// Package releaseidentity resolves and derives the identity of one Go module
// release within a repository.
package releaseidentity

import (
	"context"
	"fmt"
	"os"
	"path"
	"path/filepath"
	"strings"

	"gitea.maximumdirect.net/eric/ReleaseKit/internal/git"
	"gitea.maximumdirect.net/eric/ReleaseKit/internal/versioncontract"
	"golang.org/x/mod/modfile"
	"golang.org/x/mod/module"
)

// Selection identifies the normalized repository and selected module
// directories. ModuleDir is an absolute path; ModuleSubdir is its canonical
// repository-relative slash path, with "." representing the root.
type Selection struct {
	RepositoryDir string
	ModuleDir     string
	ModuleSubdir  string
}

// Identity is the release identity derived from one selected module.
type Identity struct {
	Selection
	ModulePath string
	Version    string
	Tag        string
	NotePath   string
}

// ResolveSelection normalizes a repository-relative module selection and
// confirms its final filesystem location remains inside the repository.
func ResolveSelection(repositoryDir, moduleDir string) (Selection, error) {
	if moduleDir == "" {
		moduleDir = "."
	}
	if filepath.IsAbs(moduleDir) {
		return Selection{}, fmt.Errorf("module directory must be repository-relative: %q", moduleDir)
	}
	repositoryAbs, err := filepath.Abs(repositoryDir)
	if err != nil {
		return Selection{}, fmt.Errorf("resolve repository directory: %w", err)
	}
	repositoryAbs, err = filepath.EvalSymlinks(repositoryAbs)
	if err != nil {
		return Selection{}, fmt.Errorf("resolve repository directory %q: %w", repositoryDir, err)
	}
	moduleAbs, err := filepath.Abs(filepath.Join(repositoryAbs, moduleDir))
	if err != nil {
		return Selection{}, fmt.Errorf("resolve module directory %q: %w", moduleDir, err)
	}
	moduleAbs, err = filepath.EvalSymlinks(moduleAbs)
	if err != nil {
		return Selection{}, fmt.Errorf("resolve module directory %q: %w", moduleDir, err)
	}
	relative, err := filepath.Rel(repositoryAbs, moduleAbs)
	if err != nil {
		return Selection{}, fmt.Errorf("make module directory relative to repository: %w", err)
	}
	if relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) || filepath.IsAbs(relative) {
		return Selection{}, fmt.Errorf("module directory %q resolves outside repository %q", moduleDir, repositoryDir)
	}
	if relative == "." {
		return Selection{RepositoryDir: repositoryAbs, ModuleDir: moduleAbs, ModuleSubdir: "."}, nil
	}
	return Selection{RepositoryDir: repositoryAbs, ModuleDir: moduleAbs, ModuleSubdir: filepath.ToSlash(relative)}, nil
}

// FromCommitted derives identity from the selected module declaration at a
// captured commit. The selected module must contain its own go.mod.
func FromCommitted(ctx context.Context, repositoryDir, moduleDir, version, commit string) (Identity, error) {
	selection, err := ResolveSelection(repositoryDir, moduleDir)
	if err != nil {
		return Identity{}, err
	}
	return FromCommittedSelection(ctx, selection, version, commit)
}

// FromCommittedSelection derives identity from a previously resolved module
// selection and the selected module declaration at a captured commit.
func FromCommittedSelection(ctx context.Context, selection Selection, version, commit string) (Identity, error) {
	if err := ctx.Err(); err != nil {
		return Identity{}, fmt.Errorf("resolve committed release identity: %w", err)
	}
	declaration, err := git.ReadCommitFile(ctx, selection.RepositoryDir, commit, moduleFilePath(selection.ModuleSubdir))
	if err != nil {
		return Identity{}, fmt.Errorf("read selected module declaration at %s: %w", commit, err)
	}
	return Derive(selection, declaration, version)
}

// FromWorkingTree derives identity from the selected module declaration in the
// working tree. The selected module must contain its own go.mod.
func FromWorkingTree(ctx context.Context, repositoryDir, moduleDir, version string) (Identity, error) {
	selection, err := ResolveSelection(repositoryDir, moduleDir)
	if err != nil {
		return Identity{}, err
	}
	if err := ctx.Err(); err != nil {
		return Identity{}, fmt.Errorf("resolve working-tree release identity: %w", err)
	}
	declarationPath := filepath.Join(selection.ModuleDir, "go.mod")
	declaration, err := os.ReadFile(declarationPath)
	if err != nil {
		return Identity{}, fmt.Errorf("read selected module declaration %q: %w", declarationPath, err)
	}
	return Derive(selection, declaration, version)
}

// Derive applies Go module path-major rules and derives the standard full tag
// and module-local release-note path from a declaration and selection.
func Derive(selection Selection, declaration []byte, version string) (Identity, error) {
	if selection.RepositoryDir == "" || selection.ModuleDir == "" || selection.ModuleSubdir == "" {
		return Identity{}, fmt.Errorf("incomplete module selection")
	}
	modulePath := modfile.ModulePath(declaration)
	if modulePath == "" {
		return Identity{}, fmt.Errorf("selected module declaration has no valid module directive")
	}
	if err := module.CheckPath(modulePath); err != nil {
		return Identity{}, fmt.Errorf("invalid module path %q: %w", modulePath, err)
	}
	_, pathMajor, ok := module.SplitPathVersion(modulePath)
	if !ok {
		return Identity{}, fmt.Errorf("module path %q has an invalid major-version suffix", modulePath)
	}
	if !versioncontract.ValidStableVersion(version) {
		return Identity{}, fmt.Errorf("invalid stable release version %q", version)
	}
	if err := module.CheckPathMajor(version, pathMajor); err != nil {
		return Identity{}, fmt.Errorf("version %q is incompatible with module path %q: %w", version, modulePath, err)
	}

	tagPrefix := selection.ModuleSubdir
	if strings.HasPrefix(pathMajor, "/") && tagPrefix != "." && path.Base(tagPrefix) == strings.TrimPrefix(pathMajor, "/") {
		tagPrefix = path.Dir(tagPrefix)
	}
	tag := version
	if tagPrefix != "." {
		tag = path.Join(tagPrefix, version)
	}
	notePath := path.Join(selection.ModuleSubdir, "docs", "releases", version+".md")
	return Identity{
		Selection:  selection,
		ModulePath: modulePath,
		Version:    version,
		Tag:        tag,
		NotePath:   notePath,
	}, nil
}

func moduleFilePath(moduleSubdir string) string {
	if moduleSubdir == "." {
		return "go.mod"
	}
	return path.Join(moduleSubdir, "go.mod")
}