Add tools API and errors package (#4)

* Add tools API and errors package

* Delegate `TextLines` to the `Lines()` method

* Refactor `IndexOutOfBoundsError` and `IllegalArgumentError`

* Refactor `calculateTextLines` to ignore rune (int32)

* Refactor `String()` to drop the empty‐slice guard

* Return -1 instead of 0 on error

* Use `any` type instead of `interface{}`

* Add Copilot prompt file

* Use overflow-safe midpoint calc

* Use `string` instead of `[]rune`

* Address code review suggestions

Author: snelusha

.github/prompts/migration.prompt.md

@@ -0,0 +1,151 @@
+# Task
+
+Convert the selected Java source into idiomatic Go **with minimal semantic drift**.
+
+- Map packages to directories and package names.
+- Map classes (including abstract) and their fields/methods to Go **interface** and **unexported implementation structs**.
+- Preserve logic while adapting to Go naming, encapsulation and error handling.
+- Keep diffs focused: one Java file -> one Go file (small support code if strictly necessary).
+
+## Inputs
+
+- Java file(s) to convert.
+- Any related types the file depends on.
+- Package path target within the Go module.
+
+## Outputs
+
+- A new Go file with idiomatic code.
+
+## Package & File Mapping
+
+1. **One-to-one element mapping**
+    - **Files**: Map each Java source file `<Name>.java` to to one go file, named `lowercase-with-dashes.go` (e.g., `FooBar.java` -> `foo-bar.go`). Be consistent within the package.
+    - **Classes/Interfaces**: For each Java class or interface define a **Go interface** named after the Java type (exported) and an **unexported implementation struct**.
+    - **Methods**: Map each java method to a Go method or function. For overloads, pick distinct names (e.g., `Advance`, `AdvanceN`).
+
+2. **Package structure**
+    - Java: `package com.example.foo.something;`
+    - Go: directory `foo/something` with `package something` in the file.
+
+## Types & Encapsulation
+
+### Classes -> Interface + Impl Struct
+
+- Define an **interface** named exactly after the Java class, e.g. `Foo`.
+- Define a constructor `NewFoo(...) Foo` returning the interface.
+- Define an **unexported** struct `fooImpl` implements the interface. Prefer composition/embedding for reuse.
+
+### Fields -> Struct Fields
+
+- Private/encapsulated state lives in unexported struct fields (e.g., `bar int`).
+- Provide getters/setters as interface methods only if the Java API requires them to be public. Avoid exporting fields directly unless they are immutable configuration.
+
+### Constructors
+
+For each Java public constructor:
+
+```go
+func NewFoo(params) Foo {
+    return &fooImpl{/* initialize fields */}
+}
+```
+
+- If multiple Java constructors exist, use either distinct names (`NewFooWithParams`) **or** the functional options pattern for optional params (avoid overloading).
+
+### Methods & Receivers
+
+- **Non-mutating** -> value receiver if the struct is small and the method is read-only.
+- **Mutating** -> pointer receiver.
+- Prefer returning `(T, error)` over panicking; translate Java exceptions to `error` values when they cross API boundaries.
+
+### Abstract classes
+
+```go
+type Foo interface {
+    // abstract methods + requires accessors
+}
+
+type fooBase struct {/* shared fields */}
+
+func (b fooBase) GetX() T { return b.x }
+
+type fooImpl struct {
+    fooBase
+}
+func NewFoo(params) Foo {
+    return &fooImpl{
+        fooBase: fooBase{/* initialize shared fields */},
+    }
+}
+```
+
+### Method overloading
+
+Java:
+
+```java
+void advance();
+void advance(int n);
+```
+
+Go:
+
+```go
+func (r *readerImpl) Advance() {}
+func (r *readerImpl) AdvanceN(n int) {}
+```
+
+### Equality / Hashing
+
+- If Java only overrides `equals()`/`hashCode()`, in Go prefer:
+  - Use direct `==` for comparable structs; or
+  - Provide an explicit **lookup key**:
+
+    ```go
+    type FooLookupKey struct { Bar int; Baz string }
+    func (f *fooImpl) FooLookupKey() FooLookupKey { return FooLookupKey{f.bar, f.baz} }
+    ```
+
+## Naming & Comments
+
+- **Packages:** short, lowercase, single word (`something`).  
+- **Exports:** capitalize to export. Keep names concise and avoid stutter (prefer `something.Reader` with type name `Reader`).
+
+## Error Handling
+
+- Always return `(T, error)` for fallible operations. Don't use `panic` for normal control flow.
+- Wrap lower-level errors with context using `fmt.Errorf("op: %w", err)` so callers can use `errors.Is/As`.
+- Match errors with `errors.Is` (sentinels) or `errors.As` (typed errors with additional context).
+
+### Java exceptions -> Go typed errors
+
+- Define Java-specific exceptions as typed errros in `common/errors/errors.go` (package `errors`).
+
+```go
+package errors
+
+import "fmt"
+
+type IndexOutOfBoundsError struct {
+	index  int
+	length int
+}
+
+func (e IndexOutOfBoundsError) Error() string {
+	return fmt.Sprintf("Index %d out of bounds for length %d", e.index, e.length)
+}
+
+func (e IndexOutOfBoundsError) GetIndex() int  { return e.index }
+func (e IndexOutOfBoundsError) GetLength() int { return e.length }
+```
+
+## Generics
+
+- Map Java generics to Go generics when needed.
+
+## Guardrails & Non goals
+
+- **Do no** add file headers or license comments.
+- **Do not** introduce new public APIs unless requires by the Java surface.
+- **Do not** add comments unless the Java source has them.

common/errors/errors.go

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2025, WSO2 LLC. (http://www.wso2.com).
+ *
+ * WSO2 LLC. licenses this file to you under the Apache License,
+ * Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package errors
+
+import "fmt"
+
+type IndexOutOfBoundsError struct {
+	index  int
+	length int
+}
+
+func (e IndexOutOfBoundsError) Error() string {
+	return fmt.Sprintf("Index %d out of bounds for length %d", e.index, e.length)
+}
+
+func (e IndexOutOfBoundsError) GetIndex() int {
+	return e.index
+}
+
+func (e IndexOutOfBoundsError) GetLength() int {
+	return e.length
+}
+
+func NewIndexOutOfBoundsError(index, length int) *IndexOutOfBoundsError {
+	return &IndexOutOfBoundsError{
+		index:  index,
+		length: length,
+	}
+}
+
+type IllegalArgumentError struct {
+	argument any
+}
+
+func (e IllegalArgumentError) Error() string {
+	return fmt.Sprintf("Illegal argument: %v", e.argument)
+}
+
+func (e IllegalArgumentError) GetArgument() any {
+	return e.argument
+}
+
+func NewIllegalArgumentError(argument any) *IllegalArgumentError {
+	return &IllegalArgumentError{
+		argument: argument,
+	}
+}

tools/diagnostics/default-diagnostic.go

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2025, WSO2 LLC. (http://www.wso2.com).
+ *
+ * WSO2 LLC. licenses this file to you under the Apache License,
+ * Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package diagnostics
+
+import (
+	"fmt"
+
+	"ballerina-lang-go/tools/text"
+)
+
+// DefaultDiagnostic is an internal implementation of the Diagnostic interface that is used by the DiagnosticFactory
+// to create diagnostics.
+type DefaultDiagnostic interface {
+	Diagnostic
+}
+
+type defaultDiagnosticImpl struct {
+	diagnosticBase
+	diagnosticInfo DiagnosticInfo
+	location       Location
+	properties     []DiagnosticProperty[any]
+	message        string
+}
+
+func NewDefaultDiagnostic(diagnosticInfo DiagnosticInfo, location Location, properties []DiagnosticProperty[any], args ...any) DefaultDiagnostic {
+	message := formatMessage(diagnosticInfo.MessageFormat(), args...)
+	return &defaultDiagnosticImpl{
+		diagnosticInfo: diagnosticInfo,
+		location:       location,
+		properties:     properties,
+		message:        message,
+	}
+}
+
+func (dd *defaultDiagnosticImpl) Location() Location {
+	return dd.location
+}
+
+func (dd *defaultDiagnosticImpl) DiagnosticInfo() DiagnosticInfo {
+	return dd.diagnosticInfo
+}
+
+func (dd *defaultDiagnosticImpl) Message() string {
+	return dd.message
+}
+
+func (dd *defaultDiagnosticImpl) Properties() []DiagnosticProperty[any] {
+	return dd.properties
+}
+
+func (dd *defaultDiagnosticImpl) String() string {
+	lineRange := dd.location.LineRange()
+	filePath := lineRange.FileName()
+
+	startLine := lineRange.StartLine()
+	endLine := lineRange.EndLine()
+
+	oneBasedStartLine := text.LinePositionFromLineAndOffset(startLine.Line()+1, startLine.Offset()+1)
+	oneBasedEndLine := text.LinePositionFromLineAndOffset(endLine.Line()+1, endLine.Offset()+1)
+	oneBasedLineRange := text.LineRangeFromLinePositions(filePath, oneBasedStartLine, oneBasedEndLine)
+
+	return fmt.Sprintf("%s [%s:%s] %s",
+		dd.diagnosticInfo.Severity().String(),
+		filePath,
+		oneBasedLineRange.String(),
+		dd.Message())
+}
+
+func formatMessage(format string, args ...any) string {
+	if len(args) == 0 {
+		return format
+	}
+	return fmt.Sprintf(format, args...)
+}

tools/diagnostics/diagnostic-code.go

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2025, WSO2 LLC. (http://www.wso2.com).
+ *
+ * WSO2 LLC. licenses this file to you under the Apache License,
+ * Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package diagnostics
+
+// DiagnosticCode represents a diagnostic code.
+// Diagnostic code uniquely identifies a diagnostic.
+type DiagnosticCode interface {
+	Severity() DiagnosticSeverity
+	DiagnosticId() string
+	MessageKey() string
+}

tools/diagnostics/diagnostic-factory.go

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2025, WSO2 LLC. (http://www.wso2.com).
+ *
+ * WSO2 LLC. licenses this file to you under the Apache License,
+ * Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package diagnostics
+
+// CreateDiagnostic creates a Diagnostic instance from the given details.
+//
+// Parameters:
+//   - diagnosticInfo: static diagnostic information
+//   - location: the location of the diagnostic
+//   - args: arguments to diagnostic message format
+//
+// Returns a Diagnostic instance.
+func CreateDiagnostic(diagnosticInfo DiagnosticInfo, location Location, args ...any) Diagnostic {
+	return NewDefaultDiagnostic(diagnosticInfo, location, nil, args...)
+}
+
+// CreateDiagnosticWithProperties creates a Diagnostic instance from the given details.
+//
+// Parameters:
+//   - diagnosticInfo: static diagnostic information
+//   - location: the location of the diagnostic
+//   - properties: properties associated with the diagnostic
+//   - args: arguments to diagnostic message format
+//
+// Returns a Diagnostic instance.
+func CreateDiagnosticWithProperties(diagnosticInfo DiagnosticInfo, location Location, properties []DiagnosticProperty[any], args ...any) Diagnostic {
+	return NewDefaultDiagnostic(diagnosticInfo, location, properties, args...)
+}