:
+        check si == 100
+
+
 suite "Stress Testing & Final Validation":
 
   test "Chaos Shrinking: Deeply nested structural minimization":