// Package schema provides JSON Schema generation from Go structs.
// It produces standard JSON Schema as map[string]any, with no provider-specific types.
package schema

import (
	"reflect"
	"strings"
)

// FromStruct generates a JSON Schema object from a Go struct.
// Struct tags used:
//   - `json:"name"` — sets the property name (standard Go JSON convention)
//   - `description:"..."` — sets the property description
//   - `enum:"a,b,c"` — restricts string values to the given set
//
// Pointer fields are treated as optional; non-pointer fields are required.
// Anonymous (embedded) struct fields are flattened into the parent.
func FromStruct(v any) map[string]any {
	t := reflect.TypeOf(v)
	if t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	if t.Kind() != reflect.Struct {
		panic("schema.FromStruct expects a struct or pointer to struct")
	}
	return objectSchema(t)
}

func objectSchema(t reflect.Type) map[string]any {
	properties := map[string]any{}
	var required []string

	for i := 0; i < t.NumField(); i++ {
		field := t.Field(i)

		// Skip unexported fields
		if !field.IsExported() {
			continue
		}

		// Flatten anonymous (embedded) structs
		if field.Anonymous {
			ft := field.Type
			if ft.Kind() == reflect.Ptr {
				ft = ft.Elem()
			}
			if ft.Kind() == reflect.Struct {
				embedded := objectSchema(ft)
				if props, ok := embedded["properties"].(map[string]any); ok {
					for k, v := range props {
						properties[k] = v
					}
				}
				if req, ok := embedded["required"].([]string); ok {
					required = append(required, req...)
				}
			}
			continue
		}

		name := fieldName(field)
		isRequired := true
		ft := field.Type

		if ft.Kind() == reflect.Ptr {
			ft = ft.Elem()
			isRequired = false
		}

		prop := fieldSchema(field, ft)
		properties[name] = prop

		if isRequired {
			required = append(required, name)
		}
	}

	result := map[string]any{
		"type":       "object",
		"properties": properties,
	}
	if len(required) > 0 {
		result["required"] = required
	}
	return result
}

func fieldSchema(field reflect.StructField, ft reflect.Type) map[string]any {
	prop := map[string]any{}

	// Check for enum tag first
	if enumTag, ok := field.Tag.Lookup("enum"); ok {
		vals := parseEnum(enumTag)
		prop["type"] = "string"
		prop["enum"] = vals
		if desc, ok := field.Tag.Lookup("description"); ok {
			prop["description"] = desc
		}
		return prop
	}

	switch ft.Kind() {
	case reflect.String:
		prop["type"] = "string"
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		prop["type"] = "integer"
	case reflect.Float32, reflect.Float64:
		prop["type"] = "number"
	case reflect.Bool:
		prop["type"] = "boolean"
	case reflect.Struct:
		return objectSchema(ft)
	case reflect.Slice:
		prop["type"] = "array"
		elemType := ft.Elem()
		if elemType.Kind() == reflect.Ptr {
			elemType = elemType.Elem()
		}
		prop["items"] = typeSchema(elemType)
	case reflect.Map:
		prop["type"] = "object"
		if ft.Key().Kind() == reflect.String {
			valType := ft.Elem()
			if valType.Kind() == reflect.Ptr {
				valType = valType.Elem()
			}
			prop["additionalProperties"] = typeSchema(valType)
		}
	default:
		prop["type"] = "string" // fallback
	}

	if desc, ok := field.Tag.Lookup("description"); ok {
		prop["description"] = desc
	}

	return prop
}

func typeSchema(t reflect.Type) map[string]any {
	switch t.Kind() {
	case reflect.String:
		return map[string]any{"type": "string"}
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		return map[string]any{"type": "integer"}
	case reflect.Float32, reflect.Float64:
		return map[string]any{"type": "number"}
	case reflect.Bool:
		return map[string]any{"type": "boolean"}
	case reflect.Struct:
		return objectSchema(t)
	case reflect.Slice:
		elemType := t.Elem()
		if elemType.Kind() == reflect.Ptr {
			elemType = elemType.Elem()
		}
		return map[string]any{
			"type":  "array",
			"items": typeSchema(elemType),
		}
	default:
		return map[string]any{"type": "string"}
	}
}

func fieldName(f reflect.StructField) string {
	if tag, ok := f.Tag.Lookup("json"); ok {
		parts := strings.SplitN(tag, ",", 2)
		if parts[0] != "" && parts[0] != "-" {
			return parts[0]
		}
	}
	return f.Name
}

func parseEnum(tag string) []string {
	parts := strings.Split(tag, ",")
	var vals []string
	for _, p := range parts {
		p = strings.TrimSpace(p)
		if p != "" {
			vals = append(vals, p)
		}
	}
	return vals
}