fix: allow increment of 1 in Monotonic entropy for io.Reader sources

MonotonicEntropy.random rejected a validly drawn value of 0 in its
rejection-sampling loop, making an increment of exactly 1 unreachable
for entropy sources without a fast Int63n path (e.g. crypto/rand.Reader).
For inc == 2 this made the increment fully deterministic (always 2),
contradicting the documented "random number between 1 and inc inclusive"
contract. Restructure the loop to match the crypto/rand.Int pattern it is
adapted from, so 0 is a valid draw and the result is correctly uniform
over [1, inc].

ulid_test.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"io"
 	"math"
+	"math/big"
 	"math/rand"
 	"strings"
 	"testing"
@@ -629,6 +630,65 @@ func TestMonotonic(t *testing.T) {
 	}
 }
 
+// TestMonotonicIncrementRange is a regression test for a bug where
+// MonotonicEntropy.random incorrectly rejected a masked draw of 0, making an
+// increment of 1 unreachable and, for inc == 2, making the increment fully
+// deterministic (always 2). It drives Monotonic with a reader that yields an
+// exact, controlled byte sequence so the test result does not depend on
+// chance draws from a real entropy source.
+//
+// With inc == 2, random() reads one byte at a time and keeps only its low 2
+// bits (see MonotonicEntropy.random). A masked value of 0 must yield an
+// increment of 1, a masked value of 1 must yield an increment of 2, and
+// masked values of 2 or 3 are out of range and must be rejected and redrawn.
+func TestMonotonicIncrementRange(t *testing.T) {
+	t.Parallel()
+
+	// The first 10 bytes seed the initial entropy for the first ULID; their
+	// bit pattern doesn't matter to random(), only that the resulting value
+	// is non-zero and far from overflow. The remaining bytes are consumed by
+	// random(), one at a time (or more, on rejection), to compute each
+	// subsequent increment.
+	seed := append([]byte{0x01}, bytes.Repeat([]byte{0x00}, 9)...)
+	draws := []byte{0x00, 0x01, 0x02, 0x03, 0x00}
+	reader := bytes.NewReader(append(seed, draws...))
+
+	entropy := ulid.Monotonic(reader, 2)
+
+	prev, err := ulid.New(123, entropy)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// draw byte 0x00 -> masked 0 -> accepted -> increment 1
+	// draw byte 0x01 -> masked 1 -> accepted -> increment 2
+	// draw byte 0x02 -> masked 2 -> rejected (>= inc), redraw
+	// draw byte 0x03 -> masked 3 -> rejected (>= inc), redraw
+	// draw byte 0x00 -> masked 0 -> accepted -> increment 1
+	wantDeltas := []uint64{1, 2, 1}
+
+	for i, want := range wantDeltas {
+		next, err := ulid.New(123, entropy)
+		if err != nil {
+			t.Fatalf("draw %d: %v", i, err)
+		}
+
+		if prev.Compare(next) >= 0 {
+			t.Fatalf("draw %d: monotonicity violated: prev=%v next=%v", i, prev, next)
+		}
+
+		delta := new(big.Int).Sub(
+			new(big.Int).SetBytes(next.Entropy()),
+			new(big.Int).SetBytes(prev.Entropy()),
+		).Uint64()
+		if delta != want {
+			t.Fatalf("draw %d: delta = %d, want %d", i, delta, want)
+		}
+
+		prev = next
+	}
+}
+
 func TestMonotonicOverflow(t *testing.T) {
 	t.Parallel()