package kernel

import (
	"context"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path"
	"strings"

	"cos.googlesource.com/cos/tools.git/src/pkg/cos"
	"cos.googlesource.com/cos/tools.git/src/pkg/git"
	"github.com/golang/glog"
)

const (
	AppliedPatchesFile = "cos-kernel-applied-patches.txt"
	PatchBaseFile      = "cos-kernel-patch-base.txt"
)

// Prepare sets up the local kernel directory for compilation.
//
// This will first download the compiler toolchain corresponding to the
// provided kernel variant config, apply any patches from the variant config
// which are not yet applied, and prepare the .config file using the base
// kernel config and fragments from the variant config.
func Prepare(ctx context.Context, downloader cos.ArtifactsDownloader, conf KernelVariantConfig) error {
	if err := InstallCompilerToolchainAtomic(ctx, downloader, conf.Toolchain); err != nil {
		return fmt.Errorf("could not download compiler toolchain: %v", err)
	}

	if len(conf.Patches) > 0 {
		patchRoots := []string{".", "google/patches"}
		patchPaths, err := resolvePatches(conf.Patches, patchRoots)
		if err != nil {
			return fmt.Errorf("could not resolve all patches: %v", err)
		}
		if err := applyUnappliedPatches(patchPaths, AppliedPatchesFile); err != nil {
			return fmt.Errorf("could not apply the provided patches: %v", err)
		}
	}

	if len(conf.Configs) == 0 {
		return fmt.Errorf("must provide at least one config to prepare")
	}

	if err := Kmake(conf, conf.Configs); err != nil {
		return fmt.Errorf("failed to prepare config %s: %v", conf.Configs[0], err)
	}

	return nil
}

// resolvePatches gets the paths to all patches matching a provided list of paths.
//
// Each provided path may be the path to a single patch or a directory containing patches.
//
// This will check for paths relative to each provided root directory in order, returning
// the patches from the first path that is found.
func resolvePatches(paths, roots []string) ([]string, error) {
	var patches []string
	for _, p := range paths {
		found := false
		for _, root := range roots {
			rootRelativePath := path.Join(root, p)
			pathPatches, err := resolvePathPatches(rootRelativePath)
			if err == nil {
				patches = append(patches, pathPatches...)
				found = true
				break
			}

			if !errors.Is(err, fs.ErrNotExist) {
				return nil, fmt.Errorf("error statting patch path %s: %v", p, err)
			}
		}

		if !found {
			return nil, fmt.Errorf("could not find patch path %s relative to any root: %v", p, roots)
		}
	}

	return patches, nil
}

// resolvePathPatches gets the paths to all patches matching a path.
//
// The path can either be the path to a single patch or the name of a
// directory containing patches.
func resolvePathPatches(p string) ([]string, error) {
	info, err := os.Stat(p)
	if err != nil {
		return nil, err
	}

	var patches []string
	if info.IsDir() {
		entries, err := os.ReadDir(p)
		if err != nil {
			return nil, fmt.Errorf("could not read patch dir %s: %v", p, err)
		}
		for _, entry := range entries {
			patches = append(patches, path.Join(p, entry.Name()))
		}
	} else {
		patches = append(patches, p)
	}
	return patches, nil
}

// applyUnappliedPatches applies all the patches from which have not been applied yet from a list of paths.
//
// Each path can be the path to a single patch or a directory containing patches.
//
// If the repo in the current directory isn't clean, this will exit immediatly.
//
// Before applying patches, this will check for an appliedPatchesFile and read its contents. Any
// patch listed in that file will not be reapplied.
// When this function is done writing patches, it will append the list of patches it applied to
// the appliedPatchesFile, creating it if it doesn't already exist.
func applyUnappliedPatches(patches []string, appliedPatchesFile string) error {
	clean, err := git.IsClean()
	if err != nil {
		return fmt.Errorf("refusing to apply patches; failed to check if git repo was clean: %v", err)
	}
	if !clean {
		return fmt.Errorf("git repo is unclean; refusing to apply patches; please stash or reset any changed files")
	}

	contents, err := os.ReadFile(appliedPatchesFile)
	if err != nil && !errors.Is(err, fs.ErrNotExist) {
		return fmt.Errorf("failed to read %s: %v", appliedPatchesFile, err)
	}
	appliedPatches := strings.Split(strings.Trim(string(contents), "\n"), "\n")

	// Get all of the patches from the patch list which are not yet applied.
	unappliedPatches := orderedSetDifference(patches, appliedPatches)
	if unappliedPatches == nil {
		return nil
	}

	glog.Infof(
		"[ applying new patches ] %s\n",
		strings.Join(unappliedPatches, "\n"),
	)
	if err := git.ApplyPatches(unappliedPatches, os.Stderr); err != nil {
		return fmt.Errorf("failed to apply unapplied patches: %v", err)
	}

	patchesLines := strings.Join(patches, "\n")
	allPatches := append(appliedPatches, unappliedPatches...)
	allPatchesLines := []byte(strings.Join(allPatches, "\n"))
	if err := os.WriteFile(appliedPatchesFile, allPatchesLines, 0666); err != nil {
		return fmt.Errorf("failed to write new patches lines: %v", err)
	}

	if err := git.Add([]string{appliedPatchesFile}, os.Stderr); err != nil {
		return fmt.Errorf("failed to `git add` %s: %v", appliedPatchesFile, err)
	}

	commitMessage := fmt.Sprintf(
		"prepare: Applied patches\n\nApplied the following patches:\n%s\n",
		patchesLines,
	)
	if err := git.Commit(commitMessage, os.Stderr); err != nil {
		return fmt.Errorf("failed to commit patch info: %v", err)
	}

	return nil
}

// orderedSetDifference returns all of the items in set A which are not in
// set B, while preserving the order of the items in A.
func orderedSetDifference(a, b []string) []string {
	setB := make(map[string]bool)
	for _, item := range b {
		setB[item] = true
	}
	var difference []string
	for _, item := range a {
		_, found := setB[item]
		if !found {
			difference = append(difference, item)
		}
	}

	return difference
}

// PrepareBranch switches to a new branch and sets up the local kernel
// directory for compilation.
//
// See `Prepare` for details on how the branch will be prepared.
//
// The new branch will be marked with a `cos-kernel-branch-start.txt` file,
// which contains the name of the branch. This can be used as a reference
// when creating patches from the new branch.
func PrepareBranch(ctx context.Context, downloader cos.ArtifactsDownloader, conf KernelVariantConfig, branch string) error {
	if err := git.SwitchToNewBranch(branch, os.Stderr); err != nil {
		return fmt.Errorf("could not switch to branch %s to prepare it: %v", branch, err)
	}

	if err := Prepare(ctx, downloader, conf); err != nil {
		return fmt.Errorf("failed to prepare branch %s: %v", branch, err)
	}

	if err := createPatchBase(PatchBaseFile, branch); err != nil {
		return fmt.Errorf("failed to create patch base: %v", err)
	}

	return nil
}

// createPatchBase creates the patch base file for a feature and commits it to the repo.
func createPatchBase(patchBaseFile, feature string) error {
	if err := os.WriteFile(patchBaseFile, []byte(feature), 0664); err != nil {
		return fmt.Errorf("failed to write %s to mark patch base for feature %s: %v", patchBaseFile, feature, err)
	}

	if err := git.Add([]string{patchBaseFile}, os.Stderr); err != nil {
		return fmt.Errorf("failed to add patch base file %s: %v", patchBaseFile, err)
	}

	commitMessage := fmt.Sprintf("prepare: patch base %s", feature)
	if err := git.Commit(commitMessage, os.Stderr); err != nil {
		return fmt.Errorf("failed to commit patch base: %v", err)
	}

	return nil
}

// ReadPatchBase reads the patch base file created by PrepareBranch.
func ReadPatchBase() (string, error) {
	contents, err := os.ReadFile(PatchBaseFile)
	if err != nil {
		return "", fmt.Errorf("failed to get patch base: %v", err)
	}
	return strings.TrimSpace(string(contents)), nil
}

// FindPatchBaseCommit finds the hash of the base commit for a branch
// created by PrepareBranch(..., branch).
func FindPatchBaseCommit(branch string) (string, error) {
	pattern := fmt.Sprintf("^prepare: patch base %s$", branch)
	commit, err := git.FindCommitByPattern(pattern)
	if err != nil {
		return "", fmt.Errorf("error searching for pattern %s: %v", pattern, err)
	}
	return commit, nil
}