fix: guard InRange from empty/inverted ranges

pkg/random/random.go

@@ -14,5 +14,8 @@ func String(length int) string {
 }
 
 func InRange(min, max int) int {
+	if max <= min {
+		return min
+	}
 	return rand.Intn(max-min) + min
 }

pkg/random/random_test.go

@@ -0,0 +1,82 @@
+package random
+
+import (
+	"strings"
+	"testing"
+)
+
+const lowerASCIILetters = "abcdefghijklmnopqrstuvwxyz"
+
+func TestStringLength(t *testing.T) {
+	tests := []struct {
+		name string
+		n    int
+	}{
+		{"zero", 0},
+		{"one", 1},
+		{"typical", 12},
+		{"long", 1000},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := String(tc.n); len(got) != tc.n {
+				t.Fatalf("String(%d) length = %d, want %d", tc.n, len(got), tc.n)
+			}
+		})
+	}
+}
+
+func TestStringCharset(t *testing.T) {
+	const n = 4096
+	got := String(n)
+	for _, r := range got {
+		if !strings.ContainsRune(lowerASCIILetters, r) {
+			t.Fatalf("String(%d) produced rune %q outside lowercase a-z", n, r)
+		}
+	}
+}
+
+func TestStringIsRandom(t *testing.T) {
+	const n = 16
+	distinct := 0
+	const samples = 100
+	seen := make(map[string]struct{}, samples)
+	for range samples {
+		s := String(n)
+		if _, ok := seen[s]; !ok {
+			distinct++
+			seen[s] = struct{}{}
+		}
+	}
+	if distinct < samples-1 {
+		t.Fatalf("String(%d) not sufficiently random: %d distinct out of %d", n, distinct, samples)
+	}
+}
+
+func TestInRangeBounds(t *testing.T) {
+	const minVal, maxVal = 13000, 17000
+	const samples = 1000
+	for range samples {
+		got := InRange(minVal, maxVal)
+		if got < minVal || got >= maxVal {
+			t.Fatalf("InRange(%d, %d) = %d, want [%d, %d)", minVal, maxVal, got, minVal, maxVal)
+		}
+	}
+}
+
+func TestInRangeDegenerateReturnsMin(t *testing.T) {
+	tests := []struct {
+		name     string
+		min, max int
+	}{
+		{"equal bounds", 13000, 13000},
+		{"inverted bounds", 17000, 13000},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := InRange(tc.min, tc.max); got != tc.min {
+				t.Fatalf("InRange(%d, %d) = %d, want %d", tc.min, tc.max, got, tc.min)
+			}
+		})
+	}
+}