Fix typo in comment and add Evaluate function for hash comparison

Corrected a typo in a comment within `nilsimsa.go`. Introduced a new `Evaluate` function in `evaluate.go` to compute the bitwise difference between two Nilsimsa hash values, ensuring input validation and proper bit counting logic.

evaluate.go

@@ -0,0 +1,35 @@
+package nilsimsa
+
+import (
+	"fmt"
+)
+
+func Evaluate(a, b []byte) (int, error) {
+	// Check if the lengths of the strings are the same
+	if len(a) != len(b) {
+		return 0, fmt.Errorf("byte strings are of different lengths %d and %d", len(a), len(b))
+	}
+	if len(a) != 32 || len(b) != 32 {
+		return 0, fmt.Errorf("input nilsimsa hashes must be 32 bytes each, got %d and %d",
+			len(a), len(b))
+	}
+
+	// Count the differing bits
+	bitDiffCount := 0
+	for i := 0; i < len(a); i++ {
+		// XOR the bytes and count the number of `1` bits
+		xor := a[i] ^ b[i]
+		bitDiffCount += countBits(xor)
+	}
+	return bitDiffCount, nil
+}
+
+// Count the number of `1` bits in a byte
+func countBits(x byte) int {
+	count := 0
+	for x > 0 {
+		count += int(x & 1) // Add the last bit
+		x >>= 1             // Right shift the bits
+	}
+	return count
+}

nilsimsa.go

@@ -127,7 +127,7 @@ type Digest struct {
 // New create a new Nilsimsa hash diget
 func New() hash.Hash {
 	d := new(Digest)
-	// Note that no memory is allocate other than the struct itself. It is better to embedd
+	// Note that no memory is allocate other than the struct itself. It is better to embed
 	// last4Array into the struct itself since it's maximum size is know already
 	//	d.last4  = d.last4Array[:0] //creating the slice by re-slicing last4Array
 	return d