Fix double hashing hash table infinite loop bug (#207)

symboltable/double_hash_table.go

@@ -39,6 +39,26 @@ func isPrime(n int) bool {
 	return true
 }
 
+// gcd computes the greatest common divisor of two numbers.
+// It implements the Euclidean algorithm.
+func gcd(a, b uint64) uint64 {
+	// Ensure a ≥ b
+	a, b = max(a, b), min(a, b)
+
+	/*
+	 * Let the quotient be q and the remainder be r, so that a = b × q + r
+	 * Replace a with b and b with r
+	 * Repeat this until the remainder r becomes 0
+	 * The GCD is the last non-zero remainder
+	 */
+
+	for b != 0 {
+		a, b = b, a%b
+	}
+
+	return a
+}
+
 // doubleHashTable is a hash table with double hashing for conflict resolution.
 type doubleHashTable[K, V any] struct {
 	entries []*hashTableEntry[K, V]
@@ -129,23 +149,29 @@ func (ht *doubleHashTable[K, V]) probe(key K) func() int {
 	// M must be a power of 2
 	M, P := uint64(ht.m), uint64(ht.p)
 	h1 := h & (M - 1) // [0, M-1]
-	var h2 uint64
 
-	var i, next uint64
+	var h2, i, next uint64
 
 	return func() int {
-		switch i {
-		case 0:
+		if i == 0 {
 			next = h1
-		case 1:
-			// The secondary hash value, h₂(k), must satisfy the following properties:
-			//   1. h₂(k) ≠ 0: Ensures progress in probing and prevents infinite loops.
-			//   2. h₂(k) and m are relatively prime: Guarantees the entire table is cycled through for any key.
-			//   3. Independence from the primary hash, h₁(k): Avoids overlapping collision paths.
-			//   4. Efficient computation: Suitable for high-performance hash table operations.
-			h2 = P - (h % P) // [1, p] (p < m)
-			next = (h1 + h2) % M
-		default:
+		} else {
+			if i == 1 {
+				// The secondary hash value, h₂(k), must satisfy the following properties:
+				//   1. h₂(k) ≠ 0: Ensures progress in probing and prevents infinite loops.
+				//   2. h₂(k) and m are coprime: Guarantees the entire table is cycled through for any key.
+				//   3. Independence from the primary hash, h₁(k): Avoids overlapping collision paths.
+				//   4. Efficient computation: Suitable for high-performance hash table operations.
+				h2 = P - (h % P) // [1, p] (p < m)
+
+				// The hash table size (M) and the secondary hash must be relatively prime.
+				// If they share a common divisor greater than 1, the sequence generated by (h1 + i*h2) % M
+				// will only cover a subset of the indices in [0, M-1] and an infinite loop occurs.
+				if gcd(M, h2) != 1 {
+					h2++
+				}
+			}
+
 			next = (h1 + i*h2) % M
 		}
 

symboltable/double_hash_table_test.go

@@ -5,6 +5,7 @@ import (
 
 	. "github.com/moorara/algo/generic"
 	. "github.com/moorara/algo/hash"
+	"github.com/stretchr/testify/assert"
 )
 
 func getDoubleHashTableTests() []symbolTableTest[string, int] {
@@ -87,6 +88,93 @@ func getDoubleHashTableTests() []symbolTableTest[string, int] {
 	return tests
 }
 
+func TestIsPrime(t *testing.T) {
+	tests := []struct {
+		name            string
+		n               int
+		expectedIsPrime bool
+	}{
+		{
+			name:            "Negative",
+			n:               -1,
+			expectedIsPrime: false,
+		},
+		{
+			name:            "Zero",
+			n:               0,
+			expectedIsPrime: false,
+		},
+		{
+			name:            "One",
+			n:               1,
+			expectedIsPrime: false,
+		},
+		{
+			name:            "PrimeLessThan10",
+			n:               7,
+			expectedIsPrime: true,
+		},
+		{
+			name:            "NotPrimeLessThan10",
+			n:               8,
+			expectedIsPrime: false,
+		},
+		{
+			name:            "PrimeLessThan100",
+			n:               97,
+			expectedIsPrime: true,
+		},
+		{
+			name:            "NotPrimeLessThan100",
+			n:               64,
+			expectedIsPrime: false,
+		},
+		{
+			name:            "PrimeLessThan1000",
+			n:               997,
+			expectedIsPrime: true,
+		},
+		{
+			name:            "NotPrimeLessThan1000",
+			n:               666,
+			expectedIsPrime: false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expectedIsPrime, isPrime(tc.n))
+		})
+	}
+}
+
+func TestGCD(t *testing.T) {
+	tests := []struct {
+		name        string
+		a, b        uint64
+		expectedGCD uint64
+	}{
+		{
+			name:        "GCDOne",
+			a:           64,
+			b:           61,
+			expectedGCD: 1,
+		},
+		{
+			name:        "GCDGreaterThanOne",
+			a:           48,
+			b:           64,
+			expectedGCD: 16,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.expectedGCD, gcd(tc.a, tc.b))
+		})
+	}
+}
+
 func TestDoubleHashTable(t *testing.T) {
 	tests := getDoubleHashTableTests()