Fix some more minor bugs and tests

Though I can not see why you would want to.....
But there's no reason you would not be able to

processor/json.go

@@ -7,7 +7,6 @@ import (
 	"modify/processor/jsonpath"
 	"os"
 	"path/filepath"
-	"strings"
 
 	lua "github.com/yuin/gopher-lua"
 )
@@ -68,6 +67,7 @@ func (p *JSONProcessor) ProcessContent(content string, pattern string, luaExpr s
 		return content, 0, 0, nil
 	}
 
+	modCount := 0
 	for _, node := range nodes {
 		log.Printf("Processing node at path: %s with value: %v", node.Path, node.Value)
 
@@ -85,10 +85,14 @@ func (p *JSONProcessor) ProcessContent(content string, pattern string, luaExpr s
 		}
 		log.Printf("Converted node value to Lua: %v", node.Value)
 
+		originalScript := luaExpr
+		fullScript := BuildLuaScript(luaExpr)
+		log.Printf("Original script: %q, Full script: %q", originalScript, fullScript)
+
 		// Execute Lua script
-		log.Printf("Executing Lua script: %s", luaExpr)
-		if err := L.DoString(luaExpr); err != nil {
-			return content, len(nodes), 0, fmt.Errorf("error executing Lua %s: %v", luaExpr, err)
+		log.Printf("Executing Lua script: %q", fullScript)
+		if err := L.DoString(fullScript); err != nil {
+			return content, len(nodes), 0, fmt.Errorf("error executing Lua %q: %v", fullScript, err)
 		}
 		log.Println("Lua script executed successfully.")
 
@@ -99,51 +103,29 @@ func (p *JSONProcessor) ProcessContent(content string, pattern string, luaExpr s
 		}
 		log.Printf("Retrieved modified value from Lua: %v", result)
 
+		modified := false
+		modified = L.GetGlobal("modified").String() == "true"
+		if !modified {
+			log.Printf("No changes made to node at path: %s", node.Path)
+			continue
+		}
+
 		// Apply the modification to the JSON data
 		err = p.updateJSONValue(jsonData, node.Path, result)
 		if err != nil {
 			return content, len(nodes), 0, fmt.Errorf("error updating JSON: %v", err)
 		}
 		log.Printf("Updated JSON at path: %s with new value: %v", node.Path, result)
+		modCount++
 	}
 
 	// Convert the modified JSON back to a string with same formatting
 	var jsonBytes []byte
-	if indent, err := detectJsonIndentation(content); err == nil && indent != "" {
-		// Use detected indentation for output formatting
-		jsonBytes, err = json.MarshalIndent(jsonData, "", indent)
-	} else {
-		// Fall back to standard 2-space indent
 	jsonBytes, err = json.MarshalIndent(jsonData, "", "  ")
+	if err != nil {
+		return content, modCount, matchCount, fmt.Errorf("error marshalling JSON: %v", err)
 	}
-
-	// We changed all the nodes trust me bro
-	return string(jsonBytes), len(nodes), len(nodes), nil
-}
-
-// detectJsonIndentation tries to determine the indentation used in the original JSON
-func detectJsonIndentation(content string) (string, error) {
-	lines := strings.Split(content, "\n")
-	if len(lines) < 2 {
-		return "", fmt.Errorf("not enough lines to detect indentation")
-	}
-
-	// Look for the first indented line
-	for i := 1; i < len(lines); i++ {
-		line := lines[i]
-		trimmed := strings.TrimSpace(line)
-		if trimmed == "" {
-			continue
-		}
-
-		// Calculate leading whitespace
-		indent := line[:len(line)-len(trimmed)]
-		if len(indent) > 0 {
-			return indent, nil
-		}
-	}
-
-	return "", fmt.Errorf("no indentation detected")
+	return string(jsonBytes), modCount, matchCount, nil
 }
 
 // / 	Selects from the root node
@@ -165,6 +147,52 @@ func detectJsonIndentation(content string) (string, error) {
 
 // updateJSONValue updates a value in the JSON structure based on its JSONPath
 func (p *JSONProcessor) updateJSONValue(jsonData interface{}, path string, newValue interface{}) error {
+	// Special handling for root node
+	if path == "$" {
+		// For the root node, we'll copy the value to the jsonData reference
+		// This is a special case since we can't directly replace the interface{} variable
+
+		// We need to handle different types of root elements
+		switch rootValue := newValue.(type) {
+		case map[string]interface{}:
+			// For objects, we need to copy over all keys
+			rootMap, ok := jsonData.(map[string]interface{})
+			if !ok {
+				// If the original wasn't a map, completely replace it with the new map
+				// This is handled by the jsonpath.Set function
+				return jsonpath.Set(jsonData, path, newValue)
+			}
+
+			// Clear the original map
+			for k := range rootMap {
+				delete(rootMap, k)
+			}
+
+			// Copy all keys from the new map
+			for k, v := range rootValue {
+				rootMap[k] = v
+			}
+			return nil
+
+		case []interface{}:
+			// For arrays, we need to handle similarly
+			rootArray, ok := jsonData.([]interface{})
+			if !ok {
+				// If the original wasn't an array, use jsonpath.Set
+				return jsonpath.Set(jsonData, path, newValue)
+			}
+
+			// Clear and recreate the array
+			*&rootArray = rootValue
+			return nil
+
+		default:
+			// For other types, use jsonpath.Set
+			return jsonpath.Set(jsonData, path, newValue)
+		}
+	}
+
+	// For non-root paths, use the regular Set method
 	err := jsonpath.Set(jsonData, path, newValue)
 	if err != nil {
 		return fmt.Errorf("failed to update JSON value at path '%s': %w", path, err)

processor/json_test.go

@@ -133,24 +133,24 @@ func TestJSONProcessor_Process_StringValues(t *testing.T) {
 	t.Logf("Result: %s", result)
 	t.Logf("Match count: %d, Mod count: %d", matchCount, modCount)
 
-	if matchCount != 3 {
-		t.Errorf("Expected 3 matches, got %d", matchCount)
+	if matchCount != 1 {
+		t.Errorf("Expected 1 matches, got %d", matchCount)
 	}
 
-	if modCount != 3 {
-		t.Errorf("Expected 3 modifications, got %d", modCount)
+	if modCount != 1 {
+		t.Errorf("Expected 1 modifications, got %d", modCount)
 	}
 
 	// Check that all expected values are in the result
-	if !strings.Contains(result, `"maxItems": "200"`) {
+	if !strings.Contains(result, `"maxItems": 200`) {
 		t.Errorf("Result missing expected value: maxItems=200")
 	}
 
-	if !strings.Contains(result, `"itemTimeoutSecs": "60"`) {
+	if !strings.Contains(result, `"itemTimeoutSecs": 60`) {
 		t.Errorf("Result missing expected value: itemTimeoutSecs=60")
 	}
 
-	if !strings.Contains(result, `"retryCount": "10"`) {
+	if !strings.Contains(result, `"retryCount": 10`) {
 		t.Errorf("Result missing expected value: retryCount=10")
 	}
 }
@@ -265,13 +265,13 @@ func TestJSONProcessor_NestedModifications(t *testing.T) {
 			"book": [
 				{
 					"category": "reference",
-					"title": "Learn Go in 24 Hours",
-					"price": 13.188
+					"price": 13.188,
+					"title": "Learn Go in 24 Hours"
 				},
 				{
 					"category": "fiction",
-					"title": "The Go Developer",
-					"price": 10.788
+					"price": 10.788,
+					"title": "The Go Developer"
 				}
 			]
 		}
@@ -373,20 +373,20 @@ func TestJSONProcessor_ComplexScript(t *testing.T) {
 	expected := `{
 		"products": [
 			{
+				"discount": 0.1,
 				"name": "Basic Widget",
-				"price": 8.991,
-				"discount": 0.1
+				"price": 8.991
 			},
 			{
+				"discount": 0.05,
 				"name": "Premium Widget",
-				"price": 18.9905,
-				"discount": 0.05
+				"price": 18.9905
 			}
 		]
 	}`
 
 	p := &JSONProcessor{}
-	result, modCount, matchCount, err := p.ProcessContent(content, "$.products[*]", "v.price = v.price * (1 - v.discount)")
+	result, modCount, matchCount, err := p.ProcessContent(content, "$.products[*]", "v.price = round(v.price * (1 - v.discount), 4)")
 
 	if err != nil {
 		t.Fatalf("Error processing content: %v", err)
@@ -419,11 +419,24 @@ func TestJSONProcessor_SpecificItemUpdate(t *testing.T) {
 		]
 	}`
 
-	expected := `{
+	expected := `
+	{
 		"items": [
-			{"id": 1, "name": "Item 1", "stock": 10},
-			{"id": 2, "name": "Item 2", "stock": 15},
-			{"id": 3, "name": "Item 3", "stock": 0}
+			{
+				"id": 1,
+				"name": "Item 1",
+				"stock": 10
+			},
+			{
+				"id": 2,
+				"name": "Item 2",
+				"stock": 15
+			},
+			{
+				"id": 3,
+				"name": "Item 3",
+				"stock": 0
+			}
 		]
 	} `
 
@@ -454,7 +467,9 @@ func TestJSONProcessor_SpecificItemUpdate(t *testing.T) {
 // TestJSONProcessor_RootElementUpdate tests updating the root element
 func TestJSONProcessor_RootElementUpdate(t *testing.T) {
 	content := `{"value": 100}`
-	expected := `{"value": 200}`
+	expected := `{
+		"value": 200
+	}`
 
 	p := &JSONProcessor{}
 	result, modCount, matchCount, err := p.ProcessContent(content, "$.value", "v=v*2")
@@ -471,7 +486,11 @@ func TestJSONProcessor_RootElementUpdate(t *testing.T) {
 		t.Errorf("Expected 1 modification, got %d", modCount)
 	}
 
-	if result != expected {
+	// Normalize whitespace for comparison
+	normalizedResult := normalizeWhitespace(result)
+	normalizedExpected := normalizeWhitespace(expected)
+
+	if normalizedResult != normalizedExpected {
 		t.Errorf("Expected content to be:\n%s\n\nGot:\n%s", expected, result)
 	}
 }
@@ -487,9 +506,9 @@ func TestJSONProcessor_AddNewField(t *testing.T) {
 
 	expected := `{
 		"user": {
-			"name": "John",
 			"age": 30,
-			"email": "john@example.com"
+			"email": "john@example.com",
+			"name": "John"
 		}
 	}`
 
@@ -529,8 +548,8 @@ func TestJSONProcessor_RemoveField(t *testing.T) {
 
 	expected := `{
 		"user": {
-			"name": "John",
-			"email": "john@example.com"
+			"email": "john@example.com",
+			"name": "John"
 		}
 	}`
 
@@ -565,7 +584,12 @@ func TestJSONProcessor_ArrayManipulation(t *testing.T) {
 	}`
 
 	expected := ` {
-		"tags": ["GO", "JSON", "LUA", "testing"]
+		"tags": [
+			"GO",
+			"JSON",
+			"LUA",
+			"testing"
+		]
 	}`
 
 	p := &JSONProcessor{}
@@ -624,28 +648,29 @@ func TestJSONProcessor_ConditionalModification(t *testing.T) {
 	expected := `{
 		"products": [
 			{
+				"inStock": true,
 				"name": "Product A",
-				"price": 9.891,
-				"inStock": true
+				"price": 9.891
 			},
 			{
+				"inStock": false,
 				"name": "Product B",
-				"price": 5.99,
-				"inStock": false
+				"price": 5.99
 			},
 			{
+				"inStock": true,
 				"name": "Product C",
-				"price": 14.391,
-				"inStock": true
+				"price": 14.391
 			}
 		]
 	}`
 
 	p := &JSONProcessor{}
 	result, modCount, matchCount, err := p.ProcessContent(content, "$.products[*]", `
-		if v.inStock then
-			v.price = v.price * 0.9
+		if not v.inStock then
+			return false
 		end
+		v.price = v.price * 0.9
 	`)
 
 	if err != nil {
@@ -695,15 +720,15 @@ func TestJSONProcessor_DeepNesting(t *testing.T) {
 			"departments": {
 				"engineering": {
 					"teams": {
-						"frontend": {
-							"members": 12,
-							"projects": 5,
-							"status": "active"
-						},
 						"backend": {
 							"members": 8,
 							"projects": 3,
 							"status": "active"
+						},
+						"frontend": {
+							"members": 12,
+							"projects": 5,
+							"status": "active"
 						}
 					}
 				}
@@ -763,31 +788,31 @@ func TestJSONProcessor_ComplexTransformation(t *testing.T) {
 
 	expected := `{
 		"order": {
-			"items": [
-				{
-					"product": "Widget A",
-					"quantity": 5,
-					"price": 10.0,
-					"total": 50.0,
-					"discounted_total": 45.0
-				},
-				{
-					"product": "Widget B",
-					"quantity": 3,
-					"price": 15.0,
-					"total": 45.0,
-					"discounted_total": 40.5
-				}
-			],
 			"customer": {
 				"name": "John Smith",
 				"tier": "gold"
 			},
+			"items": [
+				{
+					"discounted_total": 45,
+					"price": 10,
+					"product": "Widget A",
+					"quantity": 5,
+					"total": 50
+				},
+				{
+					"discounted_total": 40.5,
+					"price": 15,
+					"product": "Widget B",
+					"quantity": 3,
+					"total": 45
+				}
+			],
 			"summary": {
-				"total_items": 8,
-				"subtotal": 95.0,
 				"discount": 9.5,
-				"total": 85.5
+				"subtotal": 95,
+				"total": 85.5,
+				"total_items": 8
 			}
 		}
 	}`
@@ -912,16 +937,16 @@ func TestJSONProcessor_RestructuringData(t *testing.T) {
 		"people": {
 			"developers": [
 				{
+					"age": 25,
 					"id": 1,
-					"name": "Alice",
-					"age": 25
+					"name": "Alice"
 				}
 			],
 			"managers": [
 				{
+					"age": 30,
 					"id": 2,
-					"name": "Bob",
-					"age": 30
+					"name": "Bob"
 				}
 			]
 		}
@@ -982,7 +1007,13 @@ func TestJSONProcessor_FilteringArrayElements(t *testing.T) {
 	}`
 
 	expected := `{
-		"numbers": [2, 4, 6, 8, 10]
+		"numbers": [
+			2,
+			4,
+			6,
+			8,
+			10
+		]
 	}`
 
 	p := &JSONProcessor{}
@@ -1017,3 +1048,51 @@ func TestJSONProcessor_FilteringArrayElements(t *testing.T) {
 		t.Errorf("Expected content to be:\n%s\n\nGot:\n%s", expected, result)
 	}
 }
+
+// TestJSONProcessor_RootNodeModification tests modifying the root node directly
+func TestJSONProcessor_RootNodeModification(t *testing.T) {
+	content := `{
+		"name": "original",
+		"value": 100
+	}`
+
+	expected := `{
+		"description": "This is a completely modified root",
+		"name": "modified",
+		"values": [
+			1,
+			2,
+			3
+		]
+	}`
+
+	p := &JSONProcessor{}
+	result, modCount, matchCount, err := p.ProcessContent(content, "$", `
+		-- Completely replace the root node
+		v = {
+			name = "modified",
+			description = "This is a completely modified root",
+			values = {1, 2, 3}
+		}
+	`)
+
+	if err != nil {
+		t.Fatalf("Error processing content: %v", err)
+	}
+
+	if matchCount != 1 {
+		t.Errorf("Expected 1 match, got %d", matchCount)
+	}
+
+	if modCount != 1 {
+		t.Errorf("Expected 1 modification, got %d", modCount)
+	}
+
+	// Normalize whitespace for comparison
+	normalizedResult := normalizeWhitespace(result)
+	normalizedExpected := normalizeWhitespace(expected)
+
+	if normalizedResult != normalizedExpected {
+		t.Errorf("Expected content to be:\n%s\n\nGot:\n%s", expected, result)
+	}
+}

processor/jsonpath/jsonpath.go

@@ -138,10 +138,6 @@ func Set(data interface{}, path string, value interface{}) error {
 		return fmt.Errorf("failed to parse JSONPath %q: %w", path, err)
 	}
 
-	if len(steps) <= 1 {
-		return fmt.Errorf("cannot set root node; the provided path %q is invalid", path)
-	}
-
 	success := false
 	err = setWithPath(data, steps, &success, value, "$", ModifyFirstMode)
 	if err != nil {
@@ -157,10 +153,6 @@ func SetAll(data interface{}, path string, value interface{}) error {
 		return fmt.Errorf("failed to parse JSONPath %q: %w", path, err)
 	}
 
-	if len(steps) <= 1 {
-		return fmt.Errorf("cannot set root node; the provided path %q is invalid", path)
-	}
-
 	success := false
 	err = setWithPath(data, steps, &success, value, "$", ModifyAllMode)
 	if err != nil {
@@ -178,17 +170,20 @@ func setWithPath(node interface{}, steps []JSONStep, success *bool, value interf
 	// Skip root step
 	actualSteps := steps
 	if len(steps) > 0 && steps[0].Type == RootStep {
-		if len(steps) == 1 {
-			return fmt.Errorf("cannot set root node; the provided path %q is invalid", currentPath)
-		}
 		actualSteps = steps[1:]
 	}
 
-	// Process the first step
+	// If we have no steps left, we're setting the root value
 	if len(actualSteps) == 0 {
-		return fmt.Errorf("cannot set root node; no steps provided for path %q", currentPath)
+		// For the root node, we need to handle it differently depending on what's passed in
+		// since we can't directly replace the interface{} variable
+
+		// We'll signal success and let the JSONProcessor handle updating the root
+		*success = true
+		return nil
 	}
 
+	// Process the first step
 	step := actualSteps[0]
 	remainingSteps := actualSteps[1:]
 	isLastStep := len(remainingSteps) == 0

processor/processor.go

@@ -2,7 +2,6 @@ package processor
 
 import (
 	"fmt"
-	"strconv"
 	"strings"
 
 	lua "github.com/yuin/gopher-lua"
@@ -81,6 +80,8 @@ func ToLua(L *lua.LState, data interface{}) (lua.LValue, error) {
 		return lua.LBool(v), nil
 	case float64:
 		return lua.LNumber(v), nil
+	case nil:
+		return lua.LNil, nil
 	default:
 		return nil, fmt.Errorf("unsupported data type: %T", data)
 	}
@@ -89,29 +90,31 @@ func ToLua(L *lua.LState, data interface{}) (lua.LValue, error) {
 // FromLua converts a Lua table to a struct or map recursively
 func FromLua(L *lua.LState, luaValue lua.LValue) (interface{}, error) {
 	switch v := luaValue.(type) {
+	// Well shit...
+	// Tables in lua are both maps and arrays
+	// As arrays they are ordered and as maps, obviously, not
+	// So when we parse them to a go map we fuck up the order for arrays
+	// We have to find a better way....
 	case *lua.LTable:
-		result := make(map[string]interface{})
-		v.ForEach(func(key lua.LValue, value lua.LValue) {
-			result[key.String()], _ = FromLua(L, value)
-		})
-		// This may be a bit wasteful...
-		// Hopefully it won't run often enough to matter
-		isArray := true
-		for key := range result {
-			_, err := strconv.Atoi(key)
+		isArray, err := IsLuaTableArray(L, v)
 		if err != nil {
-				isArray = false
-				break
-			}
+			return nil, err
 		}
 		if isArray {
-			list := make([]interface{}, 0, len(result))
-			for _, value := range result {
-				list = append(list, value)
-			}
-			return list, nil
-		}
+			result := make([]interface{}, 0)
+			v.ForEach(func(key lua.LValue, value lua.LValue) {
+				converted, _ := FromLua(L, value)
+				result = append(result, converted)
+			})
 			return result, nil
+		} else {
+			result := make(map[string]interface{})
+			v.ForEach(func(key lua.LValue, value lua.LValue) {
+				converted, _ := FromLua(L, value)
+				result[key.String()] = converted
+			})
+			return result, nil
+		}
 	case lua.LString:
 		return string(v), nil
 	case lua.LBool:
@@ -123,13 +126,34 @@ func FromLua(L *lua.LState, luaValue lua.LValue) (interface{}, error) {
 	}
 }
 
+func IsLuaTableArray(L *lua.LState, v *lua.LTable) (bool, error) {
+	L.SetGlobal("table_to_check", v)
+
+	// Use our predefined helper function from InitLuaHelpers
+	err := L.DoString(`is_array = isArray(table_to_check)`)
+	if err != nil {
+		return false, fmt.Errorf("error determining if table is array: %w", err)
+	}
+
+	// Check the result of our Lua function
+	isArray := L.GetGlobal("is_array")
+	// LVIsFalse returns true if a given LValue is a nil or false otherwise false.
+	if !lua.LVIsFalse(isArray) {
+		return true, nil
+	}
+	return false, nil
+}
+
 // InitLuaHelpers initializes common Lua helper functions
 func InitLuaHelpers(L *lua.LState) error {
 	helperScript := `
 -- Custom Lua helpers for math operations
 function min(a, b) return math.min(a, b) end
 function max(a, b) return math.max(a, b) end
-function round(x) return math.floor(x + 0.5) end
+function round(x, n)
+	if n == nil then n = 0 end
+	return math.floor(x * 10^n + 0.5) / 10^n
+end
 function floor(x) return math.floor(x) end
 function ceil(x) return math.ceil(x) end
 function upper(s) return string.upper(s) end
@@ -149,6 +173,22 @@ end
 function is_number(str)
     return tonumber(str) ~= nil
 end
+
+function isArray(t)
+    if type(t) ~= "table" then return false end
+    local max = 0
+    local count = 0
+    for k, _ in pairs(t) do
+        if type(k) ~= "number" or k < 1 or math.floor(k) ~= k then
+            return false
+        end
+        max = math.max(max, k)
+        count = count + 1
+    end
+    return max == count
+end
+
+modified = false
 `
 	if err := L.DoString(helperScript); err != nil {
 		return fmt.Errorf("error loading helper functions: %v", err)
@@ -167,8 +207,7 @@ func LimitString(s string, maxLen int) string {
 	return s[:maxLen-3] + "..."
 }
 
-// BuildLuaScript prepares a Lua expression from shorthand notation
-func BuildLuaScript(luaExpr string) string {
+func PrependLuaAssignment(luaExpr string) string {
 	// Auto-prepend v1 for expressions starting with operators
 	if strings.HasPrefix(luaExpr, "*") ||
 		strings.HasPrefix(luaExpr, "/") ||
@@ -186,10 +225,30 @@ func BuildLuaScript(luaExpr string) string {
 	if !strings.Contains(luaExpr, "=") {
 		luaExpr = "v1 = " + luaExpr
 	}
-
 	return luaExpr
 }
 
+// BuildLuaScript prepares a Lua expression from shorthand notation
+func BuildLuaScript(luaExpr string) string {
+	luaExpr = PrependLuaAssignment(luaExpr)
+
+	// This allows the user to specify whether or not they modified a value
+	// If they do nothing we assume they did modify (no return at all)
+	// If they return before our return then they themselves specify what they did
+	// If nothing is returned lua assumes nil
+	// So we can say our value was modified if the return value is either nil or true
+	// If the return value is false then the user wants to keep the original
+	fullScript := fmt.Sprintf(`
+		function run()
+			%s
+		end
+		local res = run()
+		modified = res == nil or res
+	`, luaExpr)
+
+	return fullScript
+}
+
 // Max returns the maximum of two integers
 func Max(a, b int) int {
 	if a > b {

processor/regex_test.go

@@ -35,10 +35,11 @@ func TestBuildLuaScript(t *testing.T) {
 		{"v1 * 2", "v1 = v1 * 2"},
 		{"v1 * v2", "v1 = v1 * v2"},
 		{"v1 / v2", "v1 = v1 / v2"},
+		{"12", "v1 = 12"},
 	}
 
 	for _, c := range cases {
-		result := BuildLuaScript(c.input)
+		result := PrependLuaAssignment(c.input)
 		if result != c.expected {
 			t.Errorf("BuildLuaScript(%q): expected %q, got %q", c.input, c.expected, result)
 		}

processor/xpath/xpath.go

@@ -0,0 +1,98 @@
+package xpath
+
+import "errors"
+
+// XPathStep represents a single step in an XPath expression
+type XPathStep struct {
+	Type      StepType
+	Name      string
+	Predicate *Predicate
+}
+
+// StepType defines the type of XPath step
+type StepType int
+
+const (
+	// RootStep represents the root step (/)
+	RootStep StepType = iota
+	// ChildStep represents a child element step (element)
+	ChildStep
+	// RecursiveDescentStep represents a recursive descent step (//)
+	RecursiveDescentStep
+	// WildcardStep represents a wildcard step (*)
+	WildcardStep
+	// PredicateStep represents a predicate condition step ([...])
+	PredicateStep
+)
+
+// PredicateType defines the type of XPath predicate
+type PredicateType int
+
+const (
+	// IndexPredicate represents an index predicate [n]
+	IndexPredicate PredicateType = iota
+	// LastPredicate represents a last() function predicate
+	LastPredicate
+	// LastMinusPredicate represents a last()-n predicate
+	LastMinusPredicate
+	// PositionPredicate represents position()-based predicates
+	PositionPredicate
+	// AttributeExistsPredicate represents [@attr] predicate
+	AttributeExistsPredicate
+	// AttributeEqualsPredicate represents [@attr='value'] predicate
+	AttributeEqualsPredicate
+	// ComparisonPredicate represents element comparison predicates
+	ComparisonPredicate
+)
+
+// Predicate represents a condition in XPath
+type Predicate struct {
+	Type       PredicateType
+	Index      int
+	Offset     int
+	Attribute  string
+	Value      string
+	Expression string
+}
+
+// XMLNode represents a node in the result set with its value and path
+type XMLNode struct {
+	Value interface{}
+	Path  string
+}
+
+// ParseXPath parses an XPath expression into a series of steps
+func ParseXPath(path string) ([]XPathStep, error) {
+	if path == "" {
+		return nil, errors.New("empty path")
+	}
+
+	// This is just a placeholder implementation for the tests
+	// The actual implementation would parse the XPath expression
+	return nil, errors.New("not implemented")
+}
+
+// Get retrieves nodes from XML data using an XPath expression
+func Get(data interface{}, path string) ([]XMLNode, error) {
+	if data == "" {
+		return nil, errors.New("empty XML data")
+	}
+
+	// This is just a placeholder implementation for the tests
+	// The actual implementation would evaluate the XPath against the XML
+	return nil, errors.New("not implemented")
+}
+
+// Set updates a node in the XML data using an XPath expression
+func Set(xmlData string, path string, value interface{}) (string, error) {
+	// This is just a placeholder implementation for the tests
+	// The actual implementation would modify the XML based on the XPath
+	return "", errors.New("not implemented")
+}
+
+// SetAll updates all nodes matching an XPath expression in the XML data
+func SetAll(xmlData string, path string, value interface{}) (string, error) {
+	// This is just a placeholder implementation for the tests
+	// The actual implementation would modify all matching nodes
+	return "", errors.New("not implemented")
+}

processor/xpath/xpath_test.go

@@ -0,0 +1,545 @@
+package xpath
+
+import (
+	"reflect"
+	"testing"
+)
+
+// XML test data as a string for our tests
+var testXML = `
+<store>
+  <book category="fiction">
+    <title lang="en">The Fellowship of the Ring</title>
+    <author>J.R.R. Tolkien</author>
+    <year>1954</year>
+    <price>22.99</price>
+  </book>
+  <book category="fiction">
+    <title lang="en">The Two Towers</title>
+    <author>J.R.R. Tolkien</author>
+    <year>1954</year>
+    <price>23.45</price>
+  </book>
+  <book category="technical">
+    <title lang="en">Learning XML</title>
+    <author>Erik T. Ray</author>
+    <year>2003</year>
+    <price>39.95</price>
+  </book>
+  <bicycle>
+    <color>red</color>
+    <price>199.95</price>
+  </bicycle>
+</store>
+`
+
+func TestParser(t *testing.T) {
+	tests := []struct {
+		path    string
+		steps   []XPathStep
+		wantErr bool
+	}{
+		{
+			path: "/store/bicycle/color",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "bicycle"},
+				{Type: ChildStep, Name: "color"},
+			},
+		},
+		{
+			path: "//price",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: RecursiveDescentStep, Name: "price"},
+			},
+		},
+		{
+			path: "/store/book/*",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: WildcardStep},
+			},
+		},
+		{
+			path: "/store/book[1]/title",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: PredicateStep, Predicate: &Predicate{Type: IndexPredicate, Index: 1}},
+				{Type: ChildStep, Name: "title"},
+			},
+		},
+		{
+			path: "//title[@lang]",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: RecursiveDescentStep, Name: "title"},
+				{Type: PredicateStep, Predicate: &Predicate{Type: AttributeExistsPredicate, Attribute: "lang"}},
+			},
+		},
+		{
+			path: "//title[@lang='en']",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: RecursiveDescentStep, Name: "title"},
+				{Type: PredicateStep, Predicate: &Predicate{
+					Type:      AttributeEqualsPredicate,
+					Attribute: "lang",
+					Value:     "en",
+				}},
+			},
+		},
+		{
+			path: "/store/book[price>35.00]/title",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: PredicateStep, Predicate: &Predicate{
+					Type:       ComparisonPredicate,
+					Expression: "price>35.00",
+				}},
+				{Type: ChildStep, Name: "title"},
+			},
+		},
+		{
+			path: "/store/book[last()]",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: PredicateStep, Predicate: &Predicate{Type: LastPredicate}},
+			},
+		},
+		{
+			path: "/store/book[last()-1]",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: PredicateStep, Predicate: &Predicate{
+					Type:   LastMinusPredicate,
+					Offset: 1,
+				}},
+			},
+		},
+		{
+			path: "/store/book[position()<3]",
+			steps: []XPathStep{
+				{Type: RootStep},
+				{Type: ChildStep, Name: "store"},
+				{Type: ChildStep, Name: "book"},
+				{Type: PredicateStep, Predicate: &Predicate{
+					Type:       PositionPredicate,
+					Expression: "position()<3",
+				}},
+			},
+		},
+		{
+			path:    "invalid/path",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.path, func(t *testing.T) {
+			steps, err := ParseXPath(tt.path)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("ParseXPath() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if !tt.wantErr && !reflect.DeepEqual(steps, tt.steps) {
+				t.Errorf("ParseXPath() steps = %+v, want %+v", steps, tt.steps)
+			}
+		})
+	}
+}
+
+func TestEvaluator(t *testing.T) {
+	tests := []struct {
+		name     string
+		path     string
+		expected []XMLNode
+		error    bool
+	}{
+		{
+			name: "simple_element_access",
+			path: "/store/bicycle/color",
+			expected: []XMLNode{
+				{Value: "red", Path: "/store/bicycle/color"},
+			},
+		},
+		{
+			name: "recursive_element_access",
+			path: "//price",
+			expected: []XMLNode{
+				{Value: "22.99", Path: "/store/book[1]/price"},
+				{Value: "23.45", Path: "/store/book[2]/price"},
+				{Value: "39.95", Path: "/store/book[3]/price"},
+				{Value: "199.95", Path: "/store/bicycle/price"},
+			},
+		},
+		{
+			name: "wildcard_element_access",
+			path: "/store/book[1]/*",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+				{Value: "J.R.R. Tolkien", Path: "/store/book[1]/author"},
+				{Value: "1954", Path: "/store/book[1]/year"},
+				{Value: "22.99", Path: "/store/book[1]/price"},
+			},
+		},
+		{
+			name: "indexed_element_access",
+			path: "/store/book[1]/title",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+			},
+		},
+		{
+			name: "attribute_exists_predicate",
+			path: "//title[@lang]",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+				{Value: "The Two Towers", Path: "/store/book[2]/title"},
+				{Value: "Learning XML", Path: "/store/book[3]/title"},
+			},
+		},
+		{
+			name: "attribute_equals_predicate",
+			path: "//title[@lang='en']",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+				{Value: "The Two Towers", Path: "/store/book[2]/title"},
+				{Value: "Learning XML", Path: "/store/book[3]/title"},
+			},
+		},
+		{
+			name: "value_comparison_predicate",
+			path: "/store/book[price>35.00]/title",
+			expected: []XMLNode{
+				{Value: "Learning XML", Path: "/store/book[3]/title"},
+			},
+		},
+		{
+			name: "last_predicate",
+			path: "/store/book[last()]/title",
+			expected: []XMLNode{
+				{Value: "Learning XML", Path: "/store/book[3]/title"},
+			},
+		},
+		{
+			name: "last_minus_predicate",
+			path: "/store/book[last()-1]/title",
+			expected: []XMLNode{
+				{Value: "The Two Towers", Path: "/store/book[2]/title"},
+			},
+		},
+		{
+			name: "position_predicate",
+			path: "/store/book[position()<3]/title",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+				{Value: "The Two Towers", Path: "/store/book[2]/title"},
+			},
+		},
+		{
+			name:     "all_elements",
+			path:     "//*",
+			expected: []XMLNode{
+				// For brevity, we'll just check the count, not all values
+			},
+		},
+		{
+			name:     "invalid_index",
+			path:     "/store/book[10]/title",
+			expected: []XMLNode{},
+			error:    true,
+		},
+		{
+			name:     "nonexistent_element",
+			path:     "/store/nonexistent",
+			expected: []XMLNode{},
+			error:    true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := Get(testXML, tt.path)
+			if err != nil {
+				if !tt.error {
+					t.Errorf("Get() returned error: %v", err)
+				}
+				return
+			}
+
+			// Special handling for the "//*" test case
+			if tt.path == "//*" {
+				// Just check that we got multiple elements, not the specific count
+				if len(result) < 10 { // We expect at least 10 elements
+					t.Errorf("Expected multiple elements for '//*', got %d", len(result))
+				}
+				return
+			}
+
+			if len(result) != len(tt.expected) {
+				t.Errorf("Expected %d items, got %d", len(tt.expected), len(result))
+				return
+			}
+
+			// Validate both values and paths
+			for i, e := range tt.expected {
+				if i < len(result) {
+					if !reflect.DeepEqual(result[i].Value, e.Value) {
+						t.Errorf("Value at [%d]: got %v, expected %v", i, result[i].Value, e.Value)
+					}
+					if result[i].Path != e.Path {
+						t.Errorf("Path at [%d]: got %s, expected %s", i, result[i].Path, e.Path)
+					}
+				}
+			}
+		})
+	}
+}
+
+func TestEdgeCases(t *testing.T) {
+	t.Run("empty_data", func(t *testing.T) {
+		result, err := Get("", "/store/book")
+		if err == nil {
+			t.Errorf("Expected error for empty data")
+			return
+		}
+		if len(result) > 0 {
+			t.Errorf("Expected empty result, got %v", result)
+		}
+	})
+
+	t.Run("empty_path", func(t *testing.T) {
+		_, err := ParseXPath("")
+		if err == nil {
+			t.Error("Expected error for empty path")
+		}
+	})
+
+	t.Run("invalid_xml", func(t *testing.T) {
+		_, err := Get("<invalid>xml", "/store")
+		if err == nil {
+			t.Error("Expected error for invalid XML")
+		}
+	})
+
+	t.Run("current_node", func(t *testing.T) {
+		result, err := Get(testXML, "/store/book[1]/.")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 1 {
+			t.Errorf("Expected 1 result, got %d", len(result))
+		}
+	})
+
+	t.Run("attributes", func(t *testing.T) {
+		result, err := Get(testXML, "/store/book[1]/title/@lang")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 1 || result[0].Value != "en" {
+			t.Errorf("Expected 'en', got %v", result)
+		}
+	})
+}
+
+func TestGetWithPaths(t *testing.T) {
+	tests := []struct {
+		name     string
+		path     string
+		expected []XMLNode
+	}{
+		{
+			name: "simple_element_access",
+			path: "/store/bicycle/color",
+			expected: []XMLNode{
+				{Value: "red", Path: "/store/bicycle/color"},
+			},
+		},
+		{
+			name: "indexed_element_access",
+			path: "/store/book[1]/title",
+			expected: []XMLNode{
+				{Value: "The Fellowship of the Ring", Path: "/store/book[1]/title"},
+			},
+		},
+		{
+			name: "recursive_element_access",
+			path: "//price",
+			expected: []XMLNode{
+				{Value: "22.99", Path: "/store/book[1]/price"},
+				{Value: "23.45", Path: "/store/book[2]/price"},
+				{Value: "39.95", Path: "/store/book[3]/price"},
+				{Value: "199.95", Path: "/store/bicycle/price"},
+			},
+		},
+		{
+			name: "attribute_access",
+			path: "/store/book[1]/title/@lang",
+			expected: []XMLNode{
+				{Value: "en", Path: "/store/book[1]/title/@lang"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := Get(testXML, tt.path)
+			if err != nil {
+				t.Errorf("Get() returned error: %v", err)
+				return
+			}
+
+			// Check if lengths match
+			if len(result) != len(tt.expected) {
+				t.Errorf("Get() returned %d items, expected %d", len(result), len(tt.expected))
+				return
+			}
+
+			// For each expected item, find its match in the results and verify both value and path
+			for _, expected := range tt.expected {
+				found := false
+				for _, r := range result {
+					// First verify the value matches
+					if reflect.DeepEqual(r.Value, expected.Value) {
+						found = true
+						// Then verify the path matches
+						if r.Path != expected.Path {
+							t.Errorf("Path mismatch for value %v: got %s, expected %s", r.Value, r.Path, expected.Path)
+						}
+						break
+					}
+				}
+				if !found {
+					t.Errorf("Expected node with value %v and path %s not found in results", expected.Value, expected.Path)
+				}
+			}
+		})
+	}
+}
+
+func TestSet(t *testing.T) {
+	t.Run("simple element", func(t *testing.T) {
+		xmlData := `<root><name>John</name></root>`
+		newXML, err := Set(xmlData, "/root/name", "Jane")
+		if err != nil {
+			t.Errorf("Set() returned error: %v", err)
+			return
+		}
+
+		// Verify the change
+		result, err := Get(newXML, "/root/name")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 1 || result[0].Value != "Jane" {
+			t.Errorf("Set() failed: expected name to be 'Jane', got %v", result)
+		}
+	})
+
+	t.Run("attribute", func(t *testing.T) {
+		xmlData := `<root><element id="123"></element></root>`
+		newXML, err := Set(xmlData, "/root/element/@id", "456")
+		if err != nil {
+			t.Errorf("Set() returned error: %v", err)
+			return
+		}
+
+		// Verify the change
+		result, err := Get(newXML, "/root/element/@id")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 1 || result[0].Value != "456" {
+			t.Errorf("Set() failed: expected id to be '456', got %v", result)
+		}
+	})
+
+	t.Run("indexed element", func(t *testing.T) {
+		xmlData := `<root><items><item>first</item><item>second</item></items></root>`
+		newXML, err := Set(xmlData, "/root/items/item[1]", "changed")
+		if err != nil {
+			t.Errorf("Set() returned error: %v", err)
+			return
+		}
+
+		// Verify the change
+		result, err := Get(newXML, "/root/items/item[1]")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 1 || result[0].Value != "changed" {
+			t.Errorf("Set() failed: expected item to be 'changed', got %v", result)
+		}
+	})
+}
+
+func TestSetAll(t *testing.T) {
+	t.Run("multiple elements", func(t *testing.T) {
+		xmlData := `<root><items><item>first</item><item>second</item></items></root>`
+		newXML, err := SetAll(xmlData, "//item", "changed")
+		if err != nil {
+			t.Errorf("SetAll() returned error: %v", err)
+			return
+		}
+
+		// Verify all items are changed
+		result, err := Get(newXML, "//item")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 2 {
+			t.Errorf("Expected 2 results, got %d", len(result))
+			return
+		}
+
+		for i, node := range result {
+			if node.Value != "changed" {
+				t.Errorf("Item %d not changed, got %v", i+1, node.Value)
+			}
+		}
+	})
+
+	t.Run("attributes", func(t *testing.T) {
+		xmlData := `<root><item id="1"/><item id="2"/></root>`
+		newXML, err := SetAll(xmlData, "//item/@id", "new")
+		if err != nil {
+			t.Errorf("SetAll() returned error: %v", err)
+			return
+		}
+
+		// Verify all attributes are changed
+		result, err := Get(newXML, "//item/@id")
+		if err != nil {
+			t.Errorf("Get() returned error: %v", err)
+			return
+		}
+		if len(result) != 2 {
+			t.Errorf("Expected 2 results, got %d", len(result))
+			return
+		}
+
+		for i, node := range result {
+			if node.Value != "new" {
+				t.Errorf("Attribute %d not changed, got %v", i+1, node.Value)
+			}
+		}
+	})
+}