fix(read): prevent panic on negative slice index in a key path

GetValue and Exists validated only the upper bound of a numeric key-path
segment, so a negative index such as "arr.-1" passed the check and indexed
the slice with -1, causing an index-out-of-range panic. Add a lower-bound
check (i < 0) to the []int, []string and []any cases in both functions.

read.go

@@ -63,19 +63,19 @@ func (c *Config) Exists(key string, findByPath ...bool) (ok bool) {
 			}
 		case []int: // is array(is from Set)
 			i, err := strconv.Atoi(k)
-			if err != nil || i >= len(typeData) {
+			if err != nil || i < 0 || i >= len(typeData) {
 				return false
 			}
 			item = typeData[i]
 		case []string: // is array(is from Set)
 			i, err := strconv.Atoi(k)
-			if err != nil || i >= len(typeData) {
+			if err != nil || i < 0 || i >= len(typeData) {
 				return false
 			}
 			item = typeData[i]
 		case []any: // is array(load from file)
 			i, err := strconv.Atoi(k)
-			if err != nil || i >= len(typeData) {
+			if err != nil || i < 0 || i >= len(typeData) {
 				return false
 			}
 			item = typeData[i]
@@ -214,7 +214,7 @@ func (c *Config) GetValue(key string, findByPath ...bool) (value any, ok bool) {
 			i, err := strconv.Atoi(k)
 
 			// check slice index
-			if err != nil || len(typeData) <= i {
+			if err != nil || i < 0 || len(typeData) <= i {
 				ok = false
 				c.addError(err)
 				return
@@ -223,7 +223,7 @@ func (c *Config) GetValue(key string, findByPath ...bool) (value any, ok bool) {
 			item = typeData[i]
 		case []string: // is array(is from Set)
 			i, err := strconv.Atoi(k)
-			if err != nil || len(typeData) <= i {
+			if err != nil || i < 0 || len(typeData) <= i {
 				ok = false
 				c.addError(err)
 				return
@@ -232,7 +232,7 @@ func (c *Config) GetValue(key string, findByPath ...bool) (value any, ok bool) {
 			item = typeData[i]
 		case []any: // is array(load from file)
 			i, err := strconv.Atoi(k)
-			if err != nil || len(typeData) <= i {
+			if err != nil || i < 0 || len(typeData) <= i {
 				ok = false
 				c.addError(err)
 				return

read_test.go

@@ -499,3 +499,24 @@ func TestConfig_Duration_getter(t *testing.T) {
 	is.Eq(7*time.Second, c.Duration("missing", 7*time.Second))
 	is.Eq(time.Duration(0), c.Duration("missing"))
 }
+
+func TestConfig_GetValue_negativeIndex(t *testing.T) {
+	is := assert.New(t)
+
+	c := New("test-neg")
+	err := c.LoadData(map[string]any{
+		"arr": []any{"a", "b", "c"},
+	})
+	is.NoErr(err)
+
+	// A negative slice index must not panic; it is simply not found.
+	is.NotPanics(func() {
+		v, ok := c.GetValue("arr.-1")
+		is.False(ok)
+		is.Nil(v)
+	})
+
+	is.NotPanics(func() {
+		is.False(c.Exists("arr.-1"))
+	})
+}

testdata/issues59.ini

@@ -1,4 +1,4 @@
-; exported at 2025-08-20 14:11:56
+; exported at 2026-09-06 00:43:11
 
 age = 123
 baseKey = value
@@ -9,8 +9,8 @@ multiWords = hello world
 name = app
 
 [map1]
+key = val
 key1 = val1
 key2 = val2
-key = val