/* Copyright 2016 The Bazel Authors. All rights reserved.

Licensed 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 main

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"path/filepath"

	"github.com/bazelbuild/bazel-gazelle/config"
	"github.com/bazelbuild/bazel-gazelle/rule"
	"github.com/pmezard/go-difflib/difflib"
)

var errExit = fmt.Errorf("encountered changes while running diff")

func diffFile(c *config.Config, f *rule.File) error {
	rel, err := filepath.Rel(c.RepoRoot, f.Path)
	if err != nil {
		return fmt.Errorf("error getting old path for file %q: %v", f.Path, err)
	}
	rel = filepath.ToSlash(rel)

	// The epoch timestamp is assumed to represent file creation/deletion events
	// by some tools, so use a dummy timestamp that is one ns past the epoch.
	// See https://github.com/bazelbuild/bazel-gazelle/issues/1528.
	date := "1970-01-01 00:00:00.000000001 +0000"

	diff := difflib.UnifiedDiff{
		Context:  3,
		FromDate: date,
		ToDate:   date,
	}

	newContent := f.Format()
	if bytes.Equal(newContent, f.Content) {
		// No change.
		return nil
	}

	if _, err := os.Stat(f.Path); os.IsNotExist(err) {
		diff.FromFile = "/dev/null"
	} else if err != nil {
		return fmt.Errorf("error reading original file: %v", err)
	} else if c.ReadBuildFilesDir == "" {
		diff.FromFile = rel
	} else {
		diff.FromFile = f.Path
	}

	if len(f.Content) != 0 {
		diff.A = difflib.SplitLines(string(f.Content))
	}

	diff.B = difflib.SplitLines(string(newContent))
	outPath := findOutputPath(c, f)
	if c.WriteBuildFilesDir == "" {
		diff.ToFile = rel
	} else {
		diff.ToFile = outPath
	}

	// The diff for patch creation should be stored separately from the original diff
	// so that the exit code from mode=diff is consistent with the exit code from
	// mode=fix|print
	patchDiff := diff

	// Trim off trailing newline from the end of the file.  Modern diff tools typically
	// handle this, however difflib does not and is no longer maintained, so we need to
	// handle it here.
	if len(diff.A) > 0 {
		if diff.A[len(diff.A)-1] == "\n" {
			patchDiff.A = diff.A[:len(diff.A)-1]
		}
	}
	if len(diff.B) > 0 {
		if diff.B[len(diff.B)-1] == "\n" {
			patchDiff.B = diff.B[:len(diff.B)-1]
		}
	}

	uc := getUpdateConfig(c)
	var out io.Writer = os.Stdout
	if uc.patchPath != "" {
		out = &uc.patchBuffer
	}
	if err := difflib.WriteUnifiedDiff(out, patchDiff); err != nil {
		return fmt.Errorf("error diffing %s: %v", f.Path, err)
	}
	if ds, _ := difflib.GetUnifiedDiffString(diff); ds != "" {
		return errExit
	}

	return nil
}
