// Package gormcol provides GORM model generation with type-safe column descriptors.
//
// This file handles the generation of <Model>Cols variables that provide
// type-safe column references for use in GORM queries.
package gormcol

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"unicode"
)

// fieldInfo holds information about a struct field.
type fieldInfo struct {
	GoName  string // Go struct field name
	ColName string // database column name from gorm tag
}

// structInfo holds information about a parsed Go struct.
type structInfo struct {
	Name       string      // struct name
	Table      string      // table name (derived from struct name using GORM convention)
	TableConst string      // constant name (e.g., "TableNamePsAccess")
	Fields     []fieldInfo // list of fields
	FilePath   string      // source file path
}

// toSnakeCase converts a CamelCase string to snake_case.
// E.g., "PsProductShop" -> "ps_product_shop"
func toSnakeCase(s string) string {
	var result strings.Builder
	for i, r := range s {
		if unicode.IsUpper(r) {
			if i > 0 {
				result.WriteRune('_')
			}
			result.WriteRune(unicode.ToLower(r))
		} else {
			result.WriteRune(r)
		}
	}
	return result.String()
}

// parseGormColumn extracts the column name value from a gorm tag string.
// The tag is expected to be in the format "column:name;otherTag" or similar.
// Returns the column name if found, empty string otherwise.
func parseGormColumn(tag string) string {
	for _, part := range strings.Split(tag, ";") {
		part = strings.TrimSpace(part)
		if strings.HasPrefix(part, "column:") {
			return strings.TrimPrefix(part, "column:")
		}
	}
	return ""
}

// parseModelFile parses a Go model source file and extracts struct information.
// It uses go/ast to parse the file and extract:
//   - Struct name from the type declaration
//   - Table name from the TableName constant
//   - Field names and their database column names from gorm tags
//
// Returns nil if no valid struct is found in the file.
func parseModelFile(path string) (*structInfo, error) {
	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, path, nil, parser.ParseComments)
	if err != nil {
		return nil, err
	}

	var si structInfo
	si.FilePath = path

	for _, decl := range f.Decls {
		gd, ok := decl.(*ast.GenDecl)
		if !ok {
			continue
		}

		if gd.Tok == token.CONST {
			for _, spec := range gd.Specs {
				vs, ok := spec.(*ast.ValueSpec)
				if !ok {
					continue
				}
				for i, name := range vs.Names {
					if strings.HasPrefix(name.Name, "TableName") {
						si.TableConst = name.Name
						if i < len(vs.Values) {
							if bl, ok := vs.Values[i].(*ast.BasicLit); ok {
								si.Table = strings.Trim(bl.Value, "\"")
							}
						}
					}
				}
			}
		}

		if gd.Tok != token.TYPE {
			continue
		}
		for _, spec := range gd.Specs {
			ts, ok := spec.(*ast.TypeSpec)
			if !ok {
				continue
			}
			st, ok := ts.Type.(*ast.StructType)
			if !ok {
				continue
			}

			si.Name = ts.Name.Name

			for _, field := range st.Fields.List {
				if len(field.Names) == 0 || field.Tag == nil {
					continue
				}
				goName := field.Names[0].Name
				tag := strings.Trim(field.Tag.Value, "`")

				var gormTag string
				for _, t := range strings.Split(tag, " ") {
					t = strings.TrimSpace(t)
					if strings.HasPrefix(t, "gorm:") {
						gormTag = strings.TrimPrefix(t, "gorm:")
						gormTag = strings.Trim(gormTag, "\"")
						break
					}
				}

				colName := parseGormColumn(gormTag)
				if colName == "" {
					continue
				}

				si.Fields = append(si.Fields, fieldInfo{
					GoName:  goName,
					ColName: colName,
				})
			}
		}
	}

	if si.Name == "" {
		return nil, nil
	}

	// If no TableName constant was found, derive table name from struct name using GORM convention.
	if si.Table == "" {
		si.Table = toSnakeCase(si.Name)
	}

	return &si, nil
}

// generateColsVarBlock generates the Go source code for a <Model>Cols variable.
// The generated code defines a struct with Field typed members for each
// database column, providing type-safe column references.
func generateColsVarBlock(si *structInfo) string {
	if len(si.Fields) == 0 {
		return ""
	}

	var b strings.Builder
	b.WriteString(fmt.Sprintf("\nvar %sCols = struct {\n", si.Name))
	for _, f := range si.Fields {
		b.WriteString(fmt.Sprintf("\t%s gormcol.Field\n", f.GoName))
	}
	b.WriteString("}{\n")
	for _, f := range si.Fields {
		b.WriteString(fmt.Sprintf("\t%s: gormcol.Field{Column: %q},\n", f.GoName, f.ColName))
	}
	b.WriteString("}\n")
	return b.String()
}

// findGoMod searches upward from startDir for a go.mod file.
func findGoMod(startDir string) (string, error) {
	dir := startDir
	for {
		path := filepath.Join(dir, "go.mod")
		if _, err := os.Stat(path); err == nil {
			return path, nil
		}
		parent := filepath.Dir(dir)
		if parent == dir {
			return "", fmt.Errorf("go.mod not found from %s", startDir)
		}
		dir = parent
	}
}

// readModulePath extracts the module path from a go.mod file.
func readModulePath(goModPath string) (string, error) {
	content, err := os.ReadFile(goModPath)
	if err != nil {
		return "", err
	}
	for _, line := range strings.Split(string(content), "\n") {
		line = strings.TrimSpace(line)
		if strings.HasPrefix(line, "module ") {
			return strings.TrimSpace(strings.TrimPrefix(line, "module ")), nil
		}
	}
	return "", fmt.Errorf("module directive not found in %s", goModPath)
}

// generateCols appends <Model>Cols variables to generated model files.
// It parses each .go file in the output directory, extracts struct fields
// with gorm column tags, and generates type-safe Field descriptors.
func (m *GormGen) generateCols() error {
	dir := m.cfg.OutputDir
	if !strings.HasPrefix(dir, "./") {
		dir = "./" + dir
	}

	absDir, err := filepath.Abs(dir)
	if err != nil {
		return err
	}

	entries, err := os.ReadDir(absDir)
	if err != nil {
		return err
	}

	goModPath, err := findGoMod(absDir)
	if err != nil {
		return err
	}
	modulePath, err := readModulePath(goModPath)
	if err != nil {
		return err
	}
	gormcolImport := fmt.Sprintf("%q", modulePath+"")

	var fileFilter *regexp.Regexp
	if len(m.cfg.SelectedTables) > 0 {
		pattern := "^(" + strings.Join(m.cfg.SelectedTables, "|") + ")$"
		fileFilter, err = regexp.Compile(pattern)
		if err != nil {
			return fmt.Errorf("invalid selected tables pattern: %w", err)
		}
	} else {
		fileFilter, err = regexp.Compile("^" + m.cfg.TableFilter + "$")
		if err != nil {
			return fmt.Errorf("invalid table filter regex %q: %w", m.cfg.TableFilter, err)
		}
	}

	for _, entry := range entries {
		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".go") {
			continue
		}

		// Match files by the table filter regex (strip .go suffix for matching)
		fileBase := strings.TrimSuffix(entry.Name(), ".go")
		if !fileFilter.MatchString(fileBase) {
			continue
		}

		path := filepath.Join(absDir, entry.Name())

		si, err := parseModelFile(path)
		if err != nil {
			fmt.Fprintf(os.Stderr, "warning: skipping cols for %s: %v\n", entry.Name(), err)
			continue
		}
		if si == nil || len(si.Fields) == 0 {
			continue
		}

		content, err := os.ReadFile(path)
		if err != nil {
			return err
		}

		fileContent := string(content)

		if strings.Contains(fileContent, "gormcol.Field{") {
			continue
		}

		if !strings.Contains(fileContent, gormcolImport) {
			if strings.Contains(fileContent, "import (") {
				fileContent = strings.Replace(fileContent, "import (", "import (\n\t"+gormcolImport, 1)
			} else if strings.Contains(fileContent, "package dbmodel") {
				fileContent = strings.Replace(fileContent, "package dbmodel",
					"package dbmodel\n\nimport "+gormcolImport, 1)
			}
		}

		colsBlock := generateColsVarBlock(si)
		fileContent += colsBlock

		if err := os.WriteFile(path, []byte(fileContent), 0644); err != nil {
			return err
		}

		fmt.Printf("Cols: %s\n", entry.Name())
	}

	return nil
}